| Dynamic DMA mapping Guide |
| ========================= |
| |
| David S. Miller <davem@redhat.com> |
| Richard Henderson <rth@cygnus.com> |
| Jakub Jelinek <jakub@redhat.com> |
| |
| This is a guide to device driver writers on how to use the DMA API |
| with example pseudo-code. For a concise description of the API, see |
| DMA-API.txt. |
| |
| CPU and DMA addresses |
| |
| There are several kinds of addresses involved in the DMA API, and it's |
| important to understand the differences. |
| |
| The kernel normally uses virtual addresses. Any address returned by |
| kmalloc(), vmalloc(), and similar interfaces is a virtual address and can |
| be stored in a "void *". |
| |
| The virtual memory system (TLB, page tables, etc.) translates virtual |
| addresses to CPU physical addresses, which are stored as "phys_addr_t" or |
| "resource_size_t". The kernel manages device resources like registers as |
| physical addresses. These are the addresses in /proc/iomem. The physical |
| address is not directly useful to a driver; it must use ioremap() to map |
| the space and produce a virtual address. |
| |
| I/O devices use a third kind of address: a "bus address". If a device has |
| registers at an MMIO address, or if it performs DMA to read or write system |
| memory, the addresses used by the device are bus addresses. In some |
| systems, bus addresses are identical to CPU physical addresses, but in |
| general they are not. IOMMUs and host bridges can produce arbitrary |
| mappings between physical and bus addresses. |
| |
| From a device's point of view, DMA uses the bus address space, but it may |
| be restricted to a subset of that space. For example, even if a system |
| supports 64-bit addresses for main memory and PCI BARs, it may use an IOMMU |
| so devices only need to use 32-bit DMA addresses. |
| |
| Here's a picture and some examples: |
| |
| CPU CPU Bus |
| Virtual Physical Address |
| Address Address Space |
| Space Space |
| |
| +-------+ +------+ +------+ |
| | | |MMIO | Offset | | |
| | | Virtual |Space | applied | | |
| C +-------+ --------> B +------+ ----------> +------+ A |
| | | mapping | | by host | | |
| +-----+ | | | | bridge | | +--------+ |
| | | | | +------+ | | | | |
| | CPU | | | | RAM | | | | Device | |
| | | | | | | | | | | |
| +-----+ +-------+ +------+ +------+ +--------+ |
| | | Virtual |Buffer| Mapping | | |
| X +-------+ --------> Y +------+ <---------- +------+ Z |
| | | mapping | RAM | by IOMMU |
| | | | | |
| | | | | |
| +-------+ +------+ |
| |
| During the enumeration process, the kernel learns about I/O devices and |
| their MMIO space and the host bridges that connect them to the system. For |
| example, if a PCI device has a BAR, the kernel reads the bus address (A) |
| from the BAR and converts it to a CPU physical address (B). The address B |
| is stored in a struct resource and usually exposed via /proc/iomem. When a |
| driver claims a device, it typically uses ioremap() to map physical address |
| B at a virtual address (C). It can then use, e.g., ioread32(C), to access |
| the device registers at bus address A. |
| |
| If the device supports DMA, the driver sets up a buffer using kmalloc() or |
| a similar interface, which returns a virtual address (X). The virtual |
| memory system maps X to a physical address (Y) in system RAM. The driver |
| can use virtual address X to access the buffer, but the device itself |
| cannot because DMA doesn't go through the CPU virtual memory system. |
| |
| In some simple systems, the device can do DMA directly to physical address |
| Y. But in many others, there is IOMMU hardware that translates DMA |
| addresses to physical addresses, e.g., it translates Z to Y. This is part |
| of the reason for the DMA API: the driver can give a virtual address X to |
| an interface like dma_map_single(), which sets up any required IOMMU |
| mapping and returns the DMA address Z. The driver then tells the device to |
| do DMA to Z, and the IOMMU maps it to the buffer at address Y in system |
| RAM. |
| |
| So that Linux can use the dynamic DMA mapping, it needs some help from the |
| drivers, namely it has to take into account that DMA addresses should be |
| mapped only for the time they are actually used and unmapped after the DMA |
| transfer. |
| |
| The following API will work of course even on platforms where no such |
| hardware exists. |
| |
| Note that the DMA API works with any bus independent of the underlying |
| microprocessor architecture. You should use the DMA API rather than the |
| bus-specific DMA API, i.e., use the dma_map_*() interfaces rather than the |
| pci_map_*() interfaces. |
| |
