ia64/linux-2.6.18-xen.hg

view Documentation/DMA-mapping.txt @ 912:dd42cdb0ab89

[IA64] Build blktap2 driver by default in x86 builds.

add CONFIG_XEN_BLKDEV_TAP2=y to buildconfigs/linux-defconfig_xen_ia64.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
author Isaku Yamahata <yamahata@valinux.co.jp>
date Mon Jun 29 12:09:16 2009 +0900 (2009-06-29)
parents 831230e53067
children
line source
1 Dynamic DMA mapping
2 ===================
4 David S. Miller <davem@redhat.com>
5 Richard Henderson <rth@cygnus.com>
6 Jakub Jelinek <jakub@redhat.com>
8 This document describes the DMA mapping system in terms of the pci_
9 API. For a similar API that works for generic devices, see
10 DMA-API.txt.
12 Most of the 64bit platforms have special hardware that translates bus
13 addresses (DMA addresses) into physical addresses. This is similar to
14 how page tables and/or a TLB translates virtual addresses to physical
15 addresses on a CPU. This is needed so that e.g. PCI devices can
16 access with a Single Address Cycle (32bit DMA address) any page in the
17 64bit physical address space. Previously in Linux those 64bit
18 platforms had to set artificial limits on the maximum RAM size in the
19 system, so that the virt_to_bus() static scheme works (the DMA address
20 translation tables were simply filled on bootup to map each bus
21 address to the physical page __pa(bus_to_virt())).
23 So that Linux can use the dynamic DMA mapping, it needs some help from the
24 drivers, namely it has to take into account that DMA addresses should be
25 mapped only for the time they are actually used and unmapped after the DMA
26 transfer.
28 The following API will work of course even on platforms where no such
29 hardware exists, see e.g. include/asm-i386/pci.h for how it is implemented on
30 top of the virt_to_bus interface.
32 First of all, you should make sure
34 #include <linux/pci.h>
36 is in your driver. This file will obtain for you the definition of the
37 dma_addr_t (which can hold any valid DMA address for the platform)
38 type which should be used everywhere you hold a DMA (bus) address
39 returned from the DMA mapping functions.
41 What memory is DMA'able?
43 The first piece of information you must know is what kernel memory can
44 be used with the DMA mapping facilities. There has been an unwritten
45 set of rules regarding this, and this text is an attempt to finally
46 write them down.
48 If 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
51 that memory using the addresses returned from those routines.
53 This means specifically that you may _not_ use the memory/addresses
54 returned from vmalloc() for DMA. It is possible to DMA to the
55 _underlying_ memory mapped into a vmalloc() area, but this requires
56 walking page tables to get the physical addresses, and then
57 translating each of those pages back to a kernel address using
58 something like __va(). [ EDIT: Update this when we integrate
59 Gerd Knorr's generic code which does this. ]
61 This rule also means that you may use neither kernel image addresses
62 (items in data/text/bss segments), nor module image addresses, nor
63 stack addresses for DMA. These could all be mapped somewhere entirely
64 different than the rest of physical memory. Even if those classes of
65 memory could physically work with DMA, you'd need to ensure the I/O
66 buffers were cacheline-aligned. Without that, you'd see cacheline
67 sharing problems (data corruption) on CPUs with DMA-incoherent caches.
68 (The CPU could write to one word, DMA would write to a different one
69 in the same cache line, and one of them could be overwritten.)
71 Also, this means that you cannot take the return of a kmap()
72 call and DMA to/from that. This is similar to vmalloc().
74 What about block I/O and networking buffers? The block I/O and
75 networking subsystems make sure that the buffers they use are valid
76 for you to DMA from/to.
78 DMA addressing limitations
80 Does your device have any DMA addressing limitations? For example, is
81 your device only capable of driving the low order 24-bits of address
82 on the PCI bus for SAC DMA transfers? If so, you need to inform the
83 PCI layer of this fact.
85 By default, the kernel assumes that your device can address the full
86 32-bits in a SAC cycle. For a 64-bit DAC capable device, this needs
87 to be increased. And for a device with limitations, as discussed in
88 the previous paragraph, it needs to be decreased.
