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FUJITA Tomonori216bf582010-03-10 15:23:42 -08001 Dynamic DMA mapping Guide
2 =========================
Linus Torvalds1da177e2005-04-16 15:20:36 -07003
4 David S. Miller <davem@redhat.com>
5 Richard Henderson <rth@cygnus.com>
6 Jakub Jelinek <jakub@redhat.com>
7
FUJITA Tomonori216bf582010-03-10 15:23:42 -08008This is a guide to device driver writers on how to use the DMA API
9with example pseudo-code. For a concise description of the API, see
Linus Torvalds1da177e2005-04-16 15:20:36 -070010DMA-API.txt.
11
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -060012 CPU and DMA addresses
13
14There are several kinds of addresses involved in the DMA API, and it's
15important to understand the differences.
16
17The kernel normally uses virtual addresses. Any address returned by
18kmalloc(), vmalloc(), and similar interfaces is a virtual address and can
19be stored in a "void *".
20
21The virtual memory system (TLB, page tables, etc.) translates virtual
22addresses to CPU physical addresses, which are stored as "phys_addr_t" or
23"resource_size_t". The kernel manages device resources like registers as
24physical addresses. These are the addresses in /proc/iomem. The physical
25address is not directly useful to a driver; it must use ioremap() to map
26the space and produce a virtual address.
27
Yinghai Lu3a9ad0b2015-05-27 17:23:51 -070028I/O devices use a third kind of address: a "bus address". If a device has
29registers at an MMIO address, or if it performs DMA to read or write system
30memory, the addresses used by the device are bus addresses. In some
31systems, bus addresses are identical to CPU physical addresses, but in
32general they are not. IOMMUs and host bridges can produce arbitrary
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -060033mappings between physical and bus addresses.
34
Yinghai Lu3a9ad0b2015-05-27 17:23:51 -070035From a device's point of view, DMA uses the bus address space, but it may
36be restricted to a subset of that space. For example, even if a system
37supports 64-bit addresses for main memory and PCI BARs, it may use an IOMMU
38so devices only need to use 32-bit DMA addresses.
39
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -060040Here's a picture and some examples:
41
42 CPU CPU Bus
43 Virtual Physical Address
44 Address Address Space
45 Space Space
46
47 +-------+ +------+ +------+
48 | | |MMIO | Offset | |
49 | | Virtual |Space | applied | |
50 C +-------+ --------> B +------+ ----------> +------+ A
51 | | mapping | | by host | |
52 +-----+ | | | | bridge | | +--------+
53 | | | | +------+ | | | |
54 | CPU | | | | RAM | | | | Device |
55 | | | | | | | | | |
56 +-----+ +-------+ +------+ +------+ +--------+
57 | | Virtual |Buffer| Mapping | |
58 X +-------+ --------> Y +------+ <---------- +------+ Z
59 | | mapping | RAM | by IOMMU
60 | | | |
61 | | | |
62 +-------+ +------+
63
64During the enumeration process, the kernel learns about I/O devices and
65their MMIO space and the host bridges that connect them to the system. For
66example, if a PCI device has a BAR, the kernel reads the bus address (A)
67from the BAR and converts it to a CPU physical address (B). The address B
68is stored in a struct resource and usually exposed via /proc/iomem. When a
69driver claims a device, it typically uses ioremap() to map physical address
70B at a virtual address (C). It can then use, e.g., ioread32(C), to access
71the device registers at bus address A.
72
73If the device supports DMA, the driver sets up a buffer using kmalloc() or
74a similar interface, which returns a virtual address (X). The virtual
75memory system maps X to a physical address (Y) in system RAM. The driver
76can use virtual address X to access the buffer, but the device itself
77cannot because DMA doesn't go through the CPU virtual memory system.
78
79In some simple systems, the device can do DMA directly to physical address
Yinghai Lu3a9ad0b2015-05-27 17:23:51 -070080Y. But in many others, there is IOMMU hardware that translates DMA
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -060081addresses to physical addresses, e.g., it translates Z to Y. This is part
82of the reason for the DMA API: the driver can give a virtual address X to
83an interface like dma_map_single(), which sets up any required IOMMU
Yinghai Lu3a9ad0b2015-05-27 17:23:51 -070084mapping and returns the DMA address Z. The driver then tells the device to
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -060085do DMA to Z, and the IOMMU maps it to the buffer at address Y in system
86RAM.
Linus Torvalds1da177e2005-04-16 15:20:36 -070087
88So that Linux can use the dynamic DMA mapping, it needs some help from the
89drivers, namely it has to take into account that DMA addresses should be
90mapped only for the time they are actually used and unmapped after the DMA
91transfer.
92
93The following API will work of course even on platforms where no such
FUJITA Tomonori216bf582010-03-10 15:23:42 -080094hardware exists.
95
96Note that the DMA API works with any bus independent of the underlying
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -060097microprocessor architecture. You should use the DMA API rather than the
98bus-specific DMA API, i.e., use the dma_map_*() interfaces rather than the
99pci_map_*() interfaces.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700100
101First of all, you should make sure
102
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800103#include <linux/dma-mapping.h>
Linus Torvalds1da177e2005-04-16 15:20:36 -0700104
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600105is in your driver, which provides the definition of dma_addr_t. This type
Yinghai Lu3a9ad0b2015-05-27 17:23:51 -0700106can hold any valid DMA address for the platform and should be used
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600107everywhere you hold a DMA address returned from the DMA mapping functions.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700108
109 What memory is DMA'able?
110
111The first piece of information you must know is what kernel memory can
112be used with the DMA mapping facilities. There has been an unwritten
113set of rules regarding this, and this text is an attempt to finally
114write them down.
115
116If you acquired your memory via the page allocator
117(i.e. __get_free_page*()) or the generic memory allocators
118(i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from
119that memory using the addresses returned from those routines.