| First of all, you should make sure |
| |
| #include <linux/dma-mapping.h> |
| |
| is in your driver, which provides the definition of dma_addr_t. This type |
| can hold any valid DMA address for the platform and should be used |
| everywhere you hold a DMA address returned from the DMA mapping functions. |
| |
| What memory is DMA'able? |
| |
| The first piece of information you must know is what kernel memory can |
| be used with the DMA mapping facilities. There has been an unwritten |
| set of rules regarding this, and this text is an attempt to finally |
| write them down. |
| |
| If you acquired your memory via the page allocator |
| (i.e. __get_free_page*()) or the generic memory allocators |
| (i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from |
| that memory using the addresses returned from those routines. |
| |
| This means specifically that you may _not_ use the memory/addresses |
| returned from vmalloc() for DMA. It is possible to DMA to the |
| _underlying_ memory mapped into a vmalloc() area, but this requires |
| walking page tables to get the physical addresses, and then |
| translating each of those pages back to a kernel address using |
| something like __va(). [ EDIT: Update this when we integrate |
| Gerd Knorr's generic code which does this. ] |
| |
| This rule also means that you may use neither kernel image addresses |
| (items in data/text/bss segments), nor module image addresses, nor |
| stack addresses for DMA. These could all be mapped somewhere entirely |
| different than the rest of physical memory. Even if those classes of |
| memory could physically work with DMA, you'd need to ensure the I/O |
| buffers were cacheline-aligned. Without that, you'd see cacheline |
| sharing problems (data corruption) on CPUs with DMA-incoherent caches. |
| (The CPU could write to one word, DMA would write to a different one |
| in the same cache line, and one of them could be overwritten.) |
| |
| Also, this means that you cannot take the return of a kmap() |
| call and DMA to/from that. This is similar to vmalloc(). |
| |
| What about block I/O and networking buffers? The block I/O and |
| networking subsystems make sure that the buffers they use are valid |
| for you to DMA from/to. |
| |
| DMA addressing limitations |
| |
| Does your device have any DMA addressing limitations? For example, is |
| your device only capable of driving the low order 24-bits of address? |
| If so, you need to inform the kernel of this fact. |
| |
| By default, the kernel assumes that your device can address the full |
| 32-bits. For a 64-bit capable device, this needs to be increased. |
| And for a device with limitations, as discussed in the previous |
| paragraph, it needs to be decreased. |
| |
| Special note about PCI: PCI-X specification requires PCI-X devices to |
| support 64-bit addressing (DAC) for all transactions. And at least |
| one platform (SGI SN2) requires 64-bit consistent allocations to |
| operate correctly when the IO bus is in PCI-X mode. |
| |
| For correct operation, you must interrogate the kernel in your device |
| probe routine to see if the DMA controller on the machine can properly |
| support the DMA addressing limitation your device has. It is good |
| style to do this even if your device holds the default setting, |
| because this shows that you did think about these issues wrt. your |
| device. |
| |
| The query is performed via a call to dma_set_mask_and_coherent(): |
| |
| int dma_set_mask_and_coherent(struct device *dev, u64 mask); |
| |
| which will query the mask for both streaming and coherent APIs together. |
| If you have some special requirements, then the following two separate |
| queries can be used instead: |
| |
| The query for streaming mappings is performed via a call to |
| dma_set_mask(): |
| |
| int dma_set_mask(struct device *dev, u64 mask); |
| |
| The query for consistent allocations is performed via a call |
| to dma_set_coherent_mask(): |
| |
| int dma_set_coherent_mask(struct device *dev, u64 mask); |
| |
| Here, dev is a pointer to the device struct of your device, and mask |
| is a bit mask describing which bits of an address your device |
| supports. It returns zero if your card can perform DMA properly on |
| the machine given the address mask you provided. In general, the |
| device struct of your device is embedded in the bus-specific device |
| struct of your device. For example, &pdev->dev is a pointer to the |
| device struct of a PCI device (pdev is a pointer to the PCI device |
| struct of your device). |
| |
| If it returns non-zero, your device cannot perform DMA properly on |
| this platform, and attempting to do so will result in undefined |