90 pci_alloc_consistent() by default will return 32-bit DMA addresses.
91 PCI-X specification requires PCI-X devices to support 64-bit
92 addressing (DAC) for all transactions. And at least one platform (SGI
93 SN2) requires 64-bit consistent allocations to operate correctly when
94 the IO bus is in PCI-X mode. Therefore, like with pci_set_dma_mask(),
95 it's good practice to call pci_set_consistent_dma_mask() to set the
96 appropriate mask even if your device only supports 32-bit DMA
97 (default) and especially if it's a PCI-X device.
99 For correct operation, you must interrogate the PCI layer in your
100 device probe routine to see if the PCI controller on the machine can
101 properly support the DMA addressing limitation your device has. It is
102 good style to do this even if your device holds the default setting,
103 because this shows that you did think about these issues wrt. your
104 device.
106 The query is performed via a call to pci_set_dma_mask():
108 int pci_set_dma_mask(struct pci_dev *pdev, u64 device_mask);
110 The query for consistent allocations is performed via a a call to
111 pci_set_consistent_dma_mask():
113 int pci_set_consistent_dma_mask(struct pci_dev *pdev, u64 device_mask);
115 Here, pdev is a pointer to the PCI device struct of your device, and
116 device_mask is a bit mask describing which bits of a PCI address your
117 device supports. It returns zero if your card can perform DMA
118 properly on the machine given the address mask you provided.
120 If it returns non-zero, your device can not perform DMA properly on
121 this platform, and attempting to do so will result in undefined
122 behavior. You must either use a different mask, or not use DMA.
124 This means that in the failure case, you have three options:
126 1) Use another DMA mask, if possible (see below).
127 2) Use some non-DMA mode for data transfer, if possible.
128 3) Ignore this device and do not initialize it.
130 It is recommended that your driver print a kernel KERN_WARNING message
131 when you end up performing either #2 or #3. In this manner, if a user
132 of your driver reports that performance is bad or that the device is not
133 even detected, you can ask them for the kernel messages to find out
134 exactly why.
136 The standard 32-bit addressing PCI device would do something like
137 this:
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 }
145 Another common scenario is a 64-bit capable device. The approach
146 here is to try for 64-bit DAC addressing, but back down to a
147 32-bit mask should that fail. The PCI platform code may fail the
148 64-bit mask not because the platform is not capable of 64-bit
149 addressing. Rather, it may fail in this case simply because
150 32-bit SAC addressing is done more efficiently than DAC addressing.
151 Sparc64 is one platform which behaves in this way.
153 Here is how you would handle a 64-bit capable device which can drive
154 all 64-bits when accessing streaming DMA:
156 int using_dac;
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 }
168 If a card is capable of using 64-bit consistent allocations as well,
169 the case would look like this:
171 int using_dac, consistent_using_dac;
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 }
187 pci_set_consistent_dma_mask() will always be able to set the same or a
188 smaller mask as pci_set_dma_mask(). However for the rare case that a
189 device driver only uses consistent allocations, one would have to
190 check the return value from pci_set_consistent_dma_mask().
192 If your 64-bit device is going to be an enormous consumer of DMA
193 mappings, this can be problematic since the DMA mappings are a
194 finite resource on many platforms. Please see the "DAC Addressing
195 for Address Space Hungry Devices" section near the end of this
196 document for how to handle this case.
198 Finally, if your device can only drive the low 24-bits of
199 address during PCI bus mastering you might do something like:
201 if (pci_set_dma_mask(pdev, DMA_24BIT_MASK)) {
202 printk(KERN_WARNING
203 "mydev: 24-bit DMA addressing not available.\n");
204 goto ignore_this_device;
205 }
206 [Better use DMA_24BIT_MASK instead of 0x00ffffff.
