Root/Documentation/DMA-mapping.txt

1            Dynamic DMA mapping
2            ===================
3
4         David S. Miller <davem@redhat.com>
5         Richard Henderson <rth@cygnus.com>
6          Jakub Jelinek <jakub@redhat.com>
7
8This document describes the DMA mapping system in terms of the pci_
9API. For a similar API that works for generic devices, see
10DMA-API.txt.
11
12Most of the 64bit platforms have special hardware that translates bus
13addresses (DMA addresses) into physical addresses. This is similar to
14how page tables and/or a TLB translates virtual addresses to physical
15addresses on a CPU. This is needed so that e.g. PCI devices can
16access with a Single Address Cycle (32bit DMA address) any page in the
1764bit physical address space. Previously in Linux those 64bit
18platforms had to set artificial limits on the maximum RAM size in the
19system, so that the virt_to_bus() static scheme works (the DMA address
20translation tables were simply filled on bootup to map each bus
21address to the physical page __pa(bus_to_virt())).
22
23So that Linux can use the dynamic DMA mapping, it needs some help from the
24drivers, namely it has to take into account that DMA addresses should be
25mapped only for the time they are actually used and unmapped after the DMA
26transfer.
27
28The following API will work of course even on platforms where no such
29hardware exists, see e.g. arch/x86/include/asm/pci.h for how it is implemented on
30top of the virt_to_bus interface.
31
32First of all, you should make sure
33
34#include <linux/pci.h>
35
36is in your driver. This file will obtain for you the definition of the
37dma_addr_t (which can hold any valid DMA address for the platform)
38type which should be used everywhere you hold a DMA (bus) address
39returned from the DMA mapping functions.
40
41             What memory is DMA'able?
42
43The first piece of information you must know is what kernel memory can
44be used with the DMA mapping facilities. There has been an unwritten
45set of rules regarding this, and this text is an attempt to finally
46write them down.
47
48If you acquired your memory via the page allocator
49(i.e. __get_free_page*()) or the generic memory allocators
50(i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from
51that memory using the addresses returned from those routines.
52
53This means specifically that you may _not_ use the memory/addresses
54returned from vmalloc() for DMA. It is possible to DMA to the
55_underlying_ memory mapped into a vmalloc() area, but this requires
56walking page tables to get the physical addresses, and then
57translating each of those pages back to a kernel address using
58something like __va(). [ EDIT: Update this when we integrate
59Gerd Knorr's generic code which does this. ]
60
61This rule also means that you may use neither kernel image addresses
62(items in data/text/bss segments), nor module image addresses, nor
63stack addresses for DMA. These could all be mapped somewhere entirely
64different than the rest of physical memory. Even if those classes of
65memory could physically work with DMA, you'd need to ensure the I/O
66buffers were cacheline-aligned. Without that, you'd see cacheline
67sharing problems (data corruption) on CPUs with DMA-incoherent caches.
68(The CPU could write to one word, DMA would write to a different one
69in the same cache line, and one of them could be overwritten.)
70
71Also, this means that you cannot take the return of a kmap()
72call and DMA to/from that. This is similar to vmalloc().
73
74What about block I/O and networking buffers? The block I/O and
75networking subsystems make sure that the buffers they use are valid
76for you to DMA from/to.
77
78            DMA addressing limitations
79
80Does your device have any DMA addressing limitations? For example, is
81your device only capable of driving the low order 24-bits of address
82on the PCI bus for SAC DMA transfers? If so, you need to inform the
83PCI layer of this fact.
84
85By default, the kernel assumes that your device can address the full
8632-bits in a SAC cycle. For a 64-bit DAC capable device, this needs
87to be increased. And for a device with limitations, as discussed in
88the previous paragraph, it needs to be decreased.
89
90pci_alloc_consistent() by default will return 32-bit DMA addresses.
91PCI-X specification requires PCI-X devices to support 64-bit
92addressing (DAC) for all transactions. And at least one platform (SGI
93SN2) requires 64-bit consistent allocations to operate correctly when
94the IO bus is in PCI-X mode. Therefore, like with pci_set_dma_mask(),
95it's good practice to call pci_set_consistent_dma_mask() to set the
96appropriate mask even if your device only supports 32-bit DMA
97(default) and especially if it's a PCI-X device.