| behavior. You must either use a different mask, or not use DMA. |
| |
| This means that in the failure case, you have three options: |
| |
| 1) Use another DMA mask, if possible (see below). |
| 2) Use some non-DMA mode for data transfer, if possible. |
| 3) Ignore this device and do not initialize it. |
| |
| It is recommended that your driver print a kernel KERN_WARNING message |
| when you end up performing either #2 or #3. In this manner, if a user |
| of your driver reports that performance is bad or that the device is not |
| even detected, you can ask them for the kernel messages to find out |
| exactly why. |
| |
| The standard 32-bit addressing device would do something like this: |
| |
| if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32))) { |
| dev_warn(dev, "mydev: No suitable DMA available\n"); |
| goto ignore_this_device; |
| } |
| |
| Another common scenario is a 64-bit capable device. The approach here |
| is to try for 64-bit addressing, but back down to a 32-bit mask that |
| should not fail. The kernel may fail the 64-bit mask not because the |
| platform is not capable of 64-bit addressing. Rather, it may fail in |
| this case simply because 32-bit addressing is done more efficiently |
| than 64-bit addressing. For example, Sparc64 PCI SAC addressing is |
| more efficient than DAC addressing. |
| |
| Here is how you would handle a 64-bit capable device which can drive |
| all 64-bits when accessing streaming DMA: |
| |
| int using_dac; |
| |
| if (!dma_set_mask(dev, DMA_BIT_MASK(64))) { |
| using_dac = 1; |
| } else if (!dma_set_mask(dev, DMA_BIT_MASK(32))) { |
| using_dac = 0; |
| } else { |
| dev_warn(dev, "mydev: No suitable DMA available\n"); |
| goto ignore_this_device; |
| } |
| |
| If a card is capable of using 64-bit consistent allocations as well, |
| the case would look like this: |
| |
| int using_dac, consistent_using_dac; |
| |
| if (!dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64))) { |
| using_dac = 1; |
| consistent_using_dac = 1; |
| } else if (!dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32))) { |
| using_dac = 0; |
| consistent_using_dac = 0; |
| } else { |
| dev_warn(dev, "mydev: No suitable DMA available\n"); |
| goto ignore_this_device; |
| } |
| |
| The coherent mask will always be able to set the same or a smaller mask as |
| the streaming mask. However for the rare case that a device driver only |
| uses consistent allocations, one would have to check the return value from |
| dma_set_coherent_mask(). |
| |
| Finally, if your device can only drive the low 24-bits of |
| address you might do something like: |
| |
| if (dma_set_mask(dev, DMA_BIT_MASK(24))) { |
| dev_warn(dev, "mydev: 24-bit DMA addressing not available\n"); |
| goto ignore_this_device; |
| } |
| |
| When dma_set_mask() or dma_set_mask_and_coherent() is successful, and |
| returns zero, the kernel saves away this mask you have provided. The |
| kernel will use this information later when you make DMA mappings. |
| |
| There is a case which we are aware of at this time, which is worth |
| mentioning in this documentation. If your device supports multiple |
| functions (for example a sound card provides playback and record |
| functions) and the various different functions have _different_ |
| DMA addressing limitations, you may wish to probe each mask and |
| only provide the functionality which the machine can handle. It |
| is important that the last call to dma_set_mask() be for the |
| most specific mask. |
| |
| Here is pseudo-code showing how this might be done: |
| |
| #define PLAYBACK_ADDRESS_BITS DMA_BIT_MASK(32) |
| #define RECORD_ADDRESS_BITS DMA_BIT_MASK(24) |
| |
| struct my_sound_card *card; |
| struct device *dev; |
| |
| ... |
| if (!dma_set_mask(dev, PLAYBACK_ADDRESS_BITS)) { |
| card->playback_enabled = 1; |
| } else { |
| card->playback_enabled = 0; |
| dev_warn(dev, "%s: Playback disabled due to DMA limitations\n", |
| card->name); |
| } |
| if (!dma_set_mask(dev, RECORD_ADDRESS_BITS)) { |
| card->record_enabled = 1; |
| } else { |
| card->record_enabled = 0; |
| dev_warn(dev, "%s: Record disabled due to DMA limitations\n", |
| card->name); |
| } |
| |
| A sound card was used as an example here because this genre of PCI |
| devices seems to be littered with ISA chips given a PCI front end, |
| and thus retaining the 16MB DMA addressing limitations of ISA. |
| |
| Types of DMA mappings |
| |
| There are two types of DMA mappings: |
| |
| - Consistent DMA mappings which are usually mapped at driver |
| initialization, unmapped at the end and for which the hardware should |
| guarantee that the device and the CPU can access the data |
