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Linus Torvalds1da177e2005-04-16 15:20:36 -07001 Dynamic DMA mapping
2 ===================
3
4 David S. Miller <davem@redhat.com>
5 Richard Henderson <rth@cygnus.com>
6 Jakub Jelinek <jakub@redhat.com>
7
8This document describes the DMA mapping system in terms of the pci_
9API. For a similar API that works for generic devices, see
10DMA-API.txt.
11
12Most of the 64bit platforms have special hardware that translates bus
13addresses (DMA addresses) into physical addresses. This is similar to
14how page tables and/or a TLB translates virtual addresses to physical
15addresses on a CPU. This is needed so that e.g. PCI devices can
16access with a Single Address Cycle (32bit DMA address) any page in the
1764bit physical address space. Previously in Linux those 64bit
18platforms had to set artificial limits on the maximum RAM size in the
19system, so that the virt_to_bus() static scheme works (the DMA address
20translation tables were simply filled on bootup to map each bus
21address to the physical page __pa(bus_to_virt())).
22
23So that Linux can use the dynamic DMA mapping, it needs some help from the
24drivers, namely it has to take into account that DMA addresses should be
25mapped only for the time they are actually used and unmapped after the DMA
26transfer.
27
28The following API will work of course even on platforms where no such
29hardware exists, see e.g. include/asm-i386/pci.h for how it is implemented on
30top of the virt_to_bus interface.
31
32First of all, you should make sure
33
34#include <linux/pci.h>
35
36is in your driver. This file will obtain for you the definition of the
37dma_addr_t (which can hold any valid DMA address for the platform)
38type which should be used everywhere you hold a DMA (bus) address
39returned from the DMA mapping functions.
40
41 What memory is DMA'able?
42
43The first piece of information you must know is what kernel memory can
44be used with the DMA mapping facilities. There has been an unwritten
45set of rules regarding this, and this text is an attempt to finally
46write them down.
47
48If you acquired your memory via the page allocator
49(i.e. __get_free_page*()) or the generic memory allocators
50(i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from
51that memory using the addresses returned from those routines.
52
53This means specifically that you may _not_ use the memory/addresses
54returned from vmalloc() for DMA. It is possible to DMA to the
55_underlying_ memory mapped into a vmalloc() area, but this requires
56walking page tables to get the physical addresses, and then
57translating each of those pages back to a kernel address using
58something like __va(). [ EDIT: Update this when we integrate
59Gerd Knorr's generic code which does this. ]
60
David Brownell21440d32006-04-01 10:21:52 -080061This rule also means that you may use neither kernel image addresses
62(items in data/text/bss segments), nor module image addresses, nor
63stack addresses for DMA. These could all be mapped somewhere entirely
64different than the rest of physical memory. Even if those classes of
65memory could physically work with DMA, you'd need to ensure the I/O
66buffers were cacheline-aligned. Without that, you'd see cacheline
67sharing problems (data corruption) on CPUs with DMA-incoherent caches.
68(The CPU could write to one word, DMA would write to a different one
69in the same cache line, and one of them could be overwritten.)
Linus Torvalds1da177e2005-04-16 15:20:36 -070070
71Also, this means that you cannot take the return of a kmap()
72call and DMA to/from that. This is similar to vmalloc().
73
74What about block I/O and networking buffers? The block I/O and
75networking subsystems make sure that the buffers they use are valid
76for you to DMA from/to.
77
78 DMA addressing limitations
79
80Does your device have any DMA addressing limitations? For example, is
81your device only capable of driving the low order 24-bits of address
82on the PCI bus for SAC DMA transfers? If so, you need to inform the
83PCI layer of this fact.
84
85By default, the kernel assumes that your device can address the full
8632-bits in a SAC cycle. For a 64-bit DAC capable device, this needs
87to be increased. And for a device with limitations, as discussed in
88the previous paragraph, it needs to be decreased.
89
90pci_alloc_consistent() by default will return 32-bit DMA addresses.
91PCI-X specification requires PCI-X devices to support 64-bit
92addressing (DAC) for all transactions. And at least one platform (SGI
93SN2) requires 64-bit consistent allocations to operate correctly when
94the IO bus is in PCI-X mode. Therefore, like with pci_set_dma_mask(),
95it's good practice to call pci_set_consistent_dma_mask() to set the
96appropriate mask even if your device only supports 32-bit DMA
97(default) and especially if it's a PCI-X device.