98
99For correct operation, you must interrogate the PCI layer in your
100device probe routine to see if the PCI controller on the machine can
101properly support the DMA addressing limitation your device has. It is
102good style to do this even if your device holds the default setting,
103because this shows that you did think about these issues wrt. your
104device.
105
106The query is performed via a call to pci_set_dma_mask():
107
108    int pci_set_dma_mask(struct pci_dev *pdev, u64 device_mask);
109
110The query for consistent allocations is performed via a call to
111pci_set_consistent_dma_mask():
112
113    int pci_set_consistent_dma_mask(struct pci_dev *pdev, u64 device_mask);
114
115Here, pdev is a pointer to the PCI device struct of your device, and
116device_mask is a bit mask describing which bits of a PCI address your
117device supports. It returns zero if your card can perform DMA
118properly on the machine given the address mask you provided.
119
120If it returns non-zero, your device cannot perform DMA properly on
121this platform, and attempting to do so will result in undefined
122behavior. You must either use a different mask, or not use DMA.
123
124This means that in the failure case, you have three options:
125
1261) Use another DMA mask, if possible (see below).
1272) Use some non-DMA mode for data transfer, if possible.
1283) Ignore this device and do not initialize it.
129
130It is recommended that your driver print a kernel KERN_WARNING message
131when you end up performing either #2 or #3. In this manner, if a user
132of your driver reports that performance is bad or that the device is not
133even detected, you can ask them for the kernel messages to find out
134exactly why.
135
136The standard 32-bit addressing PCI device would do something like
137this:
138
139    if (pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) {
140        printk(KERN_WARNING
141               "mydev: No suitable DMA available.\n");
142        goto ignore_this_device;
143    }
144
145Another common scenario is a 64-bit capable device. The approach
146here is to try for 64-bit DAC addressing, but back down to a
14732-bit mask should that fail. The PCI platform code may fail the
14864-bit mask not because the platform is not capable of 64-bit
149addressing. Rather, it may fail in this case simply because
15032-bit SAC addressing is done more efficiently than DAC addressing.
151Sparc64 is one platform which behaves in this way.
152
153Here is how you would handle a 64-bit capable device which can drive
154all 64-bits when accessing streaming DMA:
155
156    int using_dac;
157
158    if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(64))) {
159        using_dac = 1;
160    } else if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) {
161        using_dac = 0;
162    } else {
163        printk(KERN_WARNING
164               "mydev: No suitable DMA available.\n");
165        goto ignore_this_device;
166    }
167
168If a card is capable of using 64-bit consistent allocations as well,
169the case would look like this:
170
171    int using_dac, consistent_using_dac;
172
173    if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(64))) {
174        using_dac = 1;
175           consistent_using_dac = 1;
176        pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(64));
177    } else if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) {
178        using_dac = 0;
179        consistent_using_dac = 0;
180        pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(32));
181    } else {
182        printk(KERN_WARNING
183               "mydev: No suitable DMA available.\n");
184        goto ignore_this_device;
185    }
186
187pci_set_consistent_dma_mask() will always be able to set the same or a
188smaller mask as pci_set_dma_mask(). However for the rare case that a
189device driver only uses consistent allocations, one would have to
190check the return value from pci_set_consistent_dma_mask().
191
192Finally, if your device can only drive the low 24-bits of
193address during PCI bus mastering you might do something like:
194
195    if (pci_set_dma_mask(pdev, DMA_BIT_MASK(24))) {
196        printk(KERN_WARNING
197               "mydev: 24-bit DMA addressing not available.\n");
198        goto ignore_this_device;
199    }
200
201When pci_set_dma_mask() is successful, and returns zero, the PCI layer
202saves away this mask you have provided. The PCI layer will use this
203information later when you make DMA mappings.
204
205There is a case which we are aware of at this time, which is worth
206mentioning in this documentation. If your device supports multiple
207functions (for example a sound card provides playback and record
208functions) and the various different functions have _different_
209DMA addressing limitations, you may wish to probe each mask and
210only provide the functionality which the machine can handle. It
211is important that the last call to pci_set_dma_mask() be for the
212most specific mask.
213
214Here is pseudo-code showing how this might be done:
215
216    #define PLAYBACK_ADDRESS_BITS DMA_BIT_MASK(32)
217    #define RECORD_ADDRESS_BITS 0x00ffffff
218
219    struct my_sound_card *card;
220    struct pci_dev *pdev;
221
222    ...