| in parallel and will see updates made by each other without any |
| explicit software flushing. |
| |
| Think of "consistent" as "synchronous" or "coherent". |
| |
| The current default is to return consistent memory in the low 32 |
| bits of the DMA space. However, for future compatibility you should |
| set the consistent mask even if this default is fine for your |
| driver. |
| |
| Good examples of what to use consistent mappings for are: |
| |
| - Network card DMA ring descriptors. |
| - SCSI adapter mailbox command data structures. |
| - Device firmware microcode executed out of |
| main memory. |
| |
| The invariant these examples all require is that any CPU store |
| to memory is immediately visible to the device, and vice |
| versa. Consistent mappings guarantee this. |
| |
| IMPORTANT: Consistent DMA memory does not preclude the usage of |
| proper memory barriers. The CPU may reorder stores to |
| consistent memory just as it may normal memory. Example: |
| if it is important for the device to see the first word |
| of a descriptor updated before the second, you must do |
| something like: |
| |
| desc->word0 = address; |
| wmb(); |
| desc->word1 = DESC_VALID; |
| |
| in order to get correct behavior on all platforms. |
| |
| Also, on some platforms your driver may need to flush CPU write |
| buffers in much the same way as it needs to flush write buffers |
| found in PCI bridges (such as by reading a register's value |
| after writing it). |
| |
| - Streaming DMA mappings which are usually mapped for one DMA |
| transfer, unmapped right after it (unless you use dma_sync_* below) |
| and for which hardware can optimize for sequential accesses. |
| |
| Think of "streaming" as "asynchronous" or "outside the coherency |
| domain". |
| |
| Good examples of what to use streaming mappings for are: |
| |
| - Networking buffers transmitted/received by a device. |
| - Filesystem buffers written/read by a SCSI device. |
| |
| The interfaces for using this type of mapping were designed in |
| such a way that an implementation can make whatever performance |
| optimizations the hardware allows. To this end, when using |
| such mappings you must be explicit about what you want to happen. |
| |
| Neither type of DMA mapping has alignment restrictions that come from |
| the underlying bus, although some devices may have such restrictions. |
| Also, systems with caches that aren't DMA-coherent will work better |
| when the underlying buffers don't share cache lines with other data. |
| |
| |
| Using Consistent DMA mappings. |
| |
| To allocate and map large (PAGE_SIZE or so) consistent DMA regions, |
| you should do: |
| |
| dma_addr_t dma_handle; |
| |
| cpu_addr = dma_alloc_coherent(dev, size, &dma_handle, gfp); |
| |
| where device is a struct device *. This may be called in interrupt |
| context with the GFP_ATOMIC flag. |
| |
| Size is the length of the region you want to allocate, in bytes. |
| |
| This routine will allocate RAM for that region, so it acts similarly to |
| __get_free_pages() (but takes size instead of a page order). If your |
| driver needs regions sized smaller than a page, you may prefer using |
| the dma_pool interface, described below. |
| |
| The consistent DMA mapping interfaces, for non-NULL dev, will by |
| default return a DMA address which is 32-bit addressable. Even if the |
| device indicates (via DMA mask) that it may address the upper 32-bits, |
| consistent allocation will only return > 32-bit addresses for DMA if |
| the consistent DMA mask has been explicitly changed via |
| dma_set_coherent_mask(). This is true of the dma_pool interface as |
| well. |
| |
| dma_alloc_coherent() returns two values: the virtual address which you |
| can use to access it from the CPU and dma_handle which you pass to the |
| card. |
| |
| The CPU virtual address and the DMA address are both |
| guaranteed to be aligned to the smallest PAGE_SIZE order which |
| is greater than or equal to the requested size. This invariant |
| exists (for example) to guarantee that if you allocate a chunk |
| which is smaller than or equal to 64 kilobytes, the extent of the |
| buffer you receive will not cross a 64K boundary. |
| |
| To unmap and free such a DMA region, you call: |
| |
| dma_free_coherent(dev, size, cpu_addr, dma_handle); |
| |
| where dev, size are the same as in the above call and cpu_addr and |
| dma_handle are the values dma_alloc_coherent() returned to you. |
| This function may not be called in interrupt context. |
| |
| If your driver needs lots of smaller memory regions, you can write |
| custom code to subdivide pages returned by dma_alloc_coherent(), |
