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Diffstat (limited to 'Documentation/PCI/PCI-DMA-mapping.txt')
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diff --git a/Documentation/PCI/PCI-DMA-mapping.txt b/Documentation/PCI/PCI-DMA-mapping.txt deleted file mode 100644 index ecad88d9fe59..000000000000 --- a/Documentation/PCI/PCI-DMA-mapping.txt +++ /dev/null @@ -1,766 +0,0 @@ - Dynamic DMA mapping - =================== - - David S. Miller <davem@redhat.com> - Richard Henderson <rth@cygnus.com> - Jakub Jelinek <jakub@redhat.com> - -This document describes the DMA mapping system in terms of the pci_ -API. For a similar API that works for generic devices, see -DMA-API.txt. - -Most of the 64bit platforms have special hardware that translates bus -addresses (DMA addresses) into physical addresses. This is similar to -how page tables and/or a TLB translates virtual addresses to physical -addresses on a CPU. This is needed so that e.g. PCI devices can -access with a Single Address Cycle (32bit DMA address) any page in the -64bit physical address space. Previously in Linux those 64bit -platforms had to set artificial limits on the maximum RAM size in the -system, so that the virt_to_bus() static scheme works (the DMA address -translation tables were simply filled on bootup to map each bus -address to the physical page __pa(bus_to_virt())). - -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, see e.g. arch/x86/include/asm/pci.h for how it is implemented on -top of the virt_to_bus interface. - -First of all, you should make sure - -#include <linux/pci.h> - -is in your driver. This file will obtain for you the definition of the -dma_addr_t (which can hold any valid DMA address for the platform) -type which should be used everywhere you hold a DMA (bus) 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 -on the PCI bus for SAC DMA transfers? If so, you need to inform the -PCI layer of this fact. - -By default, the kernel assumes that your device can address the full -32-bits in a SAC cycle. For a 64-bit DAC capable device, this needs -to be increased. And for a device with limitations, as discussed in -the previous paragraph, it needs to be decreased. - -pci_alloc_consistent() by default will return 32-bit DMA addresses. -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. Therefore, like with pci_set_dma_mask(), -it's good practice to call pci_set_consistent_dma_mask() to set the -appropriate mask even if your device only supports 32-bit DMA -(default) and especially if it's a PCI-X device. - -For correct operation, you must interrogate the PCI layer in your -device probe routine to see if the PCI 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 pci_set_dma_mask(): - - int pci_set_dma_mask(struct pci_dev *pdev, u64 device_mask); - -The query for consistent allocations is performed via a call to -pci_set_consistent_dma_mask(): - - int pci_set_consistent_dma_mask(struct pci_dev *pdev, u64 device_mask); - -Here, pdev is a pointer to the PCI device struct of your device, and -device_mask is a bit mask describing which bits of a PCI address your -device supports. It returns zero if your card can perform DMA -properly on the machine given the address mask you provided. - -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 PCI device would do something like -this: - - if (pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) { - printk(KERN_WARNING - "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 DAC addressing, but back down to a -32-bit mask should that fail. The PCI platform code 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 SAC addressing is done more efficiently than DAC addressing. -Sparc64 is one platform which behaves in this way. - -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 (!pci_set_dma_mask(pdev, DMA_BIT_MASK(64))) { - using_dac = 1; - } else if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) { - using_dac = 0; - } else { - printk(KERN_WARNING - "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 (!pci_set_dma_mask(pdev, DMA_BIT_MASK(64))) { - using_dac = 1; - consistent_using_dac = 1; - pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(64)); - } else if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) { - using_dac = 0; - consistent_using_dac = 0; - pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(32)); - } else { - printk(KERN_WARNING - "mydev: No suitable DMA available.\n"); - goto ignore_this_device; - } - -pci_set_consistent_dma_mask() will always be able to set the same or a -smaller mask as pci_set_dma_mask(). However for the rare case that a -device driver only uses consistent allocations, one would have to -check the return value from pci_set_consistent_dma_mask(). - -Finally, if your device can only drive the low 24-bits of -address during PCI bus mastering you might do something like: - - if (pci_set_dma_mask(pdev, DMA_BIT_MASK(24))) { - printk(KERN_WARNING - "mydev: 24-bit DMA addressing not available.