223    if (!pci_set_dma_mask(pdev, PLAYBACK_ADDRESS_BITS)) {
224        card->playback_enabled = 1;
225    } else {
226        card->playback_enabled = 0;
227        printk(KERN_WARN "%s: Playback disabled due to DMA limitations.\n",
228               card->name);
229    }
230    if (!pci_set_dma_mask(pdev, RECORD_ADDRESS_BITS)) {
231        card->record_enabled = 1;
232    } else {
233        card->record_enabled = 0;
234        printk(KERN_WARN "%s: Record disabled due to DMA limitations.\n",
235               card->name);
236    }
237
238A sound card was used as an example here because this genre of PCI
239devices seems to be littered with ISA chips given a PCI front end,
240and thus retaining the 16MB DMA addressing limitations of ISA.
241
242            Types of DMA mappings
243
244There are two types of DMA mappings:
245
246- Consistent DMA mappings which are usually mapped at driver
247  initialization, unmapped at the end and for which the hardware should
248  guarantee that the device and the CPU can access the data
249  in parallel and will see updates made by each other without any
250  explicit software flushing.
251
252  Think of "consistent" as "synchronous" or "coherent".
253
254  The current default is to return consistent memory in the low 32
255  bits of the PCI bus space. However, for future compatibility you
256  should set the consistent mask even if this default is fine for your
257  driver.
258
259  Good examples of what to use consistent mappings for are:
260
261    - Network card DMA ring descriptors.
262    - SCSI adapter mailbox command data structures.
263    - Device firmware microcode executed out of
264      main memory.
265
266  The invariant these examples all require is that any CPU store
267  to memory is immediately visible to the device, and vice
268  versa. Consistent mappings guarantee this.
269
270  IMPORTANT: Consistent DMA memory does not preclude the usage of
271             proper memory barriers. The CPU may reorder stores to
272         consistent memory just as it may normal memory. Example:
273         if it is important for the device to see the first word
274         of a descriptor updated before the second, you must do
275         something like:
276
277        desc->word0 = address;
278        wmb();
279        desc->word1 = DESC_VALID;
280
281             in order to get correct behavior on all platforms.
282
283         Also, on some platforms your driver may need to flush CPU write
284         buffers in much the same way as it needs to flush write buffers
285         found in PCI bridges (such as by reading a register's value
286         after writing it).
287
288- Streaming DMA mappings which are usually mapped for one DMA transfer,
289  unmapped right after it (unless you use pci_dma_sync_* below) and for which
290  hardware can optimize for sequential accesses.
291
292  This of "streaming" as "asynchronous" or "outside the coherency
293  domain".
294
295  Good examples of what to use streaming mappings for are:
296
297    - Networking buffers transmitted/received by a device.
298    - Filesystem buffers written/read by a SCSI device.
299
300  The interfaces for using this type of mapping were designed in
301  such a way that an implementation can make whatever performance
302  optimizations the hardware allows. To this end, when using
303  such mappings you must be explicit about what you want to happen.
304
305Neither type of DMA mapping has alignment restrictions that come
306from PCI, although some devices may have such restrictions.
307Also, systems with caches that aren't DMA-coherent will work better
308when the underlying buffers don't share cache lines with other data.
309
310
311         Using Consistent DMA mappings.
312
313To allocate and map large (PAGE_SIZE or so) consistent DMA regions,
314you should do:
315
316    dma_addr_t dma_handle;
317
318    cpu_addr = pci_alloc_consistent(pdev, size, &dma_handle);
319
320where pdev is a struct pci_dev *. This may be called in interrupt context.
321You should use dma_alloc_coherent (see DMA-API.txt) for buses
322where devices don't have struct pci_dev (like ISA, EISA).
323
324This argument is needed because the DMA translations may be bus
325specific (and often is private to the bus which the device is attached
326to).
327
328Size is the length of the region you want to allocate, in bytes.
329
330This routine will allocate RAM for that region, so it acts similarly to
331__get_free_pages (but takes size instead of a page order). If your
332driver needs regions sized smaller than a page, you may prefer using
333the pci_pool interface, described below.