207 See linux/include/dma-mapping.h for reference.]
209 When pci_set_dma_mask() is successful, and returns zero, the PCI layer
210 saves away this mask you have provided. The PCI layer will use this
211 information later when you make DMA mappings.
213 There is a case which we are aware of at this time, which is worth
214 mentioning in this documentation. If your device supports multiple
215 functions (for example a sound card provides playback and record
216 functions) and the various different functions have _different_
217 DMA addressing limitations, you may wish to probe each mask and
218 only provide the functionality which the machine can handle. It
219 is important that the last call to pci_set_dma_mask() be for the
220 most specific mask.
222 Here is pseudo-code showing how this might be done:
224 #define PLAYBACK_ADDRESS_BITS DMA_32BIT_MASK
225 #define RECORD_ADDRESS_BITS 0x00ffffff
227 struct my_sound_card *card;
228 struct pci_dev *pdev;
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 }
246 A sound card was used as an example here because this genre of PCI
247 devices seems to be littered with ISA chips given a PCI front end,
248 and thus retaining the 16MB DMA addressing limitations of ISA.
250 Types of DMA mappings
252 There are two types of DMA mappings:
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.
260 Think of "consistent" as "synchronous" or "coherent".
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.
267 Good examples of what to use consistent mappings for are:
269 - Network card DMA ring descriptors.
270 - SCSI adapter mailbox command data structures.
271 - Device firmware microcode executed out of
272 main memory.
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.
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:
285 desc->word0 = address;
286 wmb();
287 desc->word1 = DESC_VALID;
289 in order to get correct behavior on all platforms.
291 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).
296 - 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.
300 This of "streaming" as "asynchronous" or "outside the coherency
301 domain".
303 Good examples of what to use streaming mappings for are:
305 - Networking buffers transmitted/received by a device.
306 - Filesystem buffers written/read by a SCSI device.
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.
313 Neither type of DMA mapping has alignment restrictions that come
314 from PCI, although some devices may have such restrictions.
315 Also, systems with caches that aren't DMA-coherent will work better
316 when the underlying buffers don't share cache lines with other data.
319 Using Consistent DMA mappings.
321 To allocate and map large (PAGE_SIZE or so) consistent DMA regions,
322 you should do:
324 dma_addr_t dma_handle;
326 cpu_addr = pci_alloc_consistent(dev, size, &dma_handle);
328 where dev is a struct pci_dev *. You should pass NULL for PCI like buses
329 where devices don't have struct pci_dev (like ISA, EISA). This may be
330 called in interrupt context.
332 This argument is needed because the DMA translations may be bus
333 specific (and often is private to the bus which the device is attached
334 to).
336 Size is the length of the region you want to allocate, in bytes.
338 This 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
340 driver needs regions sized smaller than a page, you may prefer using
341 the pci_pool interface, described below.
343 The consistent DMA mapping interfaces, for non-NULL dev, will by
344 default return a DMA address which is SAC (Single Address Cycle)
345 addressable. Even if the device indicates (via PCI dma mask) that it
346 may address the upper 32-bits and thus perform DAC cycles, consistent
347 allocation will only return > 32-bit PCI addresses for DMA if the
348 consistent dma mask has been explicitly changed via
349 pci_set_consistent_dma_mask(). This is true of the pci_pool interface
350 as well.
352 pci_alloc_consistent returns two values: the virtual address which you
353 can use to access it from the CPU and dma_handle which you pass to the
354 card.
356 The cpu return address and the DMA bus master address are both
357 guaranteed to be aligned to the smallest PAGE_SIZE order which
358 is greater than or equal to the requested size. This invariant
359 exists (for example) to guarantee that if you allocate a chunk
360 which is smaller than or equal to 64 kilobytes, the extent of the
361 buffer you receive will not cross a 64K boundary.