| or you can use the dma_pool API to do that. A dma_pool is like |
| a kmem_cache, but it uses dma_alloc_coherent(), not __get_free_pages(). |
| Also, it understands common hardware constraints for alignment, |
| like queue heads needing to be aligned on N byte boundaries. |
| |
| Create a dma_pool like this: |
| |
| struct dma_pool *pool; |
| |
| pool = dma_pool_create(name, dev, size, align, boundary); |
| |
| The "name" is for diagnostics (like a kmem_cache name); dev and size |
| are as above. The device's hardware alignment requirement for this |
| type of data is "align" (which is expressed in bytes, and must be a |
| power of two). If your device has no boundary crossing restrictions, |
| pass 0 for boundary; passing 4096 says memory allocated from this pool |
| must not cross 4KByte boundaries (but at that time it may be better to |
| use dma_alloc_coherent() directly instead). |
| |
| Allocate memory from a DMA pool like this: |
| |
| cpu_addr = dma_pool_alloc(pool, flags, &dma_handle); |
| |
| flags are GFP_KERNEL if blocking is permitted (not in_interrupt nor |
| holding SMP locks), GFP_ATOMIC otherwise. Like dma_alloc_coherent(), |
| this returns two values, cpu_addr and dma_handle. |
| |
| Free memory that was allocated from a dma_pool like this: |
| |
| dma_pool_free(pool, cpu_addr, dma_handle); |
| |
| where pool is what you passed to dma_pool_alloc(), and cpu_addr and |
| dma_handle are the values dma_pool_alloc() returned. This function |
| may be called in interrupt context. |
| |
| Destroy a dma_pool by calling: |
| |
| dma_pool_destroy(pool); |
| |
| Make sure you've called dma_pool_free() for all memory allocated |
| from a pool before you destroy the pool. This function may not |
| be called in interrupt context. |
| |
| DMA Direction |
| |
| The interfaces described in subsequent portions of this document |
| take a DMA direction argument, which is an integer and takes on |
| one of the following values: |
| |
| DMA_BIDIRECTIONAL |
| DMA_TO_DEVICE |
| DMA_FROM_DEVICE |
| DMA_NONE |
| |
| You should provide the exact DMA direction if you know it. |
| |
| DMA_TO_DEVICE means "from main memory to the device" |
| DMA_FROM_DEVICE means "from the device to main memory" |
| It is the direction in which the data moves during the DMA |
| transfer. |
| |
| You are _strongly_ encouraged to specify this as precisely |
| as you possibly can. |
| |
| If you absolutely cannot know the direction of the DMA transfer, |
| specify DMA_BIDIRECTIONAL. It means that the DMA can go in |
| either direction. The platform guarantees that you may legally |
| specify this, and that it will work, but this may be at the |
| cost of performance for example. |
| |
| The value DMA_NONE is to be used for debugging. One can |
| hold this in a data structure before you come to know the |
| precise direction, and this will help catch cases where your |
| direction tracking logic has failed to set things up properly. |
| |
| Another advantage of specifying this value precisely (outside of |
| potential platform-specific optimizations of such) is for debugging. |
| Some platforms actually have a write permission boolean which DMA |
| mappings can be marked with, much like page protections in the user |
| program address space. Such platforms can and do report errors in the |
| kernel logs when the DMA controller hardware detects violation of the |
| permission setting. |
| |
| Only streaming mappings specify a direction, consistent mappings |
| implicitly have a direction attribute setting of |
| DMA_BIDIRECTIONAL. |
| |
| The SCSI subsystem tells you the direction to use in the |
| 'sc_data_direction' member of the SCSI command your driver is |
| working on. |
| |
| For Networking drivers, it's a rather simple affair. For transmit |
| packets, map/unmap them with the DMA_TO_DEVICE direction |
| specifier. For receive packets, just the opposite, map/unmap them |
| with the DMA_FROM_DEVICE direction specifier. |
| |
| Using Streaming DMA mappings |
| |
| The streaming DMA mapping routines can be called from interrupt |
| context. There are two versions of each map/unmap, one which will |
| map/unmap a single memory region, and one which will map/unmap a |
| scatterlist. |
| |
| To map a single region, you do: |
| |
| struct device *dev = &my_dev->dev; |
| dma_addr_t dma_handle; |
| void *addr = buffer->ptr; |
| size_t size = buffer->len; |
| |
| dma_handle = dma_map_single(dev, addr, size, direction); |
| if (dma_mapping_error(dev, dma_handle)) { |
| /* |
| * reduce current DMA mapping usage, |
| * delay and try again later or |
| * reset driver. |
| */ |
| goto map_error_handling; |
| } |
| |
| and to unmap it: |
| |