334
335The consistent DMA mapping interfaces, for non-NULL pdev, will by
336default return a DMA address which is SAC (Single Address Cycle)
337addressable. Even if the device indicates (via PCI dma mask) that it
338may address the upper 32-bits and thus perform DAC cycles, consistent
339allocation will only return > 32-bit PCI addresses for DMA if the
340consistent dma mask has been explicitly changed via
341pci_set_consistent_dma_mask(). This is true of the pci_pool interface
342as well.
343
344pci_alloc_consistent returns two values: the virtual address which you
345can use to access it from the CPU and dma_handle which you pass to the
346card.
347
348The cpu return address and the DMA bus master address are both
349guaranteed to be aligned to the smallest PAGE_SIZE order which
350is greater than or equal to the requested size. This invariant
351exists (for example) to guarantee that if you allocate a chunk
352which is smaller than or equal to 64 kilobytes, the extent of the
353buffer you receive will not cross a 64K boundary.
354
355To unmap and free such a DMA region, you call:
356
357    pci_free_consistent(pdev, size, cpu_addr, dma_handle);
358
359where pdev, size are the same as in the above call and cpu_addr and
360dma_handle are the values pci_alloc_consistent returned to you.
361This function may not be called in interrupt context.
362
363If your driver needs lots of smaller memory regions, you can write
364custom code to subdivide pages returned by pci_alloc_consistent,
365or you can use the pci_pool API to do that. A pci_pool is like
366a kmem_cache, but it uses pci_alloc_consistent not __get_free_pages.
367Also, it understands common hardware constraints for alignment,
368like queue heads needing to be aligned on N byte boundaries.
369
370Create a pci_pool like this:
371
372    struct pci_pool *pool;
373
374    pool = pci_pool_create(name, pdev, size, align, alloc);
375
376The "name" is for diagnostics (like a kmem_cache name); pdev and size
377are as above. The device's hardware alignment requirement for this
378type of data is "align" (which is expressed in bytes, and must be a
379power of two). If your device has no boundary crossing restrictions,
380pass 0 for alloc; passing 4096 says memory allocated from this pool
381must not cross 4KByte boundaries (but at that time it may be better to
382go for pci_alloc_consistent directly instead).
383
384Allocate memory from a pci pool like this:
385
386    cpu_addr = pci_pool_alloc(pool, flags, &dma_handle);
387
388flags are SLAB_KERNEL if blocking is permitted (not in_interrupt nor
389holding SMP locks), SLAB_ATOMIC otherwise. Like pci_alloc_consistent,
390this returns two values, cpu_addr and dma_handle.
391
392Free memory that was allocated from a pci_pool like this:
393
394    pci_pool_free(pool, cpu_addr, dma_handle);
395
396where pool is what you passed to pci_pool_alloc, and cpu_addr and
397dma_handle are the values pci_pool_alloc returned. This function
398may be called in interrupt context.
399
400Destroy a pci_pool by calling:
401
402    pci_pool_destroy(pool);
403
404Make sure you've called pci_pool_free for all memory allocated
405from a pool before you destroy the pool. This function may not
406be called in interrupt context.
407
408            DMA Direction
409
410The interfaces described in subsequent portions of this document
411take a DMA direction argument, which is an integer and takes on
412one of the following values:
413
414 PCI_DMA_BIDIRECTIONAL
415 PCI_DMA_TODEVICE
416 PCI_DMA_FROMDEVICE
417 PCI_DMA_NONE
418
419One should provide the exact DMA direction if you know it.
420
421PCI_DMA_TODEVICE means "from main memory to the PCI device"
422PCI_DMA_FROMDEVICE means "from the PCI device to main memory"
423It is the direction in which the data moves during the DMA
424transfer.
425
426You are _strongly_ encouraged to specify this as precisely
427as you possibly can.
428
429If you absolutely cannot know the direction of the DMA transfer,
430specify PCI_DMA_BIDIRECTIONAL. It means that the DMA can go in
431either direction. The platform guarantees that you may legally
432specify this, and that it will work, but this may be at the
433cost of performance for example.
434
435The value PCI_DMA_NONE is to be used for debugging. One can
436hold this in a data structure before you come to know the
437precise direction, and this will help catch cases where your
438direction tracking logic has failed to set things up properly.
439
440Another advantage of specifying this value precisely (outside of
441potential platform-specific optimizations of such) is for debugging.