363 To unmap and free such a DMA region, you call:
365 pci_free_consistent(dev, size, cpu_addr, dma_handle);
367 where dev, size are the same as in the above call and cpu_addr and
368 dma_handle are the values pci_alloc_consistent returned to you.
369 This function may not be called in interrupt context.
371 If your driver needs lots of smaller memory regions, you can write
372 custom code to subdivide pages returned by pci_alloc_consistent,
373 or you can use the pci_pool API to do that. A pci_pool is like
374 a kmem_cache, but it uses pci_alloc_consistent not __get_free_pages.
375 Also, it understands common hardware constraints for alignment,
376 like queue heads needing to be aligned on N byte boundaries.
378 Create a pci_pool like this:
380 struct pci_pool *pool;
382 pool = pci_pool_create(name, dev, size, align, alloc);
384 The "name" is for diagnostics (like a kmem_cache name); dev and size
385 are as above. The device's hardware alignment requirement for this
386 type of data is "align" (which is expressed in bytes, and must be a
387 power of two). If your device has no boundary crossing restrictions,
388 pass 0 for alloc; passing 4096 says memory allocated from this pool
389 must not cross 4KByte boundaries (but at that time it may be better to
390 go for pci_alloc_consistent directly instead).
392 Allocate memory from a pci pool like this:
394 cpu_addr = pci_pool_alloc(pool, flags, &dma_handle);
396 flags are SLAB_KERNEL if blocking is permitted (not in_interrupt nor
397 holding SMP locks), SLAB_ATOMIC otherwise. Like pci_alloc_consistent,
398 this returns two values, cpu_addr and dma_handle.
400 Free memory that was allocated from a pci_pool like this:
402 pci_pool_free(pool, cpu_addr, dma_handle);
404 where pool is what you passed to pci_pool_alloc, and cpu_addr and
405 dma_handle are the values pci_pool_alloc returned. This function
406 may be called in interrupt context.
408 Destroy a pci_pool by calling:
410 pci_pool_destroy(pool);
412 Make sure you've called pci_pool_free for all memory allocated
413 from a pool before you destroy the pool. This function may not
414 be called in interrupt context.
416 DMA Direction
418 The interfaces described in subsequent portions of this document
419 take a DMA direction argument, which is an integer and takes on
420 one of the following values:
422 PCI_DMA_BIDIRECTIONAL
423 PCI_DMA_TODEVICE
424 PCI_DMA_FROMDEVICE
425 PCI_DMA_NONE
427 One should provide the exact DMA direction if you know it.
429 PCI_DMA_TODEVICE means "from main memory to the PCI device"
430 PCI_DMA_FROMDEVICE means "from the PCI device to main memory"
431 It is the direction in which the data moves during the DMA
432 transfer.
434 You are _strongly_ encouraged to specify this as precisely
435 as you possibly can.
437 If you absolutely cannot know the direction of the DMA transfer,
438 specify PCI_DMA_BIDIRECTIONAL. It means that the DMA can go in
439 either direction. The platform guarantees that you may legally
440 specify this, and that it will work, but this may be at the
441 cost of performance for example.
443 The value PCI_DMA_NONE is to be used for debugging. One can
444 hold this in a data structure before you come to know the
445 precise direction, and this will help catch cases where your
446 direction tracking logic has failed to set things up properly.
448 Another advantage of specifying this value precisely (outside of
449 potential platform-specific optimizations of such) is for debugging.
450 Some platforms actually have a write permission boolean which DMA
451 mappings can be marked with, much like page protections in the user
452 program address space. Such platforms can and do report errors in the
453 kernel logs when the PCI controller hardware detects violation of the
454 permission setting.
456 Only streaming mappings specify a direction, consistent mappings
457 implicitly have a direction attribute setting of
458 PCI_DMA_BIDIRECTIONAL.
460 The SCSI subsystem tells you the direction to use in the
461 'sc_data_direction' member of the SCSI command your driver is
462 working on.