98
99For correct operation, you must interrogate the PCI layer in your
100device probe routine to see if the PCI controller on the machine can
101properly support the DMA addressing limitation your device has. It is
102good style to do this even if your device holds the default setting,
103because this shows that you did think about these issues wrt. your
104device.
105
106The query is performed via a call to pci_set_dma_mask():
107
108 int pci_set_dma_mask(struct pci_dev *pdev, u64 device_mask);
109
110The query for consistent allocations is performed via a a call to
111pci_set_consistent_dma_mask():
112
113 int pci_set_consistent_dma_mask(struct pci_dev *pdev, u64 device_mask);
114
115Here, pdev is a pointer to the PCI device struct of your device, and
116device_mask is a bit mask describing which bits of a PCI address your
117device supports. It returns zero if your card can perform DMA
118properly on the machine given the address mask you provided.
119
120If it returns non-zero, your device can not perform DMA properly on
121this platform, and attempting to do so will result in undefined
122behavior. You must either use a different mask, or not use DMA.
123
124This means that in the failure case, you have three options:
125
1261) Use another DMA mask, if possible (see below).
1272) Use some non-DMA mode for data transfer, if possible.
1283) Ignore this device and do not initialize it.
129
130It is recommended that your driver print a kernel KERN_WARNING message
131when you end up performing either #2 or #3. In this manner, if a user
132of your driver reports that performance is bad or that the device is not
133even detected, you can ask them for the kernel messages to find out
134exactly why.
135
136The standard 32-bit addressing PCI device would do something like
137this:
138
139 if (pci_set_dma_mask(pdev, DMA_32BIT_MASK)) {
140 printk(KERN_WARNING
141 "mydev: No suitable DMA available.\n");
142 goto ignore_this_device;
143 }
144
145Another common scenario is a 64-bit capable device. The approach
146here is to try for 64-bit DAC addressing, but back down to a
14732-bit mask should that fail. The PCI platform code may fail the
14864-bit mask not because the platform is not capable of 64-bit
149addressing. Rather, it may fail in this case simply because
15032-bit SAC addressing is done more efficiently than DAC addressing.
151Sparc64 is one platform which behaves in this way.
152
153Here is how you would handle a 64-bit capable device which can drive
154all 64-bits when accessing streaming DMA:
155
156 int using_dac;
157
158 if (!pci_set_dma_mask(pdev, DMA_64BIT_MASK)) {
159 using_dac = 1;
160 } else if (!pci_set_dma_mask(pdev, DMA_32BIT_MASK)) {
161 using_dac = 0;
162 } else {
163 printk(KERN_WARNING
164 "mydev: No suitable DMA available.\n");
165 goto ignore_this_device;
166 }
167
168If a card is capable of using 64-bit consistent allocations as well,
169the case would look like this:
170
171 int using_dac, consistent_using_dac;
172
173 if (!pci_set_dma_mask(pdev, DMA_64BIT_MASK)) {
174 using_dac = 1;
175 consistent_using_dac = 1;
176 pci_set_consistent_dma_mask(pdev, DMA_64BIT_MASK);
177 } else if (!pci_set_dma_mask(pdev, DMA_32BIT_MASK)) {
178 using_dac = 0;
179 consistent_using_dac = 0;
180 pci_set_consistent_dma_mask(pdev, DMA_32BIT_MASK);
181 } else {
182 printk(KERN_WARNING
183 "mydev: No suitable DMA available.\n");
184 goto ignore_this_device;
185 }
186
187pci_set_consistent_dma_mask() will always be able to set the same or a
188smaller mask as pci_set_dma_mask(). However for the rare case that a
189device driver only uses consistent allocations, one would have to
190check the return value from pci_set_consistent_dma_mask().
191
192If your 64-bit device is going to be an enormous consumer of DMA
193mappings, this can be problematic since the DMA mappings are a
194finite resource on many platforms. Please see the "DAC Addressing
195for Address Space Hungry Devices" section near the end of this
196document for how to handle this case.
197
198Finally, if your device can only drive the low 24-bits of
199address during PCI bus mastering you might do something like:
200
Tobias Klauser56b146d2006-04-10 22:54:21 -0700201 if (pci_set_dma_mask(pdev, DMA_24BIT_MASK)) {
Linus Torvalds1da177e2005-04-16 15:20:36 -0700202 printk(KERN_WARNING
203 "mydev: 24-bit DMA addressing not available.\n");
204 goto ignore_this_device;
205 }
Matthias Gehre910638a2006-03-28 01:56:48 -0800206[Better use DMA_24BIT_MASK instead of 0x00ffffff.