120
121This means specifically that you may _not_ use the memory/addresses
122returned from vmalloc() for DMA. It is possible to DMA to the
123_underlying_ memory mapped into a vmalloc() area, but this requires
124walking page tables to get the physical addresses, and then
125translating each of those pages back to a kernel address using
126something like __va(). [ EDIT: Update this when we integrate
127Gerd Knorr's generic code which does this. ]
128
David Brownell21440d32006-04-01 10:21:52 -0800129This rule also means that you may use neither kernel image addresses
130(items in data/text/bss segments), nor module image addresses, nor
131stack addresses for DMA. These could all be mapped somewhere entirely
132different than the rest of physical memory. Even if those classes of
133memory could physically work with DMA, you'd need to ensure the I/O
134buffers were cacheline-aligned. Without that, you'd see cacheline
135sharing problems (data corruption) on CPUs with DMA-incoherent caches.
136(The CPU could write to one word, DMA would write to a different one
137in the same cache line, and one of them could be overwritten.)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700138
139Also, this means that you cannot take the return of a kmap()
140call and DMA to/from that. This is similar to vmalloc().
141
142What about block I/O and networking buffers? The block I/O and
143networking subsystems make sure that the buffers they use are valid
144for you to DMA from/to.
145
146 DMA addressing limitations
147
148Does your device have any DMA addressing limitations? For example, is
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800149your device only capable of driving the low order 24-bits of address?
150If so, you need to inform the kernel of this fact.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700151
152By default, the kernel assumes that your device can address the full
FUJITA Tomonori216bf582010-03-10 15:23:42 -080015332-bits. For a 64-bit capable device, this needs to be increased.
154And for a device with limitations, as discussed in the previous
155paragraph, it needs to be decreased.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700156
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800157Special note about PCI: PCI-X specification requires PCI-X devices to
158support 64-bit addressing (DAC) for all transactions. And at least
159one platform (SGI SN2) requires 64-bit consistent allocations to
160operate correctly when the IO bus is in PCI-X mode.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700161
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800162For correct operation, you must interrogate the kernel in your device
163probe routine to see if the DMA controller on the machine can properly
164support the DMA addressing limitation your device has. It is good
165style to do this even if your device holds the default setting,
Linus Torvalds1da177e2005-04-16 15:20:36 -0700166because this shows that you did think about these issues wrt. your
167device.
168
Russell King4aa806b2013-06-26 13:49:44 +0100169The query is performed via a call to dma_set_mask_and_coherent():
Linus Torvalds1da177e2005-04-16 15:20:36 -0700170
Russell King4aa806b2013-06-26 13:49:44 +0100171 int dma_set_mask_and_coherent(struct device *dev, u64 mask);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700172
Russell King4aa806b2013-06-26 13:49:44 +0100173which will query the mask for both streaming and coherent APIs together.
174If you have some special requirements, then the following two separate
175queries can be used instead:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700176
Russell King4aa806b2013-06-26 13:49:44 +0100177 The query for streaming mappings is performed via a call to
178 dma_set_mask():
179
180 int dma_set_mask(struct device *dev, u64 mask);
181
182 The query for consistent allocations is performed via a call
183 to dma_set_coherent_mask():
184
185 int dma_set_coherent_mask(struct device *dev, u64 mask);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700186
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800187Here, dev is a pointer to the device struct of your device, and mask
188is a bit mask describing which bits of an address your device
189supports. It returns zero if your card can perform DMA properly on
190the machine given the address mask you provided. In general, the
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600191device struct of your device is embedded in the bus-specific device
192struct of your device. For example, &pdev->dev is a pointer to the
193device struct of a PCI device (pdev is a pointer to the PCI device
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800194struct of your device).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700195
Matt LaPlante84eb8d02006-10-03 22:53:09 +0200196If it returns non-zero, your device cannot perform DMA properly on
Linus Torvalds1da177e2005-04-16 15:20:36 -0700197this platform, and attempting to do so will result in undefined
198behavior. You must either use a different mask, or not use DMA.
199
200This means that in the failure case, you have three options:
201
2021) Use another DMA mask, if possible (see below).
2032) Use some non-DMA mode for data transfer, if possible.
2043) Ignore this device and do not initialize it.
205
206It is recommended that your driver print a kernel KERN_WARNING message
207when you end up performing either #2 or #3. In this manner, if a user
208of your driver reports that performance is bad or that the device is not
209even detected, you can ask them for the kernel messages to find out
210exactly why.