464 For Networking drivers, it's a rather simple affair. For transmit
465 packets, map/unmap them with the PCI_DMA_TODEVICE direction
466 specifier. For receive packets, just the opposite, map/unmap them
467 with the PCI_DMA_FROMDEVICE direction specifier.
469 Using Streaming DMA mappings
471 The streaming DMA mapping routines can be called from interrupt
472 context. There are two versions of each map/unmap, one which will
473 map/unmap a single memory region, and one which will map/unmap a
474 scatterlist.
476 To map a single region, you do:
478 struct pci_dev *pdev = mydev->pdev;
479 dma_addr_t dma_handle;
480 void *addr = buffer->ptr;
481 size_t size = buffer->len;
483 dma_handle = pci_map_single(dev, addr, size, direction);
485 and to unmap it:
487 pci_unmap_single(dev, dma_handle, size, direction);
489 You should call pci_unmap_single when the DMA activity is finished, e.g.
490 from the interrupt which told you that the DMA transfer is done.
492 Using cpu pointers like this for single mappings has a disadvantage,
493 you cannot reference HIGHMEM memory in this way. Thus, there is a
494 map/unmap interface pair akin to pci_{map,unmap}_single. These
495 interfaces deal with page/offset pairs instead of cpu pointers.
496 Specifically:
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;
504 dma_handle = pci_map_page(dev, page, offset, size, direction);
506 ...
508 pci_unmap_page(dev, dma_handle, size, direction);
510 Here, "offset" means byte offset within the given page.
512 With scatterlists, you map a region gathered from several regions by:
514 int i, count = pci_map_sg(dev, sglist, nents, direction);
515 struct scatterlist *sg;
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 }
522 where nents is the number of entries in the sglist.
524 The implementation is free to merge several consecutive sglist entries
525 into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
526 consecutive sglist entries can be merged into one provided the first one
527 ends and the second one starts on a page boundary - in fact this is a huge
528 advantage for cards which either cannot do scatter-gather or have very
529 limited number of scatter-gather entries) and returns the actual number
530 of sg entries it mapped them to. On failure 0 is returned.
532 Then you should loop count times (note: this can be less than nents times)
533 and use sg_dma_address() and sg_dma_len() macros where you previously
534 accessed sg->address and sg->length as shown above.
536 To unmap a scatterlist, just call:
538 pci_unmap_sg(dev, sglist, nents, direction);
540 Again, make sure DMA activity has already finished.
542 PLEASE 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.
547 Every pci_map_{single,sg} call should have its pci_unmap_{single,sg}
548 counterpart, because the bus address space is a shared resource (although
549 in some ports the mapping is per each BUS so less devices contend for the
550 same bus address space) and you could render the machine unusable by eating
551 all bus addresses.
553 If you need to use the same streaming DMA region multiple times and touch
554 the data in between the DMA transfers, the buffer needs to be synced
555 properly in order for the cpu and device to see the most uptodate and
556 correct copy of the DMA buffer.
558 So, firstly, just map it with pci_map_{single,sg}, and after each DMA
559 transfer call either:
561 pci_dma_sync_single_for_cpu(dev, dma_handle, size, direction);
563 or:
565 pci_dma_sync_sg_for_cpu(dev, sglist, nents, direction);
567 as appropriate.
569 Then, if you wish to let the device get at the DMA area again,
570 finish accessing the data with the cpu, and then before actually
571 giving the buffer to the hardware call either:
573 pci_dma_sync_single_for_device(dev, dma_handle, size, direction);
575 or:
577 pci_dma_sync_sg_for_device(dev, sglist, nents, direction);
579 as appropriate.
581 After the last DMA transfer call one of the DMA unmap routines
582 pci_unmap_{single,sg}. If you don't touch the data from the first pci_map_*
583 call till pci_unmap_*, then you don't have to call the pci_dma_sync_*
584 routines at all.
586 Here is pseudo code which shows a situation in which you would need
587 to use the pci_dma_sync_*() interfaces.