| dma_unmap_single(dev, dma_handle, size, direction); |
| |
| You should call dma_mapping_error() as dma_map_single() could fail and return |
| error. Not all DMA implementations support the dma_mapping_error() interface. |
| However, it is a good practice to call dma_mapping_error() interface, which |
| will invoke the generic mapping error check interface. Doing so will ensure |
| that the mapping code will work correctly on all DMA implementations without |
| any dependency on the specifics of the underlying implementation. Using the |
| returned address without checking for errors could result in failures ranging |
| from panics to silent data corruption. A couple of examples of incorrect ways |
| to check for errors that make assumptions about the underlying DMA |
| implementation are as follows and these are applicable to dma_map_page() as |
| well. |
| |
| Incorrect example 1: |
| dma_addr_t dma_handle; |
| |
| dma_handle = dma_map_single(dev, addr, size, direction); |
| if ((dma_handle & 0xffff != 0) || (dma_handle >= 0x1000000)) { |
| goto map_error; |
| } |
| |
| Incorrect example 2: |
| dma_addr_t dma_handle; |
| |
| dma_handle = dma_map_single(dev, addr, size, direction); |
| if (dma_handle == DMA_ERROR_CODE) { |
| goto map_error; |
| } |
| |
| You should call dma_unmap_single() when the DMA activity is finished, e.g., |
| from the interrupt which told you that the DMA transfer is done. |
| |
| Using CPU pointers like this for single mappings has a disadvantage: |
| you cannot reference HIGHMEM memory in this way. Thus, there is a |
| map/unmap interface pair akin to dma_{map,unmap}_single(). These |
| interfaces deal with page/offset pairs instead of CPU pointers. |
| Specifically: |
| |
| struct device *dev = &my_dev->dev; |
| dma_addr_t dma_handle; |
| struct page *page = buffer->page; |
| unsigned long offset = buffer->offset; |
| size_t size = buffer->len; |
| |
| dma_handle = dma_map_page(dev, page, offset, size, direction); |
| if (dma_mapping_error(dev, dma_handle)) { |
| /* |
| * reduce current DMA mapping usage, |
| * delay and try again later or |
| * reset driver. |
| */ |
| goto map_error_handling; |
| } |
| |
| ... |
| |
| dma_unmap_page(dev, dma_handle, size, direction); |
| |
| Here, "offset" means byte offset within the given page. |
| |
| You should call dma_mapping_error() as dma_map_page() could fail and return |
| error as outlined under the dma_map_single() discussion. |
| |
| You should call dma_unmap_page() when the DMA activity is finished, e.g., |
| from the interrupt which told you that the DMA transfer is done. |
| |
| With scatterlists, you map a region gathered from several regions by: |
| |
| int i, count = dma_map_sg(dev, sglist, nents, direction); |
| struct scatterlist *sg; |
| |
| for_each_sg(sglist, sg, count, i) { |
| hw_address[i] = sg_dma_address(sg); |
| hw_len[i] = sg_dma_len(sg); |
| } |
| |
| where nents is the number of entries in the sglist. |
| |
| The implementation is free to merge several consecutive sglist entries |
| into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any |
| consecutive sglist entries can be merged into one provided the first one |
| ends and the second one starts on a page boundary - in fact this is a huge |
| advantage for cards which either cannot do scatter-gather or have very |
| limited number of scatter-gather entries) and returns the actual number |
| of sg entries it mapped them to. On failure 0 is returned. |
| |
| Then you should loop count times (note: this can be less than nents times) |
| and use sg_dma_address() and sg_dma_len() macros where you previously |
| accessed sg->address and sg->length as shown above. |
| |
| To unmap a scatterlist, just call: |
| |
| dma_unmap_sg(dev, sglist, nents, direction); |
| |
| Again, make sure DMA activity has already finished. |
| |
| PLEASE NOTE: The 'nents' argument to the dma_unmap_sg call must be |
| the _same_ one you passed into the dma_map_sg call, |
| it should _NOT_ be the 'count' value _returned_ from the |
| dma_map_sg call. |
| |
| Every dma_map_{single,sg}() call should have its dma_unmap_{single,sg}() |
| counterpart, because the DMA address space is a shared resource and |
| you could render the machine unusable by consuming all DMA addresses. |
| |
| If you need to use the same streaming DMA region multiple times and touch |
| the data in between the DMA transfers, the buffer needs to be synced |
| properly in order for the CPU and device to see the most up-to-date and |
| correct copy of the DMA buffer. |
| |
| So, firstly, just map it with dma_map_{single,sg}(), and after each DMA |
| transfer call either: |
| |
| dma_sync_single_for_cpu(dev, dma_handle, size, direction); |