211
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800212The standard 32-bit addressing device would do something like this:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700213
Russell King4aa806b2013-06-26 13:49:44 +0100214 if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32))) {
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600215 dev_warn(dev, "mydev: No suitable DMA available\n");
Linus Torvalds1da177e2005-04-16 15:20:36 -0700216 goto ignore_this_device;
217 }
218
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800219Another common scenario is a 64-bit capable device. The approach here
220is to try for 64-bit addressing, but back down to a 32-bit mask that
221should not fail. The kernel may fail the 64-bit mask not because the
222platform is not capable of 64-bit addressing. Rather, it may fail in
223this case simply because 32-bit addressing is done more efficiently
224than 64-bit addressing. For example, Sparc64 PCI SAC addressing is
225more efficient than DAC addressing.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700226
227Here is how you would handle a 64-bit capable device which can drive
228all 64-bits when accessing streaming DMA:
229
230 int using_dac;
231
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800232 if (!dma_set_mask(dev, DMA_BIT_MASK(64))) {
Linus Torvalds1da177e2005-04-16 15:20:36 -0700233 using_dac = 1;
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800234 } else if (!dma_set_mask(dev, DMA_BIT_MASK(32))) {
Linus Torvalds1da177e2005-04-16 15:20:36 -0700235 using_dac = 0;
236 } else {
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600237 dev_warn(dev, "mydev: No suitable DMA available\n");
Linus Torvalds1da177e2005-04-16 15:20:36 -0700238 goto ignore_this_device;
239 }
240
241If a card is capable of using 64-bit consistent allocations as well,
242the case would look like this:
243
244 int using_dac, consistent_using_dac;
245
Russell King4aa806b2013-06-26 13:49:44 +0100246 if (!dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64))) {
Linus Torvalds1da177e2005-04-16 15:20:36 -0700247 using_dac = 1;
Geert Uytterhoeven11e285d2015-05-21 13:57:07 +0200248 consistent_using_dac = 1;
Russell King4aa806b2013-06-26 13:49:44 +0100249 } else if (!dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32))) {
Linus Torvalds1da177e2005-04-16 15:20:36 -0700250 using_dac = 0;
251 consistent_using_dac = 0;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700252 } else {
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600253 dev_warn(dev, "mydev: No suitable DMA available\n");
Linus Torvalds1da177e2005-04-16 15:20:36 -0700254 goto ignore_this_device;
255 }
256
Emilio López34c815f2014-05-20 16:54:22 -0600257The coherent mask will always be able to set the same or a smaller mask as
258the streaming mask. However for the rare case that a device driver only
259uses consistent allocations, one would have to check the return value from
260dma_set_coherent_mask().
Linus Torvalds1da177e2005-04-16 15:20:36 -0700261
Linus Torvalds1da177e2005-04-16 15:20:36 -0700262Finally, if your device can only drive the low 24-bits of
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800263address you might do something like:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700264
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800265 if (dma_set_mask(dev, DMA_BIT_MASK(24))) {
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600266 dev_warn(dev, "mydev: 24-bit DMA addressing not available\n");
Linus Torvalds1da177e2005-04-16 15:20:36 -0700267 goto ignore_this_device;
268 }
269
Russell King4aa806b2013-06-26 13:49:44 +0100270When dma_set_mask() or dma_set_mask_and_coherent() is successful, and
271returns zero, the kernel saves away this mask you have provided. The
272kernel will use this information later when you make DMA mappings.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700273
274There is a case which we are aware of at this time, which is worth
275mentioning in this documentation. If your device supports multiple
276functions (for example a sound card provides playback and record
277functions) and the various different functions have _different_
278DMA addressing limitations, you may wish to probe each mask and
279only provide the functionality which the machine can handle. It
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800280is important that the last call to dma_set_mask() be for the
Linus Torvalds1da177e2005-04-16 15:20:36 -0700281most specific mask.
282
283Here is pseudo-code showing how this might be done:
284
Yang Hongyang2c5510d2009-04-06 19:01:19 -0700285 #define PLAYBACK_ADDRESS_BITS DMA_BIT_MASK(32)
Marin Mitov038f7d02009-12-06 18:30:44 -0800286 #define RECORD_ADDRESS_BITS DMA_BIT_MASK(24)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700287
288 struct my_sound_card *card;
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800289 struct device *dev;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700290
291 ...
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800292 if (!dma_set_mask(dev, PLAYBACK_ADDRESS_BITS)) {
Linus Torvalds1da177e2005-04-16 15:20:36 -0700293 card->playback_enabled = 1;
294 } else {
295 card->playback_enabled = 0;
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600296 dev_warn(dev, "%s: Playback disabled due to DMA limitations\n",
Linus Torvalds1da177e2005-04-16 15:20:36 -0700297 card->name);
298 }
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800299 if (!dma_set_mask(dev, RECORD_ADDRESS_BITS)) {
Linus Torvalds1da177e2005-04-16 15:20:36 -0700300 card->record_enabled = 1;
301 } else {
302 card->record_enabled = 0;
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600303 dev_warn(dev, "%s: Record disabled due to DMA limitations\n",
Linus Torvalds1da177e2005-04-16 15:20:36 -0700304 card->name);
305 }
306
307A sound card was used as an example here because this genre of PCI
308devices seems to be littered with ISA chips given a PCI front end,
309and thus retaining the 16MB DMA addressing limitations of ISA.
310
311 Types of DMA mappings
312
313There are two types of DMA mappings:
314
315- Consistent DMA mappings which are usually mapped at driver
316 initialization, unmapped at the end and for which the hardware should
317 guarantee that the device and the CPU can access the data
318 in parallel and will see updates made by each other without any
319 explicit software flushing.
320
321 Think of "consistent" as "synchronous" or "coherent".
322
323 The current default is to return consistent memory in the low 32
Yinghai Lu3a9ad0b2015-05-27 17:23:51 -0700324 bits of the DMA space. However, for future compatibility you should
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800325 set the consistent mask even if this default is fine for your
Linus Torvalds1da177e2005-04-16 15:20:36 -0700326 driver.
327
328 Good examples of what to use consistent mappings for are:
329
330 - Network card DMA ring descriptors.
331 - SCSI adapter mailbox command data structures.
332 - Device firmware microcode executed out of
333 main memory.
334
335 The invariant these examples all require is that any CPU store
336 to memory is immediately visible to the device, and vice
337 versa. Consistent mappings guarantee this.
338
339 IMPORTANT: Consistent DMA memory does not preclude the usage of
340 proper memory barriers. The CPU may reorder stores to
341 consistent memory just as it may normal memory. Example:
342 if it is important for the device to see the first word
343 of a descriptor updated before the second, you must do
344 something like:
345
346 desc->word0 = address;
347 wmb();
348 desc->word1 = DESC_VALID;
349
350 in order to get correct behavior on all platforms.
351
David Brownell21440d32006-04-01 10:21:52 -0800352 Also, on some platforms your driver may need to flush CPU write
353 buffers in much the same way as it needs to flush write buffers
354 found in PCI bridges (such as by reading a register's value
355 after writing it).