207See linux/include/dma-mapping.h for reference.]
Linus Torvalds1da177e2005-04-16 15:20:36 -0700208
209When pci_set_dma_mask() is successful, and returns zero, the PCI layer
210saves away this mask you have provided. The PCI layer will use this
211information later when you make DMA mappings.
212
213There is a case which we are aware of at this time, which is worth
214mentioning in this documentation. If your device supports multiple
215functions (for example a sound card provides playback and record
216functions) and the various different functions have _different_
217DMA addressing limitations, you may wish to probe each mask and
218only provide the functionality which the machine can handle. It
Tobias Klauser56b146d2006-04-10 22:54:21 -0700219is important that the last call to pci_set_dma_mask() be for the
Linus Torvalds1da177e2005-04-16 15:20:36 -0700220most specific mask.
221
222Here is pseudo-code showing how this might be done:
223
224 #define PLAYBACK_ADDRESS_BITS DMA_32BIT_MASK
225 #define RECORD_ADDRESS_BITS 0x00ffffff
226
227 struct my_sound_card *card;
228 struct pci_dev *pdev;
229
230 ...
231 if (!pci_set_dma_mask(pdev, PLAYBACK_ADDRESS_BITS)) {
232 card->playback_enabled = 1;
233 } else {
234 card->playback_enabled = 0;
235 printk(KERN_WARN "%s: Playback disabled due to DMA limitations.\n",
236 card->name);
237 }
238 if (!pci_set_dma_mask(pdev, RECORD_ADDRESS_BITS)) {
239 card->record_enabled = 1;
240 } else {
241 card->record_enabled = 0;
242 printk(KERN_WARN "%s: Record disabled due to DMA limitations.\n",
243 card->name);
244 }
245
246A sound card was used as an example here because this genre of PCI
247devices seems to be littered with ISA chips given a PCI front end,
248and thus retaining the 16MB DMA addressing limitations of ISA.
249
250 Types of DMA mappings
251
252There are two types of DMA mappings:
253
254- Consistent DMA mappings which are usually mapped at driver
255 initialization, unmapped at the end and for which the hardware should
256 guarantee that the device and the CPU can access the data
257 in parallel and will see updates made by each other without any
258 explicit software flushing.
259
260 Think of "consistent" as "synchronous" or "coherent".
261
262 The current default is to return consistent memory in the low 32
263 bits of the PCI bus space. However, for future compatibility you
264 should set the consistent mask even if this default is fine for your
265 driver.
266
267 Good examples of what to use consistent mappings for are:
268
269 - Network card DMA ring descriptors.
270 - SCSI adapter mailbox command data structures.
271 - Device firmware microcode executed out of
272 main memory.
273
274 The invariant these examples all require is that any CPU store
275 to memory is immediately visible to the device, and vice
276 versa. Consistent mappings guarantee this.
277
278 IMPORTANT: Consistent DMA memory does not preclude the usage of
279 proper memory barriers. The CPU may reorder stores to
280 consistent memory just as it may normal memory. Example:
281 if it is important for the device to see the first word
282 of a descriptor updated before the second, you must do
283 something like:
284
285 desc->word0 = address;
286 wmb();
287 desc->word1 = DESC_VALID;
288
289 in order to get correct behavior on all platforms.
290
David Brownell21440d32006-04-01 10:21:52 -0800291 Also, on some platforms your driver may need to flush CPU write
292 buffers in much the same way as it needs to flush write buffers
293 found in PCI bridges (such as by reading a register's value
294 after writing it).
295
Linus Torvalds1da177e2005-04-16 15:20:36 -0700296- Streaming DMA mappings which are usually mapped for one DMA transfer,
297 unmapped right after it (unless you use pci_dma_sync_* below) and for which
298 hardware can optimize for sequential accesses.
299
300 This of "streaming" as "asynchronous" or "outside the coherency
301 domain".
302
303 Good examples of what to use streaming mappings for are:
304
305 - Networking buffers transmitted/received by a device.
306 - Filesystem buffers written/read by a SCSI device.