442Some platforms actually have a write permission boolean which DMA
443mappings can be marked with, much like page protections in the user
444program address space. Such platforms can and do report errors in the
445kernel logs when the PCI controller hardware detects violation of the
446permission setting.
447
448Only streaming mappings specify a direction, consistent mappings
449implicitly have a direction attribute setting of
450PCI_DMA_BIDIRECTIONAL.
451
452The SCSI subsystem tells you the direction to use in the
453'sc_data_direction' member of the SCSI command your driver is
454working on.
455
456For Networking drivers, it's a rather simple affair. For transmit
457packets, map/unmap them with the PCI_DMA_TODEVICE direction
458specifier. For receive packets, just the opposite, map/unmap them
459with the PCI_DMA_FROMDEVICE direction specifier.
460
461          Using Streaming DMA mappings
462
463The streaming DMA mapping routines can be called from interrupt
464context. There are two versions of each map/unmap, one which will
465map/unmap a single memory region, and one which will map/unmap a
466scatterlist.
467
468To map a single region, you do:
469
470    struct pci_dev *pdev = mydev->pdev;
471    dma_addr_t dma_handle;
472    void *addr = buffer->ptr;
473    size_t size = buffer->len;
474
475    dma_handle = pci_map_single(pdev, addr, size, direction);
476
477and to unmap it:
478
479    pci_unmap_single(pdev, dma_handle, size, direction);
480
481You should call pci_unmap_single when the DMA activity is finished, e.g.
482from the interrupt which told you that the DMA transfer is done.
483
484Using cpu pointers like this for single mappings has a disadvantage,
485you cannot reference HIGHMEM memory in this way. Thus, there is a
486map/unmap interface pair akin to pci_{map,unmap}_single. These
487interfaces deal with page/offset pairs instead of cpu pointers.
488Specifically:
489
490    struct pci_dev *pdev = mydev->pdev;
491    dma_addr_t dma_handle;
492    struct page *page = buffer->page;
493    unsigned long offset = buffer->offset;
494    size_t size = buffer->len;
495
496    dma_handle = pci_map_page(pdev, page, offset, size, direction);
497
498    ...
499
500    pci_unmap_page(pdev, dma_handle, size, direction);
501
502Here, "offset" means byte offset within the given page.
503
504With scatterlists, you map a region gathered from several regions by:
505
506    int i, count = pci_map_sg(pdev, sglist, nents, direction);
507    struct scatterlist *sg;
508
509    for_each_sg(sglist, sg, count, i) {
510        hw_address[i] = sg_dma_address(sg);
511        hw_len[i] = sg_dma_len(sg);
512    }
513
514where nents is the number of entries in the sglist.
515
516The implementation is free to merge several consecutive sglist entries
517into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
518consecutive sglist entries can be merged into one provided the first one
519ends and the second one starts on a page boundary - in fact this is a huge
520advantage for cards which either cannot do scatter-gather or have very
521limited number of scatter-gather entries) and returns the actual number
522of sg entries it mapped them to. On failure 0 is returned.
523
524Then you should loop count times (note: this can be less than nents times)
525and use sg_dma_address() and sg_dma_len() macros where you previously
526accessed sg->address and sg->length as shown above.
527
528To unmap a scatterlist, just call:
529
530    pci_unmap_sg(pdev, sglist, nents, direction);
531
532Again, make sure DMA activity has already finished.
533
534PLEASE NOTE: The 'nents' argument to the pci_unmap_sg call must be
535              the _same_ one you passed into the pci_map_sg call,
536          it should _NOT_ be the 'count' value _returned_ from the
537              pci_map_sg call.
538
539Every pci_map_{single,sg} call should have its pci_unmap_{single,sg}
540counterpart, because the bus address space is a shared resource (although
541in some ports the mapping is per each BUS so less devices contend for the
542same bus address space) and you could render the machine unusable by eating
543all bus addresses.
544
545If you need to use the same streaming DMA region multiple times and touch
546the data in between the DMA transfers, the buffer needs to be synced
547properly in order for the cpu and device to see the most uptodate and
548correct copy of the DMA buffer.
549
550So, firstly, just map it with pci_map_{single,sg}, and after each DMA
551transfer call either:
552
553    pci_dma_sync_single_for_cpu(pdev, dma_handle, size, direction);
554
555or:
556
557    pci_dma_sync_sg_for_cpu(pdev, sglist, nents, direction);
558
559as appropriate.