356
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800357- Streaming DMA mappings which are usually mapped for one DMA
358 transfer, unmapped right after it (unless you use dma_sync_* below)
359 and for which hardware can optimize for sequential accesses.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700360
Geert Uytterhoeven11e285d2015-05-21 13:57:07 +0200361 Think of "streaming" as "asynchronous" or "outside the coherency
Linus Torvalds1da177e2005-04-16 15:20:36 -0700362 domain".
363
364 Good examples of what to use streaming mappings for are:
365
366 - Networking buffers transmitted/received by a device.
367 - Filesystem buffers written/read by a SCSI device.
368
369 The interfaces for using this type of mapping were designed in
370 such a way that an implementation can make whatever performance
371 optimizations the hardware allows. To this end, when using
372 such mappings you must be explicit about what you want to happen.
373
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800374Neither type of DMA mapping has alignment restrictions that come from
375the underlying bus, although some devices may have such restrictions.
David Brownell21440d32006-04-01 10:21:52 -0800376Also, systems with caches that aren't DMA-coherent will work better
377when the underlying buffers don't share cache lines with other data.
378
Linus Torvalds1da177e2005-04-16 15:20:36 -0700379
380 Using Consistent DMA mappings.
381
382To allocate and map large (PAGE_SIZE or so) consistent DMA regions,
383you should do:
384
385 dma_addr_t dma_handle;
386
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800387 cpu_addr = dma_alloc_coherent(dev, size, &dma_handle, gfp);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700388
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800389where device is a struct device *. This may be called in interrupt
390context with the GFP_ATOMIC flag.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700391
392Size is the length of the region you want to allocate, in bytes.
393
394This routine will allocate RAM for that region, so it acts similarly to
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600395__get_free_pages() (but takes size instead of a page order). If your
Linus Torvalds1da177e2005-04-16 15:20:36 -0700396driver needs regions sized smaller than a page, you may prefer using
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800397the dma_pool interface, described below.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700398
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800399The consistent DMA mapping interfaces, for non-NULL dev, will by
400default return a DMA address which is 32-bit addressable. Even if the
401device indicates (via DMA mask) that it may address the upper 32-bits,
402consistent allocation will only return > 32-bit addresses for DMA if
403the consistent DMA mask has been explicitly changed via
404dma_set_coherent_mask(). This is true of the dma_pool interface as
405well.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700406
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600407dma_alloc_coherent() returns two values: the virtual address which you
Linus Torvalds1da177e2005-04-16 15:20:36 -0700408can use to access it from the CPU and dma_handle which you pass to the
409card.
410
Yinghai Lu3a9ad0b2015-05-27 17:23:51 -0700411The CPU virtual address and the DMA address are both
Linus Torvalds1da177e2005-04-16 15:20:36 -0700412guaranteed to be aligned to the smallest PAGE_SIZE order which
413is greater than or equal to the requested size. This invariant
414exists (for example) to guarantee that if you allocate a chunk
415which is smaller than or equal to 64 kilobytes, the extent of the
416buffer you receive will not cross a 64K boundary.
417
418To unmap and free such a DMA region, you call:
419
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800420 dma_free_coherent(dev, size, cpu_addr, dma_handle);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700421
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800422where dev, size are the same as in the above call and cpu_addr and
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600423dma_handle are the values dma_alloc_coherent() returned to you.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700424This function may not be called in interrupt context.
425
426If your driver needs lots of smaller memory regions, you can write
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600427custom code to subdivide pages returned by dma_alloc_coherent(),
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800428or you can use the dma_pool API to do that. A dma_pool is like
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600429a kmem_cache, but it uses dma_alloc_coherent(), not __get_free_pages().
Linus Torvalds1da177e2005-04-16 15:20:36 -0700430Also, it understands common hardware constraints for alignment,
431like queue heads needing to be aligned on N byte boundaries.
432
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800433Create a dma_pool like this:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700434
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800435 struct dma_pool *pool;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700436
Gioh Kim2af9da82014-05-20 17:09:35 -0600437 pool = dma_pool_create(name, dev, size, align, boundary);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700438
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800439The "name" is for diagnostics (like a kmem_cache name); dev and size
Linus Torvalds1da177e2005-04-16 15:20:36 -0700440are as above. The device's hardware alignment requirement for this
441type of data is "align" (which is expressed in bytes, and must be a
442power of two). If your device has no boundary crossing restrictions,
Gioh Kim2af9da82014-05-20 17:09:35 -0600443pass 0 for boundary; passing 4096 says memory allocated from this pool
Linus Torvalds1da177e2005-04-16 15:20:36 -0700444must not cross 4KByte boundaries (but at that time it may be better to
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600445use dma_alloc_coherent() directly instead).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700446
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600447Allocate memory from a DMA pool like this:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700448
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800449 cpu_addr = dma_pool_alloc(pool, flags, &dma_handle);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700450
Gioh Kim2af9da82014-05-20 17:09:35 -0600451flags are GFP_KERNEL if blocking is permitted (not in_interrupt nor
452holding SMP locks), GFP_ATOMIC otherwise. Like dma_alloc_coherent(),
Linus Torvalds1da177e2005-04-16 15:20:36 -0700453this returns two values, cpu_addr and dma_handle.
454
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800455Free memory that was allocated from a dma_pool like this:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700456
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800457 dma_pool_free(pool, cpu_addr, dma_handle);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700458
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600459where pool is what you passed to dma_pool_alloc(), and cpu_addr and
460dma_handle are the values dma_pool_alloc() returned. This function
Linus Torvalds1da177e2005-04-16 15:20:36 -0700461may be called in interrupt context.