307
308 The interfaces for using this type of mapping were designed in
309 such a way that an implementation can make whatever performance
310 optimizations the hardware allows. To this end, when using
311 such mappings you must be explicit about what you want to happen.
312
313Neither type of DMA mapping has alignment restrictions that come
314from PCI, although some devices may have such restrictions.
David Brownell21440d32006-04-01 10:21:52 -0800315Also, systems with caches that aren't DMA-coherent will work better
316when the underlying buffers don't share cache lines with other data.
317
Linus Torvalds1da177e2005-04-16 15:20:36 -0700318
319 Using Consistent DMA mappings.
320
321To allocate and map large (PAGE_SIZE or so) consistent DMA regions,
322you should do:
323
324 dma_addr_t dma_handle;
325
326 cpu_addr = pci_alloc_consistent(dev, size, &dma_handle);
327
328where dev is a struct pci_dev *. You should pass NULL for PCI like buses
329where devices don't have struct pci_dev (like ISA, EISA). This may be
330called in interrupt context.
331
332This argument is needed because the DMA translations may be bus
333specific (and often is private to the bus which the device is attached
334to).
335
336Size is the length of the region you want to allocate, in bytes.
337
338This routine will allocate RAM for that region, so it acts similarly to
339__get_free_pages (but takes size instead of a page order). If your
340driver needs regions sized smaller than a page, you may prefer using
341the pci_pool interface, described below.
342
343The consistent DMA mapping interfaces, for non-NULL dev, will by
344default return a DMA address which is SAC (Single Address Cycle)
345addressable. Even if the device indicates (via PCI dma mask) that it
346may address the upper 32-bits and thus perform DAC cycles, consistent
347allocation will only return > 32-bit PCI addresses for DMA if the
348consistent dma mask has been explicitly changed via
349pci_set_consistent_dma_mask(). This is true of the pci_pool interface
350as well.
351
352pci_alloc_consistent returns two values: the virtual address which you
353can use to access it from the CPU and dma_handle which you pass to the
354card.
355
356The cpu return address and the DMA bus master address are both
357guaranteed to be aligned to the smallest PAGE_SIZE order which
358is greater than or equal to the requested size. This invariant
359exists (for example) to guarantee that if you allocate a chunk
360which is smaller than or equal to 64 kilobytes, the extent of the
361buffer you receive will not cross a 64K boundary.
362
363To unmap and free such a DMA region, you call:
364
365 pci_free_consistent(dev, size, cpu_addr, dma_handle);
366
367where dev, size are the same as in the above call and cpu_addr and
368dma_handle are the values pci_alloc_consistent returned to you.
369This function may not be called in interrupt context.
370
371If your driver needs lots of smaller memory regions, you can write
372custom code to subdivide pages returned by pci_alloc_consistent,
373or you can use the pci_pool API to do that. A pci_pool is like
374a kmem_cache, but it uses pci_alloc_consistent not __get_free_pages.
375Also, it understands common hardware constraints for alignment,
376like queue heads needing to be aligned on N byte boundaries.
377
378Create a pci_pool like this:
379
380 struct pci_pool *pool;
381
382 pool = pci_pool_create(name, dev, size, align, alloc);
383
384The "name" is for diagnostics (like a kmem_cache name); dev and size
385are as above. The device's hardware alignment requirement for this
386type of data is "align" (which is expressed in bytes, and must be a
387power of two). If your device has no boundary crossing restrictions,
388pass 0 for alloc; passing 4096 says memory allocated from this pool
389must not cross 4KByte boundaries (but at that time it may be better to
390go for pci_alloc_consistent directly instead).
391
392Allocate memory from a pci pool like this:
393
394 cpu_addr = pci_pool_alloc(pool, flags, &dma_handle);
395
396flags are SLAB_KERNEL if blocking is permitted (not in_interrupt nor
397holding SMP locks), SLAB_ATOMIC otherwise. Like pci_alloc_consistent,
398this returns two values, cpu_addr and dma_handle.
399
400Free memory that was allocated from a pci_pool like this:
401
402 pci_pool_free(pool, cpu_addr, dma_handle);
403
404where pool is what you passed to pci_pool_alloc, and cpu_addr and
405dma_handle are the values pci_pool_alloc returned. This function
406may be called in interrupt context.
407
408Destroy a pci_pool by calling:
409
410 pci_pool_destroy(pool);
411
412Make sure you've called pci_pool_free for all memory allocated
413from a pool before you destroy the pool. This function may not
414be called in interrupt context.