| |
| or: |
| |
| dma_sync_sg_for_cpu(dev, sglist, nents, direction); |
| |
| as appropriate. |
| |
| Then, if you wish to let the device get at the DMA area again, |
| finish accessing the data with the CPU, and then before actually |
| giving the buffer to the hardware call either: |
| |
| dma_sync_single_for_device(dev, dma_handle, size, direction); |
| |
| or: |
| |
| dma_sync_sg_for_device(dev, sglist, nents, direction); |
| |
| as appropriate. |
| |
| PLEASE NOTE: The 'nents' argument to dma_sync_sg_for_cpu() and |
| dma_sync_sg_for_device() must be the same passed to |
| dma_map_sg(). It is _NOT_ the count returned by |
| dma_map_sg(). |
| |
| After the last DMA transfer call one of the DMA unmap routines |
| dma_unmap_{single,sg}(). If you don't touch the data from the first |
| dma_map_*() call till dma_unmap_*(), then you don't have to call the |
| dma_sync_*() routines at all. |
| |
| Here is pseudo code which shows a situation in which you would need |
| to use the dma_sync_*() interfaces. |
| |
| my_card_setup_receive_buffer(struct my_card *cp, char *buffer, int len) |
| { |
| dma_addr_t mapping; |
| |
| mapping = dma_map_single(cp->dev, buffer, len, DMA_FROM_DEVICE); |
| if (dma_mapping_error(cp->dev, mapping)) { |
| /* |
| * reduce current DMA mapping usage, |
| * delay and try again later or |
| * reset driver. |
| */ |
| goto map_error_handling; |
| } |
| |
| cp->rx_buf = buffer; |
| cp->rx_len = len; |
| cp->rx_dma = mapping; |
| |
| give_rx_buf_to_card(cp); |
| } |
| |
| ... |
| |
| my_card_interrupt_handler(int irq, void *devid, struct pt_regs *regs) |
| { |
| struct my_card *cp = devid; |
| |
| ... |
| if (read_card_status(cp) == RX_BUF_TRANSFERRED) { |
| struct my_card_header *hp; |
| |
| /* Examine the header to see if we wish |
| * to accept the data. But synchronize |
| * the DMA transfer with the CPU first |
| * so that we see updated contents. |
| */ |
| dma_sync_single_for_cpu(&cp->dev, cp->rx_dma, |
| cp->rx_len, |
| DMA_FROM_DEVICE); |
| |
| /* Now it is safe to examine the buffer. */ |
| hp = (struct my_card_header *) cp->rx_buf; |
| if (header_is_ok(hp)) { |
| dma_unmap_single(&cp->dev, cp->rx_dma, cp->rx_len, |
| DMA_FROM_DEVICE); |
| pass_to_upper_layers(cp->rx_buf); |
| make_and_setup_new_rx_buf(cp); |
| } else { |
| /* CPU should not write to |
| * DMA_FROM_DEVICE-mapped area, |
| * so dma_sync_single_for_device() is |
| * not needed here. It would be required |
| * for DMA_BIDIRECTIONAL mapping if |
| * the memory was modified. |
| */ |
| give_rx_buf_to_card(cp); |
| } |
| } |
| } |
| |
| Drivers converted fully to this interface should not use virt_to_bus() any |
| longer, nor should they use bus_to_virt(). Some drivers have to be changed a |
| little bit, because there is no longer an equivalent to bus_to_virt() in the |
| dynamic DMA mapping scheme - you have to always store the DMA addresses |
| returned by the dma_alloc_coherent(), dma_pool_alloc(), and dma_map_single() |
| calls (dma_map_sg() stores them in the scatterlist itself if the platform |
| supports dynamic DMA mapping in hardware) in your driver structures and/or |
| in the card registers. |
| |
| All drivers should be using these interfaces with no exceptions. It |
| is planned to completely remove virt_to_bus() and bus_to_virt() as |
| they are entirely deprecated. Some ports already do not provide these |
| as it is impossible to correctly support them. |
| |
| Handling Errors |
| |
| DMA address space is limited on some architectures and an allocation |
| failure can be determined by: |
| |
| - checking if dma_alloc_coherent() returns NULL or dma_map_sg returns 0 |
| |
| - checking the dma_addr_t returned from dma_map_single() and dma_map_page() |
| by using dma_mapping_error(): |
| |
| dma_addr_t dma_handle; |
| |
| dma_handle = dma_map_single(dev, addr, size, direction); |
| if (dma_mapping_error(dev, dma_handle)) { |
| /* |
| * reduce current DMA mapping usage, |
| * delay and try again later or |
| * reset driver. |
| */ |
| goto map_error_handling; |
| } |
| |
| - unmap pages that are already mapped, when mapping error occurs in the middle |
| of a multiple page mapping attempt. These example are applicable to |
| dma_map_page() as well. |
| |
| Example 1: |
| dma_addr_t dma_handle1; |
| dma_addr_t dma_handle2; |
| |
| dma_handle1 = dma_map_single(dev, addr, size, direction); |
| if (dma_mapping_error(dev, dma_handle1)) { |
| /* |
| * reduce current DMA mapping usage, |
| * delay and try again later or |
| * reset driver. |
| */ |
| goto map_error_handling1; |
| } |
| dma_handle2 = dma_map_single(dev, addr, size, direction); |
| if (dma_mapping_error(dev, dma_handle2)) { |