462
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800463Destroy a dma_pool by calling:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700464
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800465 dma_pool_destroy(pool);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700466
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600467Make sure you've called dma_pool_free() for all memory allocated
Linus Torvalds1da177e2005-04-16 15:20:36 -0700468from a pool before you destroy the pool. This function may not
469be called in interrupt context.
470
471 DMA Direction
472
473The interfaces described in subsequent portions of this document
474take a DMA direction argument, which is an integer and takes on
475one of the following values:
476
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800477 DMA_BIDIRECTIONAL
478 DMA_TO_DEVICE
479 DMA_FROM_DEVICE
480 DMA_NONE
Linus Torvalds1da177e2005-04-16 15:20:36 -0700481
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600482You should provide the exact DMA direction if you know it.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700483
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800484DMA_TO_DEVICE means "from main memory to the device"
485DMA_FROM_DEVICE means "from the device to main memory"
Linus Torvalds1da177e2005-04-16 15:20:36 -0700486It is the direction in which the data moves during the DMA
487transfer.
488
489You are _strongly_ encouraged to specify this as precisely
490as you possibly can.
491
492If you absolutely cannot know the direction of the DMA transfer,
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800493specify DMA_BIDIRECTIONAL. It means that the DMA can go in
Linus Torvalds1da177e2005-04-16 15:20:36 -0700494either direction. The platform guarantees that you may legally
495specify this, and that it will work, but this may be at the
496cost of performance for example.
497
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800498The value DMA_NONE is to be used for debugging. One can
Linus Torvalds1da177e2005-04-16 15:20:36 -0700499hold this in a data structure before you come to know the
500precise direction, and this will help catch cases where your
501direction tracking logic has failed to set things up properly.
502
503Another advantage of specifying this value precisely (outside of
504potential platform-specific optimizations of such) is for debugging.
505Some platforms actually have a write permission boolean which DMA
506mappings can be marked with, much like page protections in the user
507program address space. Such platforms can and do report errors in the
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800508kernel logs when the DMA controller hardware detects violation of the
Linus Torvalds1da177e2005-04-16 15:20:36 -0700509permission setting.
510
511Only streaming mappings specify a direction, consistent mappings
512implicitly have a direction attribute setting of
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800513DMA_BIDIRECTIONAL.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700514
be7db052005-04-17 15:26:13 -0500515The SCSI subsystem tells you the direction to use in the
516'sc_data_direction' member of the SCSI command your driver is
517working on.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700518
519For Networking drivers, it's a rather simple affair. For transmit
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800520packets, map/unmap them with the DMA_TO_DEVICE direction
Linus Torvalds1da177e2005-04-16 15:20:36 -0700521specifier. For receive packets, just the opposite, map/unmap them
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800522with the DMA_FROM_DEVICE direction specifier.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700523
524 Using Streaming DMA mappings
525
526The streaming DMA mapping routines can be called from interrupt
527context. There are two versions of each map/unmap, one which will
528map/unmap a single memory region, and one which will map/unmap a
529scatterlist.
530
531To map a single region, you do:
532
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800533 struct device *dev = &my_dev->dev;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700534 dma_addr_t dma_handle;
535 void *addr = buffer->ptr;
536 size_t size = buffer->len;
537
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800538 dma_handle = dma_map_single(dev, addr, size, direction);
Liu Huab2dd83b2014-09-18 12:15:28 +0800539 if (dma_mapping_error(dev, dma_handle)) {
Shuah Khan8d7f62e2012-10-18 14:00:58 -0600540 /*
541 * reduce current DMA mapping usage,
542 * delay and try again later or
543 * reset driver.
544 */
545 goto map_error_handling;
546 }
Linus Torvalds1da177e2005-04-16 15:20:36 -0700547
548and to unmap it:
549
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800550 dma_unmap_single(dev, dma_handle, size, direction);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700551
Shuah Khan8d7f62e2012-10-18 14:00:58 -0600552You should call dma_mapping_error() as dma_map_single() could fail and return
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600553error. Not all DMA implementations support the dma_mapping_error() interface.
Shuah Khan8d7f62e2012-10-18 14:00:58 -0600554However, it is a good practice to call dma_mapping_error() interface, which
555will invoke the generic mapping error check interface. Doing so will ensure
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600556that the mapping code will work correctly on all DMA implementations without
Shuah Khan8d7f62e2012-10-18 14:00:58 -0600557any dependency on the specifics of the underlying implementation. Using the
558returned address without checking for errors could result in failures ranging
Shuah Khanbe62bc42013-02-27 17:03:10 -0800559from panics to silent data corruption. A couple of examples of incorrect ways
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600560to check for errors that make assumptions about the underlying DMA
Shuah Khanbe62bc42013-02-27 17:03:10 -0800561implementation are as follows and these are applicable to dma_map_page() as
562well.
Shuah Khan8d7f62e2012-10-18 14:00:58 -0600563
564Incorrect example 1:
565 dma_addr_t dma_handle;
566
567 dma_handle = dma_map_single(dev, addr, size, direction);
568 if ((dma_handle & 0xffff != 0) || (dma_handle >= 0x1000000)) {
569 goto map_error;
570 }
571
572Incorrect example 2:
573 dma_addr_t dma_handle;
574
575 dma_handle = dma_map_single(dev, addr, size, direction);
576 if (dma_handle == DMA_ERROR_CODE) {
577 goto map_error;
578 }
579
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600580You should call dma_unmap_single() when the DMA activity is finished, e.g.,
Linus Torvalds1da177e2005-04-16 15:20:36 -0700581from the interrupt which told you that the DMA transfer is done.