\n"); - goto ignore_this_device; - } - -When pci_set_dma_mask() is successful, and returns zero, the PCI layer -saves away this mask you have provided. The PCI layer 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 pci_set_dma_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 pci_dev *pdev; - - ... - if (!pci_set_dma_mask(pdev, PLAYBACK_ADDRESS_BITS)) { - card->playback_enabled = 1; - } else { - card->playback_enabled = 0; - printk(KERN_WARNING "%s: Playback disabled due to DMA limitations.\n", - card->name); - } - if (!pci_set_dma_mask(pdev, RECORD_ADDRESS_BITS)) { - card->record_enabled = 1; - } else { - card->record_enabled = 0; - printk(KERN_WARNING "%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 PCI bus 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 pci_dma_sync_* below) and for which - hardware can optimize for sequential accesses. - - This 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 PCI, 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 = pci_alloc_consistent(pdev, size, &dma_handle); - -where pdev is a struct pci_dev *. This may be called in interrupt context. -You should use dma_alloc_coherent (see DMA-API.txt) for buses -where devices don't have struct pci_dev (like ISA, EISA). - -This argument is needed because the DMA translations may be bus -specific (and often is private to the bus which the device is attached -to). - -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 pci_pool interface, described below. - -The consistent DMA mapping interfaces, for non-NULL pdev, will by -default return a DMA address which is SAC (Single Address Cycle) -addressable. Even if the device indicates (via PCI dma mask) that it -may address the upper 32-bits and thus perform DAC cycles, consistent -allocation will only return > 32-bit PCI addresses for DMA if the -consistent dma mask has been explicitly changed via -pci_set_consistent_dma_mask(). This is true of the pci_pool interface -as well. - -pci_alloc_consistent 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 return address and the DMA bus master 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: - - pci_free_consistent(pdev, size, cpu_addr, dma_handle); - -where pdev, size are the same as in the above call and cpu_addr and -dma_handle are the values pci_alloc_consistent 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 pci_alloc_consistent, -or you can use the pci_pool API to do that. A pci_pool is like -a kmem_cache, but it uses pci_alloc_consistent 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 pci_pool like this: - - struct pci_pool *pool; - - pool = pci_pool_create(name, pdev, size, align, alloc); - -The "name" is for diagnostics (like a kmem_cache name); pdev 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 alloc; passing 4096 says memory allocated from this pool -must not cross 4KByte boundaries (but at that time it may be better to -go for pci_alloc_consistent directly instead). - -Allocate memory from a pci pool like this: - - cpu_addr = pci_pool_alloc(pool, flags, &dma_handle); - -flags are SLAB_KERNEL if blocking is permitted (not in_interrupt nor -holding SMP locks), SLAB_ATOMIC otherwise. Like pci_alloc_consistent, -this returns two values, cpu_addr and dma_handle. - -Free memory that was allocated from a pci_pool like this: - - pci_pool_free(pool, cpu_addr, dma_handle); - -where pool is what you passed to pci_pool_alloc, and cpu_addr and -dma_handle are the values pci_pool_alloc returned. This function -may be called in interrupt context. - -Destroy a pci_pool by calling: - - pci_pool_destroy(pool); - -Make sure you've called pci_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: - - PCI_DMA_BIDIRECTIONAL - PCI_DMA_TODEVICE - PCI_DMA_FROMDEVICE - PCI_DMA_NONE - -One should provide the exact DMA direction if you know it. - -PCI_DMA_TODEVICE means "from main memory to the PCI device" -PCI_DMA_FROMDEVICE means "from the PCI 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 PCI_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 PCI_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 PCI controller hardware detects violation of the -permission setting. - -Only streaming mappings specify a direction, consistent mappings -implicitly have a direction attribute setting of -PCI_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 PCI_DMA_TODEVICE direction -specifier. For receive packets, just the opposite, map/unmap them -with the PCI_DMA_FROMDEVICE 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 pci_dev *pdev = mydev->pdev; - dma_addr_t dma_handle; - void *addr = buffer->ptr; - size_t size = buffer->len; - - dma_handle = pci_map_single(pdev, addr, size, direction); - -and to unmap it: - - pci_unmap_single(pdev, dma_handle, size, direction); - -You should call pci_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 pci_{map,unmap}_single. These -interfaces deal with page/offset pairs instead of cpu pointers. -Specifically: - - struct pci_dev *pdev = mydev->pdev; - dma_addr_t dma_handle; - struct page *page = buffer->page; - unsigned long offset = buffer->offset; - size_t size = buffer->len; - - dma_handle = pci_map_page(pdev, page, offset, size, direction); - - ... - - pci_unmap_page(pdev, dma_handle, size, direction); - -Here, "offset" means byte offset within the given page. - -With scatterlists, you map a region gathered from several regions by: - - int i, count = pci_map_sg(pdev, 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: - - pci_unmap_sg(pdev, sglist, nents, direction); - -Again, make sure DMA activity has already finished. - -PLEASE NOTE: The 'nents' argument to the pci_unmap_sg call must be - the _same_ one you passed into the pci_map_sg call, - it should _NOT_ be the 'count' value _returned_ from the - pci_map_sg call. - -Every pci_map_{single,sg} call should have its pci_unmap_{single,sg} -counterpart, because the bus address space is a shared resource (although -in some ports the mapping is per each BUS so less devices contend for the -same bus address space) and you could render the machine unusable by eating -all bus 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 uptodate and -correct copy of the DMA buffer. - -So, firstly, just map it with pci_map_{single,sg}, and after each DMA -transfer call either: - - pci_dma_sync_single_for_cpu(pdev, dma_handle, size, direction); - -or: - - pci_dma_sync_sg_for_cpu(pdev, 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: - - pci_dma_sync_single_for_device(pdev, dma_handle, size, direction); - -or: - - pci_dma_sync_sg_for_device(dev, sglist, nents, direction); - -as appropriate. - -After the last DMA transfer call one of the DMA unmap routines -pci_unmap_{single,sg}. If you don't touch the data from the first pci_map_* -call till pci_unmap_*, then you don't have to call the pci_dma_sync_* -routines at all. - -Here is pseudo code which shows a situation in which you would need -to use the pci_dma_sync_*() interfaces. - - my_card_setup_receive_buffer(struct my_card *cp, char *buffer, int len) - { - dma_addr_t mapping; - - mapping = pci_map_single(cp->pdev, buffer, len, PCI_DMA_FROMDEVICE); - - 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. - */ - pci_dma_sync_single_for_cpu(cp->pdev, cp->rx_dma, - cp->rx_len, - PCI_DMA_FROMDEVICE); - - /* Now it is safe to examine the buffer. */ - hp = (struct my_card_header *) cp->rx_buf; - if (header_is_ok(hp)) { - pci_unmap_single(cp->pdev, cp->rx_dma, cp->rx_len, - PCI_DMA_FROMDEVICE); - pass_to_upper_layers(cp->rx_buf); - make_and_setup_new_rx_buf(cp); - } else { - /* Just sync the buffer and give it back - * to the card. - */ - pci_dma_sync_single_for_device(cp->pdev, - cp->rx_dma, - cp->rx_len, - PCI_DMA_FROMDEVICE); - 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 pci_alloc_consistent, pci_pool_alloc, and pci_map_single -calls (pci_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 PCI 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. - - Optimizing Unmap State Space Consumption - -On many platforms, pci_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 DECLARE_PCI_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; - DECLARE_PCI_UNMAP_ADDR(mapping) - DECLARE_PCI_UNMAP_LEN(len) - }; - - NOTE: DO NOT put a semicolon at the end of the DECLARE_*() - macro. - -2) Use pci_unmap_{addr,len}_set to set these values. - Example, before: - - ringp->mapping = FOO; - ringp->len = BAR; - - after: - - pci_unmap_addr_set(ringp, mapping, FOO); - pci_unmap_len_set(ringp, len, BAR); - -3) Use pci_unmap_{addr,len} to access these values. - Example, before: - - pci_unmap_single(pdev, ringp->mapping, ringp->len, - PCI_DMA_FROMDEVICE); - - after: - - pci_unmap_single(pdev, - pci_unmap_addr(ringp, mapping), - pci_unmap_len(ringp, len), - PCI_DMA_FROMDEVICE); - -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. - - Struct scatterlist must contain, at a minimum, the following - members: - - struct page *page; - unsigned int offset; - unsigned int length; - - The base address is specified by a "page+offset" pair. - - Previous versions of struct scatterlist contained a "void *address" - field that was sometimes used instead of page+offset. As of Linux - 2.5., page+offset is always used, and the "address" field has been - deleted. - -2) More to come... - - Handling Errors - -DMA address space is limited on some architectures and an allocation -failure can be determined by: - -- checking if pci_alloc_consistent returns NULL or pci_map_sg returns 0 - -- checking the returned dma_addr_t of pci_map_single and pci_map_page - by using pci_dma_mapping_error(): - - dma_addr_t dma_handle; - - dma_handle = pci_map_single(pdev, addr, size, direction); - if (pci_dma_mapping_error(pdev, dma_handle)) { - /* - * reduce current DMA mapping usage, - * delay and try again later or - * reset driver. - */ - } - - Closing - -This document, and the API itself, would not be in it's 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> |