415
416 DMA Direction
417
418The interfaces described in subsequent portions of this document
419take a DMA direction argument, which is an integer and takes on
420one of the following values:
421
422 PCI_DMA_BIDIRECTIONAL
423 PCI_DMA_TODEVICE
424 PCI_DMA_FROMDEVICE
425 PCI_DMA_NONE
426
427One should provide the exact DMA direction if you know it.
428
429PCI_DMA_TODEVICE means "from main memory to the PCI device"
430PCI_DMA_FROMDEVICE means "from the PCI device to main memory"
431It is the direction in which the data moves during the DMA
432transfer.
433
434You are _strongly_ encouraged to specify this as precisely
435as you possibly can.
436
437If you absolutely cannot know the direction of the DMA transfer,
438specify PCI_DMA_BIDIRECTIONAL. It means that the DMA can go in
439either direction. The platform guarantees that you may legally
440specify this, and that it will work, but this may be at the
441cost of performance for example.
442
443The value PCI_DMA_NONE is to be used for debugging. One can
444hold this in a data structure before you come to know the
445precise direction, and this will help catch cases where your
446direction tracking logic has failed to set things up properly.
447
448Another advantage of specifying this value precisely (outside of
449potential platform-specific optimizations of such) is for debugging.
450Some platforms actually have a write permission boolean which DMA
451mappings can be marked with, much like page protections in the user
452program address space. Such platforms can and do report errors in the
453kernel logs when the PCI controller hardware detects violation of the
454permission setting.
455
456Only streaming mappings specify a direction, consistent mappings
457implicitly have a direction attribute setting of
458PCI_DMA_BIDIRECTIONAL.
459
be7db052005-04-17 15:26:13 -0500460The SCSI subsystem tells you the direction to use in the
461'sc_data_direction' member of the SCSI command your driver is
462working on.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700463
464For Networking drivers, it's a rather simple affair. For transmit
465packets, map/unmap them with the PCI_DMA_TODEVICE direction
466specifier. For receive packets, just the opposite, map/unmap them
467with the PCI_DMA_FROMDEVICE direction specifier.
468
469 Using Streaming DMA mappings
470
471The streaming DMA mapping routines can be called from interrupt
472context. There are two versions of each map/unmap, one which will
473map/unmap a single memory region, and one which will map/unmap a
474scatterlist.
475
476To map a single region, you do:
477
478 struct pci_dev *pdev = mydev->pdev;
479 dma_addr_t dma_handle;
480 void *addr = buffer->ptr;
481 size_t size = buffer->len;
482
483 dma_handle = pci_map_single(dev, addr, size, direction);
484
485and to unmap it:
486
487 pci_unmap_single(dev, dma_handle, size, direction);
488
489You should call pci_unmap_single when the DMA activity is finished, e.g.
490from the interrupt which told you that the DMA transfer is done.
491
492Using cpu pointers like this for single mappings has a disadvantage,
493you cannot reference HIGHMEM memory in this way. Thus, there is a
494map/unmap interface pair akin to pci_{map,unmap}_single. These
495interfaces deal with page/offset pairs instead of cpu pointers.
496Specifically:
497
498 struct pci_dev *pdev = mydev->pdev;
499 dma_addr_t dma_handle;
500 struct page *page = buffer->page;
501 unsigned long offset = buffer->offset;
502 size_t size = buffer->len;
503
504 dma_handle = pci_map_page(dev, page, offset, size, direction);
505
506 ...
507
508 pci_unmap_page(dev, dma_handle, size, direction);
509
510Here, "offset" means byte offset within the given page.
511
512With scatterlists, you map a region gathered from several regions by:
513
514 int i, count = pci_map_sg(dev, sglist, nents, direction);
515 struct scatterlist *sg;
516
517 for (i = 0, sg = sglist; i < count; i++, sg++) {
518 hw_address[i] = sg_dma_address(sg);
519 hw_len[i] = sg_dma_len(sg);
520 }
521
522where nents is the number of entries in the sglist.
523
524The implementation is free to merge several consecutive sglist entries
525into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
526consecutive sglist entries can be merged into one provided the first one
527ends and the second one starts on a page boundary - in fact this is a huge
528advantage for cards which either cannot do scatter-gather or have very
529limited number of scatter-gather entries) and returns the actual number
530of sg entries it mapped them to. On failure 0 is returned.