582
Bjorn Helgaasf311a722014-05-20 16:56:27 -0600583Using CPU pointers like this for single mappings has a disadvantage:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700584you cannot reference HIGHMEM memory in this way. Thus, there is a
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600585map/unmap interface pair akin to dma_{map,unmap}_single(). These
Bjorn Helgaasf311a722014-05-20 16:56:27 -0600586interfaces deal with page/offset pairs instead of CPU pointers.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700587Specifically:
588
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800589 struct device *dev = &my_dev->dev;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700590 dma_addr_t dma_handle;
591 struct page *page = buffer->page;
592 unsigned long offset = buffer->offset;
593 size_t size = buffer->len;
594
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800595 dma_handle = dma_map_page(dev, page, offset, size, direction);
Liu Huab2dd83b2014-09-18 12:15:28 +0800596 if (dma_mapping_error(dev, dma_handle)) {
Shuah Khan8d7f62e2012-10-18 14:00:58 -0600597 /*
598 * reduce current DMA mapping usage,
599 * delay and try again later or
600 * reset driver.
601 */
602 goto map_error_handling;
603 }
Linus Torvalds1da177e2005-04-16 15:20:36 -0700604
605 ...
606
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800607 dma_unmap_page(dev, dma_handle, size, direction);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700608
609Here, "offset" means byte offset within the given page.
610
Shuah Khan8d7f62e2012-10-18 14:00:58 -0600611You should call dma_mapping_error() as dma_map_page() could fail and return
612error as outlined under the dma_map_single() discussion.
613
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600614You should call dma_unmap_page() when the DMA activity is finished, e.g.,
Shuah Khan8d7f62e2012-10-18 14:00:58 -0600615from the interrupt which told you that the DMA transfer is done.
616
Linus Torvalds1da177e2005-04-16 15:20:36 -0700617With scatterlists, you map a region gathered from several regions by:
618
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800619 int i, count = dma_map_sg(dev, sglist, nents, direction);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700620 struct scatterlist *sg;
621
saeed bishara4c2f6d42007-08-08 13:09:00 +0200622 for_each_sg(sglist, sg, count, i) {
Linus Torvalds1da177e2005-04-16 15:20:36 -0700623 hw_address[i] = sg_dma_address(sg);
624 hw_len[i] = sg_dma_len(sg);
625 }
626
627where nents is the number of entries in the sglist.
628
629The implementation is free to merge several consecutive sglist entries
630into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
631consecutive sglist entries can be merged into one provided the first one
632ends and the second one starts on a page boundary - in fact this is a huge
633advantage for cards which either cannot do scatter-gather or have very
634limited number of scatter-gather entries) and returns the actual number
635of sg entries it mapped them to. On failure 0 is returned.
636
637Then you should loop count times (note: this can be less than nents times)
638and use sg_dma_address() and sg_dma_len() macros where you previously
639accessed sg->address and sg->length as shown above.
640
641To unmap a scatterlist, just call:
642
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800643 dma_unmap_sg(dev, sglist, nents, direction);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700644
645Again, make sure DMA activity has already finished.
646
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800647PLEASE NOTE: The 'nents' argument to the dma_unmap_sg call must be
648 the _same_ one you passed into the dma_map_sg call,
Linus Torvalds1da177e2005-04-16 15:20:36 -0700649 it should _NOT_ be the 'count' value _returned_ from the
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800650 dma_map_sg call.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700651
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600652Every dma_map_{single,sg}() call should have its dma_unmap_{single,sg}()
Yinghai Lu3a9ad0b2015-05-27 17:23:51 -0700653counterpart, because the DMA address space is a shared resource and
654you could render the machine unusable by consuming all DMA addresses.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700655
656If you need to use the same streaming DMA region multiple times and touch
657the data in between the DMA transfers, the buffer needs to be synced
Bjorn Helgaasf311a722014-05-20 16:56:27 -0600658properly in order for the CPU and device to see the most up-to-date and
Linus Torvalds1da177e2005-04-16 15:20:36 -0700659correct copy of the DMA buffer.
660
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600661So, firstly, just map it with dma_map_{single,sg}(), and after each DMA
Linus Torvalds1da177e2005-04-16 15:20:36 -0700662transfer call either:
663
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800664 dma_sync_single_for_cpu(dev, dma_handle, size, direction);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700665
666or:
667
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800668 dma_sync_sg_for_cpu(dev, sglist, nents, direction);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700669
670as appropriate.
671
672Then, if you wish to let the device get at the DMA area again,
Bjorn Helgaasf311a722014-05-20 16:56:27 -0600673finish accessing the data with the CPU, and then before actually
Linus Torvalds1da177e2005-04-16 15:20:36 -0700674giving the buffer to the hardware call either:
675
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800676 dma_sync_single_for_device(dev, dma_handle, size, direction);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700677
678or:
679
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800680 dma_sync_sg_for_device(dev, sglist, nents, direction);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700681
682as appropriate.
683
Sakari Ailus7bc590b2015-09-23 14:41:09 +0300684PLEASE NOTE: The 'nents' argument to dma_sync_sg_for_cpu() and
685 dma_sync_sg_for_device() must be the same passed to
686 dma_map_sg(). It is _NOT_ the count returned by
687 dma_map_sg().
688
Linus Torvalds1da177e2005-04-16 15:20:36 -0700689After the last DMA transfer call one of the DMA unmap routines
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600690dma_unmap_{single,sg}(). If you don't touch the data from the first
691dma_map_*() call till dma_unmap_*(), then you don't have to call the
692dma_sync_*() routines at all.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700693
694Here is pseudo code which shows a situation in which you would need
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800695to use the dma_sync_*() interfaces.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700696
697 my_card_setup_receive_buffer(struct my_card *cp, char *buffer, int len)
698 {
699 dma_addr_t mapping;
700
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800701 mapping = dma_map_single(cp->dev, buffer, len, DMA_FROM_DEVICE);
Liu Huab2dd83b2014-09-18 12:15:28 +0800702 if (dma_mapping_error(cp->dev, dma_handle)) {
Shuah Khan8d7f62e2012-10-18 14:00:58 -0600703 /*
704 * reduce current DMA mapping usage,
705 * delay and try again later or
706 * reset driver.