589 my_card_setup_receive_buffer(struct my_card *cp, char *buffer, int len)
590 {
591 dma_addr_t mapping;
593 mapping = pci_map_single(cp->pdev, buffer, len, PCI_DMA_FROMDEVICE);
595 cp->rx_buf = buffer;
596 cp->rx_len = len;
597 cp->rx_dma = mapping;
599 give_rx_buf_to_card(cp);
600 }
602 ...
604 my_card_interrupt_handler(int irq, void *devid, struct pt_regs *regs)
605 {
606 struct my_card *cp = devid;
608 ...
609 if (read_card_status(cp) == RX_BUF_TRANSFERRED) {
610 struct my_card_header *hp;
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);
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 }
641 Drivers converted fully to this interface should not use virt_to_bus any
642 longer, nor should they use bus_to_virt. Some drivers have to be changed a
643 little bit, because there is no longer an equivalent to bus_to_virt in the
644 dynamic DMA mapping scheme - you have to always store the DMA addresses
645 returned by the pci_alloc_consistent, pci_pool_alloc, and pci_map_single
646 calls (pci_map_sg stores them in the scatterlist itself if the platform
647 supports dynamic DMA mapping in hardware) in your driver structures and/or
648 in the card registers.
650 All PCI drivers should be using these interfaces with no exceptions.
651 It is planned to completely remove virt_to_bus() and bus_to_virt() as
652 they are entirely deprecated. Some ports already do not provide these
653 as it is impossible to correctly support them.
655 64-bit DMA and DAC cycle support
657 Do you understand all of the text above? Great, then you already
658 know how to use 64-bit DMA addressing under Linux. Simply make
659 the appropriate pci_set_dma_mask() calls based upon your cards
660 capabilities, then use the mapping APIs above.
662 It is that simple.
664 Well, not for some odd devices. See the next section for information
665 about that.
667 DAC Addressing for Address Space Hungry Devices
669 There exists a class of devices which do not mesh well with the PCI
670 DMA mapping API. By definition these "mappings" are a finite
671 resource. The number of total available mappings per bus is platform
672 specific, but there will always be a reasonable amount.
674 What is "reasonable"? Reasonable means that networking and block I/O
675 devices need not worry about using too many mappings.
677 As an example of a problematic device, consider compute cluster cards.
678 They can potentially need to access gigabytes of memory at once via
679 DMA. Dynamic mappings are unsuitable for this kind of access pattern.
681 To this end we've provided a small API by which a device driver
682 may use DAC cycles to directly address all of physical memory.
683 Not all platforms support this, but most do. It is easy to determine
684 whether the platform will work properly at probe time.
686 First, understand that there may be a SEVERE performance penalty for
687 using these interfaces on some platforms. Therefore, you MUST only
688 use these interfaces if it is absolutely required. %99 of devices can
689 use the normal APIs without any problems.
691 Note that for streaming type mappings you must either use these
692 interfaces, or the dynamic mapping interfaces above. You may not mix
693 usage of both for the same device. Such an act is illegal and is
694 guaranteed to put a banana in your tailpipe.
696 However, consistent mappings may in fact be used in conjunction with
697 these interfaces. Remember that, as defined, consistent mappings are
698 always going to be SAC addressable.
700 The first thing your driver needs to do is query the PCI platform
701 layer if it is capable of handling your devices DAC addressing
702 capabilities:
704 int pci_dac_dma_supported(struct pci_dev *hwdev, u64 mask);
706 You may not use the following interfaces if this routine fails.
708 Next, DMA addresses using this API are kept track of using the
709 dma64_addr_t type. It is guaranteed to be big enough to hold any
710 DAC address the platform layer will give to you from the following
711 routines. If you have consistent mappings as well, you still
712 use plain dma_addr_t to keep track of those.
714 All mappings obtained here will be direct. The mappings are not
715 translated, and this is the purpose of this dialect of the DMA API.