531
532Then you should loop count times (note: this can be less than nents times)
533and use sg_dma_address() and sg_dma_len() macros where you previously
534accessed sg->address and sg->length as shown above.
535
536To unmap a scatterlist, just call:
537
538 pci_unmap_sg(dev, sglist, nents, direction);
539
540Again, make sure DMA activity has already finished.
541
542PLEASE NOTE: The 'nents' argument to the pci_unmap_sg call must be
543 the _same_ one you passed into the pci_map_sg call,
544 it should _NOT_ be the 'count' value _returned_ from the
545 pci_map_sg call.
546
547Every pci_map_{single,sg} call should have its pci_unmap_{single,sg}
548counterpart, because the bus address space is a shared resource (although
549in some ports the mapping is per each BUS so less devices contend for the
550same bus address space) and you could render the machine unusable by eating
551all bus addresses.
552
553If you need to use the same streaming DMA region multiple times and touch
554the data in between the DMA transfers, the buffer needs to be synced
555properly in order for the cpu and device to see the most uptodate and
556correct copy of the DMA buffer.
557
558So, firstly, just map it with pci_map_{single,sg}, and after each DMA
559transfer call either:
560
561 pci_dma_sync_single_for_cpu(dev, dma_handle, size, direction);
562
563or:
564
565 pci_dma_sync_sg_for_cpu(dev, sglist, nents, direction);
566
567as appropriate.
568
569Then, if you wish to let the device get at the DMA area again,
570finish accessing the data with the cpu, and then before actually
571giving the buffer to the hardware call either:
572
573 pci_dma_sync_single_for_device(dev, dma_handle, size, direction);
574
575or:
576
577 pci_dma_sync_sg_for_device(dev, sglist, nents, direction);
578
579as appropriate.
580
581After the last DMA transfer call one of the DMA unmap routines
582pci_unmap_{single,sg}. If you don't touch the data from the first pci_map_*
583call till pci_unmap_*, then you don't have to call the pci_dma_sync_*
584routines at all.
585
586Here is pseudo code which shows a situation in which you would need
587to use the pci_dma_sync_*() interfaces.
588
589 my_card_setup_receive_buffer(struct my_card *cp, char *buffer, int len)
590 {
591 dma_addr_t mapping;
592
593 mapping = pci_map_single(cp->pdev, buffer, len, PCI_DMA_FROMDEVICE);
594
595 cp->rx_buf = buffer;
596 cp->rx_len = len;
597 cp->rx_dma = mapping;
598
599 give_rx_buf_to_card(cp);
600 }
601
602 ...
603
604 my_card_interrupt_handler(int irq, void *devid, struct pt_regs *regs)
605 {
606 struct my_card *cp = devid;
607
608 ...
609 if (read_card_status(cp) == RX_BUF_TRANSFERRED) {
610 struct my_card_header *hp;
611
612 /* Examine the header to see if we wish
613 * to accept the data. But synchronize
614 * the DMA transfer with the CPU first
615 * so that we see updated contents.
616 */
617 pci_dma_sync_single_for_cpu(cp->pdev, cp->rx_dma,
618 cp->rx_len,
619 PCI_DMA_FROMDEVICE);
620
621 /* Now it is safe to examine the buffer. */
622 hp = (struct my_card_header *) cp->rx_buf;
623 if (header_is_ok(hp)) {
624 pci_unmap_single(cp->pdev, cp->rx_dma, cp->rx_len,
625 PCI_DMA_FROMDEVICE);
626 pass_to_upper_layers(cp->rx_buf);
627 make_and_setup_new_rx_buf(cp);
628 } else {
629 /* Just sync the buffer and give it back
630 * to the card.
631 */
632 pci_dma_sync_single_for_device(cp->pdev,
633 cp->rx_dma,
634 cp->rx_len,
635 PCI_DMA_FROMDEVICE);
636 give_rx_buf_to_card(cp);
637 }
638 }
639 }
640
641Drivers converted fully to this interface should not use virt_to_bus any
642longer, nor should they use bus_to_virt. Some drivers have to be changed a
643little bit, because there is no longer an equivalent to bus_to_virt in the
644dynamic DMA mapping scheme - you have to always store the DMA addresses
645returned by the pci_alloc_consistent, pci_pool_alloc, and pci_map_single
646calls (pci_map_sg stores them in the scatterlist itself if the platform
647supports dynamic DMA mapping in hardware) in your driver structures and/or
648in the card registers.