| /* |
| * reduce current DMA mapping usage, |
| * delay and try again later or |
| * reset driver. |
| */ |
| goto map_error_handling2; |
| } |
| |
| ... |
| |
| map_error_handling2: |
| dma_unmap_single(dma_handle1); |
| map_error_handling1: |
| |
| Example 2: (if buffers are allocated in a loop, unmap all mapped buffers when |
| mapping error is detected in the middle) |
| |
| dma_addr_t dma_addr; |
| dma_addr_t array[DMA_BUFFERS]; |
| int save_index = 0; |
| |
| for (i = 0; i < DMA_BUFFERS; i++) { |
| |
| ... |
| |
| dma_addr = dma_map_single(dev, addr, size, direction); |
| if (dma_mapping_error(dev, dma_addr)) { |
| /* |
| * reduce current DMA mapping usage, |
| * delay and try again later or |
| * reset driver. |
| */ |
| goto map_error_handling; |
| } |
| array[i].dma_addr = dma_addr; |
| save_index++; |
| } |
| |
| ... |
| |
| map_error_handling: |
| |
| for (i = 0; i < save_index; i++) { |
| |
| ... |
| |
| dma_unmap_single(array[i].dma_addr); |
| } |
| |
| Networking drivers must call dev_kfree_skb() to free the socket buffer |
| and return NETDEV_TX_OK if the DMA mapping fails on the transmit hook |
| (ndo_start_xmit). This means that the socket buffer is just dropped in |
| the failure case. |
| |
| SCSI drivers must return SCSI_MLQUEUE_HOST_BUSY if the DMA mapping |
| fails in the queuecommand hook. This means that the SCSI subsystem |
| passes the command to the driver again later. |
| |
| Optimizing Unmap State Space Consumption |
| |
| On many platforms, dma_unmap_{single,page}() is simply a nop. |
| Therefore, keeping track of the mapping address and length is a waste |
| of space. Instead of filling your drivers up with ifdefs and the like |
| to "work around" this (which would defeat the whole purpose of a |
| portable API) the following facilities are provided. |
| |
| Actually, instead of describing the macros one by one, we'll |
| transform some example code. |
| |
| 1) Use DEFINE_DMA_UNMAP_{ADDR,LEN} in state saving structures. |
| Example, before: |
| |
| struct ring_state { |
| struct sk_buff *skb; |
| dma_addr_t mapping; |
| __u32 len; |
| }; |
| |
| after: |
| |
| struct ring_state { |
| struct sk_buff *skb; |
| DEFINE_DMA_UNMAP_ADDR(mapping); |
| DEFINE_DMA_UNMAP_LEN(len); |
| }; |
| |
| 2) Use dma_unmap_{addr,len}_set() to set these values. |
| Example, before: |
| |
| ringp->mapping = FOO; |
| ringp->len = BAR; |
| |
| after: |
| |
| dma_unmap_addr_set(ringp, mapping, FOO); |
| dma_unmap_len_set(ringp, len, BAR); |
| |
| 3) Use dma_unmap_{addr,len}() to access these values. |
| Example, before: |
| |
| dma_unmap_single(dev, ringp->mapping, ringp->len, |
| DMA_FROM_DEVICE); |
| |
| after: |
| |
| dma_unmap_single(dev, |
| dma_unmap_addr(ringp, mapping), |
| dma_unmap_len(ringp, len), |
| DMA_FROM_DEVICE); |
| |
| It really should be self-explanatory. We treat the ADDR and LEN |
| separately, because it is possible for an implementation to only |
| need the address in order to perform the unmap operation. |
| |
| Platform Issues |
| |
| If you are just writing drivers for Linux and do not maintain |
| an architecture port for the kernel, you can safely skip down |
| to "Closing". |
| |
| 1) Struct scatterlist requirements. |
| |
| You need to enable CONFIG_NEED_SG_DMA_LENGTH if the architecture |
| supports IOMMUs (including software IOMMU). |
| |
| 2) ARCH_DMA_MINALIGN |
| |
| Architectures must ensure that kmalloc'ed buffer is |
| DMA-safe. Drivers and subsystems depend on it. If an architecture |
| isn't fully DMA-coherent (i.e. hardware doesn't ensure that data in |
| the CPU cache is identical to data in main memory), |
| ARCH_DMA_MINALIGN must be set so that the memory allocator |
| makes sure that kmalloc'ed buffer doesn't share a cache line with |
| the others. See arch/arm/include/asm/cache.h as an example. |
| |
| Note that ARCH_DMA_MINALIGN is about DMA memory alignment |
| constraints. You don't need to worry about the architecture data |
| alignment constraints (e.g. the alignment constraints about 64-bit |
| objects). |
| |
| Closing |
| |
| This document, and the API itself, would not be in its current |
| form without the feedback and suggestions from numerous individuals. |
| We would like to specifically mention, in no particular order, the |
| following people: |
| |
| Russell King <rmk@arm.linux.org.uk> |
| Leo Dagum <dagum@barrel.engr.sgi.com> |
| Ralf Baechle <ralf@oss.sgi.com> |
| Grant Grundler <grundler@cup.hp.com> |
| Jay Estabrook <Jay.Estabrook@compaq.com> |
| Thomas Sailer <sailer@ife.ee.ethz.ch> |
| Andrea Arcangeli <andrea@suse.de> |
| Jens Axboe <jens.axboe@oracle.com> |
| David Mosberger-Tang <davidm@hpl.hp.com> |