707 */
708 goto map_error_handling;
709 }
Linus Torvalds1da177e2005-04-16 15:20:36 -0700710
711 cp->rx_buf = buffer;
712 cp->rx_len = len;
713 cp->rx_dma = mapping;
714
715 give_rx_buf_to_card(cp);
716 }
717
718 ...
719
720 my_card_interrupt_handler(int irq, void *devid, struct pt_regs *regs)
721 {
722 struct my_card *cp = devid;
723
724 ...
725 if (read_card_status(cp) == RX_BUF_TRANSFERRED) {
726 struct my_card_header *hp;
727
728 /* Examine the header to see if we wish
729 * to accept the data. But synchronize
730 * the DMA transfer with the CPU first
731 * so that we see updated contents.
732 */
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800733 dma_sync_single_for_cpu(&cp->dev, cp->rx_dma,
734 cp->rx_len,
735 DMA_FROM_DEVICE);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700736
737 /* Now it is safe to examine the buffer. */
738 hp = (struct my_card_header *) cp->rx_buf;
739 if (header_is_ok(hp)) {
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800740 dma_unmap_single(&cp->dev, cp->rx_dma, cp->rx_len,
741 DMA_FROM_DEVICE);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700742 pass_to_upper_layers(cp->rx_buf);
743 make_and_setup_new_rx_buf(cp);
744 } else {
Michal Miroslaw3f0fb4e2011-07-26 16:08:51 -0700745 /* CPU should not write to
746 * DMA_FROM_DEVICE-mapped area,
747 * so dma_sync_single_for_device() is
748 * not needed here. It would be required
749 * for DMA_BIDIRECTIONAL mapping if
750 * the memory was modified.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700751 */
Linus Torvalds1da177e2005-04-16 15:20:36 -0700752 give_rx_buf_to_card(cp);
753 }
754 }
755 }
756
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600757Drivers converted fully to this interface should not use virt_to_bus() any
758longer, nor should they use bus_to_virt(). Some drivers have to be changed a
759little bit, because there is no longer an equivalent to bus_to_virt() in the
Linus Torvalds1da177e2005-04-16 15:20:36 -0700760dynamic DMA mapping scheme - you have to always store the DMA addresses
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600761returned by the dma_alloc_coherent(), dma_pool_alloc(), and dma_map_single()
762calls (dma_map_sg() stores them in the scatterlist itself if the platform
Linus Torvalds1da177e2005-04-16 15:20:36 -0700763supports dynamic DMA mapping in hardware) in your driver structures and/or
764in the card registers.
765
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800766All drivers should be using these interfaces with no exceptions. It
767is planned to completely remove virt_to_bus() and bus_to_virt() as
Linus Torvalds1da177e2005-04-16 15:20:36 -0700768they are entirely deprecated. Some ports already do not provide these
769as it is impossible to correctly support them.
770
FUJITA Tomonori4ae9ca82010-05-26 14:44:22 -0700771 Handling Errors
772
773DMA address space is limited on some architectures and an allocation
774failure can be determined by:
775
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600776- checking if dma_alloc_coherent() returns NULL or dma_map_sg returns 0
FUJITA Tomonori4ae9ca82010-05-26 14:44:22 -0700777
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600778- checking the dma_addr_t returned from dma_map_single() and dma_map_page()
FUJITA Tomonori4ae9ca82010-05-26 14:44:22 -0700779 by using dma_mapping_error():
780
781 dma_addr_t dma_handle;
782
783 dma_handle = dma_map_single(dev, addr, size, direction);
784 if (dma_mapping_error(dev, dma_handle)) {
785 /*
786 * reduce current DMA mapping usage,
787 * delay and try again later or
788 * reset driver.
789 */
Shuah Khan8d7f62e2012-10-18 14:00:58 -0600790 goto map_error_handling;
791 }
792
793- unmap pages that are already mapped, when mapping error occurs in the middle
794 of a multiple page mapping attempt. These example are applicable to
795 dma_map_page() as well.
796
797Example 1:
798 dma_addr_t dma_handle1;
799 dma_addr_t dma_handle2;
800
801 dma_handle1 = dma_map_single(dev, addr, size, direction);
802 if (dma_mapping_error(dev, dma_handle1)) {
803 /*
804 * reduce current DMA mapping usage,
805 * delay and try again later or
806 * reset driver.
807 */
808 goto map_error_handling1;
809 }
810 dma_handle2 = dma_map_single(dev, addr, size, direction);
811 if (dma_mapping_error(dev, dma_handle2)) {
812 /*
813 * reduce current DMA mapping usage,
814 * delay and try again later or
815 * reset driver.
816 */
817 goto map_error_handling2;
818 }
819
820 ...
821
822 map_error_handling2:
823 dma_unmap_single(dma_handle1);
824 map_error_handling1:
825
Andrew Morton11cd3db2013-02-27 17:05:43 -0800826Example 2: (if buffers are allocated in a loop, unmap all mapped buffers when
Shuah Khan8d7f62e2012-10-18 14:00:58 -0600827 mapping error is detected in the middle)
828
829 dma_addr_t dma_addr;
830 dma_addr_t array[DMA_BUFFERS];
831 int save_index = 0;
832
833 for (i = 0; i < DMA_BUFFERS; i++) {
834
835 ...
836
837 dma_addr = dma_map_single(dev, addr, size, direction);
838 if (dma_mapping_error(dev, dma_addr)) {
839 /*
840 * reduce current DMA mapping usage,
841 * delay and try again later or
842 * reset driver.
843 */
844 goto map_error_handling;
845 }
846 array[i].dma_addr = dma_addr;
847 save_index++;
848 }
849
850 ...