649
650All PCI drivers should be using these interfaces with no exceptions.
651It is planned to completely remove virt_to_bus() and bus_to_virt() as
652they are entirely deprecated. Some ports already do not provide these
653as it is impossible to correctly support them.
654
655 64-bit DMA and DAC cycle support
656
657Do you understand all of the text above? Great, then you already
658know how to use 64-bit DMA addressing under Linux. Simply make
659the appropriate pci_set_dma_mask() calls based upon your cards
660capabilities, then use the mapping APIs above.
661
662It is that simple.
663
664Well, not for some odd devices. See the next section for information
665about that.
666
667 DAC Addressing for Address Space Hungry Devices
668
669There exists a class of devices which do not mesh well with the PCI
670DMA mapping API. By definition these "mappings" are a finite
671resource. The number of total available mappings per bus is platform
672specific, but there will always be a reasonable amount.
673
674What is "reasonable"? Reasonable means that networking and block I/O
675devices need not worry about using too many mappings.
676
677As an example of a problematic device, consider compute cluster cards.
678They can potentially need to access gigabytes of memory at once via
679DMA. Dynamic mappings are unsuitable for this kind of access pattern.
680
681To this end we've provided a small API by which a device driver
682may use DAC cycles to directly address all of physical memory.
683Not all platforms support this, but most do. It is easy to determine
684whether the platform will work properly at probe time.
685
686First, understand that there may be a SEVERE performance penalty for
687using these interfaces on some platforms. Therefore, you MUST only
688use these interfaces if it is absolutely required. %99 of devices can
689use the normal APIs without any problems.
690
691Note that for streaming type mappings you must either use these
692interfaces, or the dynamic mapping interfaces above. You may not mix
693usage of both for the same device. Such an act is illegal and is
694guaranteed to put a banana in your tailpipe.
695
696However, consistent mappings may in fact be used in conjunction with
697these interfaces. Remember that, as defined, consistent mappings are
698always going to be SAC addressable.
699
700The first thing your driver needs to do is query the PCI platform
701layer with your devices DAC addressing capabilities:
702
703 int pci_dac_set_dma_mask(struct pci_dev *pdev, u64 mask);
704
705This routine behaves identically to pci_set_dma_mask. You may not
706use the following interfaces if this routine fails.
707
708Next, DMA addresses using this API are kept track of using the
709dma64_addr_t type. It is guaranteed to be big enough to hold any
710DAC address the platform layer will give to you from the following
711routines. If you have consistent mappings as well, you still
712use plain dma_addr_t to keep track of those.
713
714All mappings obtained here will be direct. The mappings are not
715translated, and this is the purpose of this dialect of the DMA API.
716
717All routines work with page/offset pairs. This is the _ONLY_ way to
718portably refer to any piece of memory. If you have a cpu pointer
719(which may be validly DMA'd too) you may easily obtain the page
720and offset using something like this:
721
722 struct page *page = virt_to_page(ptr);
723 unsigned long offset = offset_in_page(ptr);
724
725Here are the interfaces:
726
727 dma64_addr_t pci_dac_page_to_dma(struct pci_dev *pdev,
728 struct page *page,
729 unsigned long offset,
730 int direction);
731
732The DAC address for the tuple PAGE/OFFSET are returned. The direction
733argument is the same as for pci_{map,unmap}_single(). The same rules
734for cpu/device access apply here as for the streaming mapping
735interfaces. To reiterate:
736
737 The cpu may touch the buffer before pci_dac_page_to_dma.
738 The device may touch the buffer after pci_dac_page_to_dma
739 is made, but the cpu may NOT.
740
741When the DMA transfer is complete, invoke:
742
743 void pci_dac_dma_sync_single_for_cpu(struct pci_dev *pdev,
744 dma64_addr_t dma_addr,
745 size_t len, int direction);
746
747This must be done before the CPU looks at the buffer again.
748This interface behaves identically to pci_dma_sync_{single,sg}_for_cpu().
749
750And likewise, if you wish to let the device get back at the buffer after
751the cpu has read/written it, invoke:
752
753 void pci_dac_dma_sync_single_for_device(struct pci_dev *pdev,
754 dma64_addr_t dma_addr,
755 size_t len, int direction);
756
757before letting the device access the DMA area again.