717 All routines work with page/offset pairs. This is the _ONLY_ way to
718 portably 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
720 and offset using something like this:
722 struct page *page = virt_to_page(ptr);
723 unsigned long offset = offset_in_page(ptr);
725 Here are the interfaces:
727 dma64_addr_t pci_dac_page_to_dma(struct pci_dev *pdev,
728 struct page *page,
729 unsigned long offset,
730 int direction);
732 The DAC address for the tuple PAGE/OFFSET are returned. The direction
733 argument is the same as for pci_{map,unmap}_single(). The same rules
734 for cpu/device access apply here as for the streaming mapping
735 interfaces. To reiterate:
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.
741 When the DMA transfer is complete, invoke:
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);
747 This must be done before the CPU looks at the buffer again.
748 This interface behaves identically to pci_dma_sync_{single,sg}_for_cpu().
750 And likewise, if you wish to let the device get back at the buffer after
751 the cpu has read/written it, invoke:
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);
757 before letting the device access the DMA area again.
759 If you need to get back to the PAGE/OFFSET tuple from a dma64_addr_t
760 the following interfaces are provided:
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);
767 This is possible with the DAC interfaces purely because they are
768 not translated in any way.
770 Optimizing Unmap State Space Consumption
772 On many platforms, pci_unmap_{single,page}() is simply a nop.
773 Therefore, keeping track of the mapping address and length is a waste
774 of space. Instead of filling your drivers up with ifdefs and the like
775 to "work around" this (which would defeat the whole purpose of a
776 portable API) the following facilities are provided.
778 Actually, instead of describing the macros one by one, we'll
779 transform some example code.
781 1) Use DECLARE_PCI_UNMAP_{ADDR,LEN} in state saving structures.
782 Example, before:
784 struct ring_state {
785 struct sk_buff *skb;
786 dma_addr_t mapping;
787 __u32 len;
788 };
790 after:
792 struct ring_state {
793 struct sk_buff *skb;
794 DECLARE_PCI_UNMAP_ADDR(mapping)
795 DECLARE_PCI_UNMAP_LEN(len)
796 };
798 NOTE: DO NOT put a semicolon at the end of the DECLARE_*()
799 macro.
801 2) Use pci_unmap_{addr,len}_set to set these values.
802 Example, before:
804 ringp->mapping = FOO;
805 ringp->len = BAR;
807 after:
809 pci_unmap_addr_set(ringp, mapping, FOO);
810 pci_unmap_len_set(ringp, len, BAR);
812 3) Use pci_unmap_{addr,len} to access these values.
813 Example, before:
815 pci_unmap_single(pdev, ringp->mapping, ringp->len,
816 PCI_DMA_FROMDEVICE);
818 after:
820 pci_unmap_single(pdev,
821 pci_unmap_addr(ringp, mapping),
822 pci_unmap_len(ringp, len),
823 PCI_DMA_FROMDEVICE);
825 It really should be self-explanatory. We treat the ADDR and LEN
826 separately, because it is possible for an implementation to only
827 need the address in order to perform the unmap operation.
829 Platform Issues
831 If you are just writing drivers for Linux and do not maintain
832 an architecture port for the kernel, you can safely skip down
833 to "Closing".
835 1) Struct scatterlist requirements.
837 Struct scatterlist must contain, at a minimum, the following
838 members:
840 struct page *page;
841 unsigned int offset;
842 unsigned int length;
844 The base address is specified by a "page+offset" pair.
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.
851 2) More to come...
853 Handling Errors
855 DMA address space is limited on some architectures and an allocation
856 failure can be determined by:
858 - checking if pci_alloc_consistent returns NULL or pci_map_sg returns 0
860 - checking the returned dma_addr_t of pci_map_single and pci_map_page
861 by using pci_dma_mapping_error():
863 dma_addr_t dma_handle;
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 }
874 Closing
876 This document, and the API itself, would not be in it's current
877 form without the feedback and suggestions from numerous individuals.
878 We would like to specifically mention, in no particular order, the
879 following people:
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>