851
852 map_error_handling:
853
854 for (i = 0; i < save_index; i++) {
855
856 ...
857
858 dma_unmap_single(array[i].dma_addr);
FUJITA Tomonori4ae9ca82010-05-26 14:44:22 -0700859 }
860
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -0600861Networking drivers must call dev_kfree_skb() to free the socket buffer
FUJITA Tomonori4ae9ca82010-05-26 14:44:22 -0700862and return NETDEV_TX_OK if the DMA mapping fails on the transmit hook
863(ndo_start_xmit). This means that the socket buffer is just dropped in
864the failure case.
865
866SCSI drivers must return SCSI_MLQUEUE_HOST_BUSY if the DMA mapping
867fails in the queuecommand hook. This means that the SCSI subsystem
868passes the command to the driver again later.
869
Linus Torvalds1da177e2005-04-16 15:20:36 -0700870 Optimizing Unmap State Space Consumption
871
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800872On many platforms, dma_unmap_{single,page}() is simply a nop.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700873Therefore, keeping track of the mapping address and length is a waste
874of space. Instead of filling your drivers up with ifdefs and the like
875to "work around" this (which would defeat the whole purpose of a
876portable API) the following facilities are provided.
877
878Actually, instead of describing the macros one by one, we'll
879transform some example code.
880
FUJITA Tomonori216bf582010-03-10 15:23:42 -08008811) Use DEFINE_DMA_UNMAP_{ADDR,LEN} in state saving structures.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700882 Example, before:
883
884 struct ring_state {
885 struct sk_buff *skb;
886 dma_addr_t mapping;
887 __u32 len;
888 };
889
890 after:
891
892 struct ring_state {
893 struct sk_buff *skb;
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800894 DEFINE_DMA_UNMAP_ADDR(mapping);
895 DEFINE_DMA_UNMAP_LEN(len);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700896 };
897
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -06008982) Use dma_unmap_{addr,len}_set() to set these values.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700899 Example, before:
900
901 ringp->mapping = FOO;
902 ringp->len = BAR;
903
904 after:
905
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800906 dma_unmap_addr_set(ringp, mapping, FOO);
907 dma_unmap_len_set(ringp, len, BAR);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700908
Bjorn Helgaas77f2ea22014-04-30 11:20:53 -06009093) Use dma_unmap_{addr,len}() to access these values.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700910 Example, before:
911
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800912 dma_unmap_single(dev, ringp->mapping, ringp->len,
913 DMA_FROM_DEVICE);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700914
915 after:
916
FUJITA Tomonori216bf582010-03-10 15:23:42 -0800917 dma_unmap_single(dev,
918 dma_unmap_addr(ringp, mapping),
919 dma_unmap_len(ringp, len),
920 DMA_FROM_DEVICE);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700921
922It really should be self-explanatory. We treat the ADDR and LEN
923separately, because it is possible for an implementation to only
924need the address in order to perform the unmap operation.
925
926 Platform Issues
927
928If you are just writing drivers for Linux and do not maintain
929an architecture port for the kernel, you can safely skip down
930to "Closing".
931
9321) Struct scatterlist requirements.
933
FUJITA Tomonorib02de872010-05-26 14:44:21 -0700934 Don't invent the architecture specific struct scatterlist; just use
935 <asm-generic/scatterlist.h>. You need to enable
936 CONFIG_NEED_SG_DMA_LENGTH if the architecture supports IOMMUs
937 (including software IOMMU).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700938
FUJITA Tomonorice00f7f2010-08-14 16:36:17 +09009392) ARCH_DMA_MINALIGN
FUJITA Tomonori2fd74e22010-05-26 14:44:23 -0700940
941 Architectures must ensure that kmalloc'ed buffer is
942 DMA-safe. Drivers and subsystems depend on it. If an architecture
943 isn't fully DMA-coherent (i.e. hardware doesn't ensure that data in
944 the CPU cache is identical to data in main memory),
FUJITA Tomonorice00f7f2010-08-14 16:36:17 +0900945 ARCH_DMA_MINALIGN must be set so that the memory allocator
FUJITA Tomonori2fd74e22010-05-26 14:44:23 -0700946 makes sure that kmalloc'ed buffer doesn't share a cache line with
947 the others. See arch/arm/include/asm/cache.h as an example.
948
FUJITA Tomonorice00f7f2010-08-14 16:36:17 +0900949 Note that ARCH_DMA_MINALIGN is about DMA memory alignment
FUJITA Tomonori2fd74e22010-05-26 14:44:23 -0700950 constraints. You don't need to worry about the architecture data
951 alignment constraints (e.g. the alignment constraints about 64-bit
952 objects).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700953
Linus Torvalds1da177e2005-04-16 15:20:36 -0700954 Closing
955
Francis Galieguea33f3222010-04-23 00:08:02 +0200956This document, and the API itself, would not be in its current
Linus Torvalds1da177e2005-04-16 15:20:36 -0700957form without the feedback and suggestions from numerous individuals.
958We would like to specifically mention, in no particular order, the
959following people:
960
961 Russell King <rmk@arm.linux.org.uk>
962 Leo Dagum <dagum@barrel.engr.sgi.com>
963 Ralf Baechle <ralf@oss.sgi.com>
964 Grant Grundler <grundler@cup.hp.com>
965 Jay Estabrook <Jay.Estabrook@compaq.com>
966 Thomas Sailer <sailer@ife.ee.ethz.ch>
967 Andrea Arcangeli <andrea@suse.de>
Rob Landley26bbb292007-10-15 11:42:52 +0200968 Jens Axboe <jens.axboe@oracle.com>
Linus Torvalds1da177e2005-04-16 15:20:36 -0700969 David Mosberger-Tang <davidm@hpl.hp.com>