758
759If you need to get back to the PAGE/OFFSET tuple from a dma64_addr_t
760the following interfaces are provided:
761
762 struct page *pci_dac_dma_to_page(struct pci_dev *pdev,
763 dma64_addr_t dma_addr);
764 unsigned long pci_dac_dma_to_offset(struct pci_dev *pdev,
765 dma64_addr_t dma_addr);
766
767This is possible with the DAC interfaces purely because they are
768not translated in any way.
769
770 Optimizing Unmap State Space Consumption
771
772On many platforms, pci_unmap_{single,page}() is simply a nop.
773Therefore, keeping track of the mapping address and length is a waste
774of space. Instead of filling your drivers up with ifdefs and the like
775to "work around" this (which would defeat the whole purpose of a
776portable API) the following facilities are provided.
777
778Actually, instead of describing the macros one by one, we'll
779transform some example code.
780
7811) Use DECLARE_PCI_UNMAP_{ADDR,LEN} in state saving structures.
782 Example, before:
783
784 struct ring_state {
785 struct sk_buff *skb;
786 dma_addr_t mapping;
787 __u32 len;
788 };
789
790 after:
791
792 struct ring_state {
793 struct sk_buff *skb;
794 DECLARE_PCI_UNMAP_ADDR(mapping)
795 DECLARE_PCI_UNMAP_LEN(len)
796 };
797
798 NOTE: DO NOT put a semicolon at the end of the DECLARE_*()
799 macro.
800
8012) Use pci_unmap_{addr,len}_set to set these values.
802 Example, before:
803
804 ringp->mapping = FOO;
805 ringp->len = BAR;
806
807 after:
808
809 pci_unmap_addr_set(ringp, mapping, FOO);
810 pci_unmap_len_set(ringp, len, BAR);
811
8123) Use pci_unmap_{addr,len} to access these values.
813 Example, before:
814
815 pci_unmap_single(pdev, ringp->mapping, ringp->len,
816 PCI_DMA_FROMDEVICE);
817
818 after:
819
820 pci_unmap_single(pdev,
821 pci_unmap_addr(ringp, mapping),
822 pci_unmap_len(ringp, len),
823 PCI_DMA_FROMDEVICE);
824
825It really should be self-explanatory. We treat the ADDR and LEN
826separately, because it is possible for an implementation to only
827need the address in order to perform the unmap operation.
828
829 Platform Issues
830
831If you are just writing drivers for Linux and do not maintain
832an architecture port for the kernel, you can safely skip down
833to "Closing".
834
8351) Struct scatterlist requirements.
836
837 Struct scatterlist must contain, at a minimum, the following
838 members:
839
840 struct page *page;
841 unsigned int offset;
842 unsigned int length;
843
844 The base address is specified by a "page+offset" pair.
845
846 Previous versions of struct scatterlist contained a "void *address"
847 field that was sometimes used instead of page+offset. As of Linux
848 2.5., page+offset is always used, and the "address" field has been
849 deleted.
850
8512) More to come...
852
853 Handling Errors
854
855DMA address space is limited on some architectures and an allocation
856failure can be determined by:
857
858- checking if pci_alloc_consistent returns NULL or pci_map_sg returns 0
859
860- checking the returned dma_addr_t of pci_map_single and pci_map_page
861 by using pci_dma_mapping_error():
862
863 dma_addr_t dma_handle;
864
865 dma_handle = pci_map_single(dev, addr, size, direction);
866 if (pci_dma_mapping_error(dma_handle)) {
867 /*
868 * reduce current DMA mapping usage,
869 * delay and try again later or
870 * reset driver.
871 */
872 }
873
874 Closing
875
876This document, and the API itself, would not be in it's current
877form without the feedback and suggestions from numerous individuals.
878We would like to specifically mention, in no particular order, the
879following people:
880
881 Russell King <rmk@arm.linux.org.uk>
882 Leo Dagum <dagum@barrel.engr.sgi.com>
883 Ralf Baechle <ralf@oss.sgi.com>
884 Grant Grundler <grundler@cup.hp.com>
885 Jay Estabrook <Jay.Estabrook@compaq.com>
886 Thomas Sailer <sailer@ife.ee.ethz.ch>
887 Andrea Arcangeli <andrea@suse.de>
888 Jens Axboe <axboe@suse.de>
889 David Mosberger-Tang <davidm@hpl.hp.com>