560
561Then, if you wish to let the device get at the DMA area again,
562finish accessing the data with the cpu, and then before actually
563giving the buffer to the hardware call either:
564
565    pci_dma_sync_single_for_device(pdev, dma_handle, size, direction);
566
567or:
568
569    pci_dma_sync_sg_for_device(dev, sglist, nents, direction);
570
571as appropriate.
572
573After the last DMA transfer call one of the DMA unmap routines
574pci_unmap_{single,sg}. If you don't touch the data from the first pci_map_*
575call till pci_unmap_*, then you don't have to call the pci_dma_sync_*
576routines at all.
577
578Here is pseudo code which shows a situation in which you would need
579to use the pci_dma_sync_*() interfaces.
580
581    my_card_setup_receive_buffer(struct my_card *cp, char *buffer, int len)
582    {
583        dma_addr_t mapping;
584
585        mapping = pci_map_single(cp->pdev, buffer, len, PCI_DMA_FROMDEVICE);
586
587        cp->rx_buf = buffer;
588        cp->rx_len = len;
589        cp->rx_dma = mapping;
590
591        give_rx_buf_to_card(cp);
592    }
593
594    ...
595
596    my_card_interrupt_handler(int irq, void *devid, struct pt_regs *regs)
597    {
598        struct my_card *cp = devid;
599
600        ...
601        if (read_card_status(cp) == RX_BUF_TRANSFERRED) {
602            struct my_card_header *hp;
603
604            /* Examine the header to see if we wish
605             * to accept the data. But synchronize
606             * the DMA transfer with the CPU first
607             * so that we see updated contents.
608             */
609            pci_dma_sync_single_for_cpu(cp->pdev, cp->rx_dma,
610                            cp->rx_len,
611                            PCI_DMA_FROMDEVICE);
612
613            /* Now it is safe to examine the buffer. */
614            hp = (struct my_card_header *) cp->rx_buf;
615            if (header_is_ok(hp)) {
616                pci_unmap_single(cp->pdev, cp->rx_dma, cp->rx_len,
617                         PCI_DMA_FROMDEVICE);
618                pass_to_upper_layers(cp->rx_buf);
619                make_and_setup_new_rx_buf(cp);
620            } else {
621                /* Just sync the buffer and give it back
622                 * to the card.
623                 */
624                pci_dma_sync_single_for_device(cp->pdev,
625                                   cp->rx_dma,
626                                   cp->rx_len,
627                                   PCI_DMA_FROMDEVICE);
628                give_rx_buf_to_card(cp);
629            }
630        }
631    }
632
633Drivers converted fully to this interface should not use virt_to_bus any
634longer, nor should they use bus_to_virt. Some drivers have to be changed a
635little bit, because there is no longer an equivalent to bus_to_virt in the
636dynamic DMA mapping scheme - you have to always store the DMA addresses
637returned by the pci_alloc_consistent, pci_pool_alloc, and pci_map_single
638calls (pci_map_sg stores them in the scatterlist itself if the platform
639supports dynamic DMA mapping in hardware) in your driver structures and/or
640in the card registers.
641
642All PCI drivers should be using these interfaces with no exceptions.
643It is planned to completely remove virt_to_bus() and bus_to_virt() as
644they are entirely deprecated. Some ports already do not provide these
645as it is impossible to correctly support them.
646
647        Optimizing Unmap State Space Consumption
648
649On many platforms, pci_unmap_{single,page}() is simply a nop.
650Therefore, keeping track of the mapping address and length is a waste
651of space. Instead of filling your drivers up with ifdefs and the like
652to "work around" this (which would defeat the whole purpose of a
653portable API) the following facilities are provided.
654
655Actually, instead of describing the macros one by one, we'll
656transform some example code.
657
6581) Use DECLARE_PCI_UNMAP_{ADDR,LEN} in state saving structures.
659   Example, before:
660
661    struct ring_state {
662        struct sk_buff *skb;
663        dma_addr_t mapping;
664        __u32 len;
665    };
666
667   after:
668
669    struct ring_state {
670        struct sk_buff *skb;
671        DECLARE_PCI_UNMAP_ADDR(mapping)
672        DECLARE_PCI_UNMAP_LEN(len)
673    };
674
675   NOTE: DO NOT put a semicolon at the end of the DECLARE_*()
676         macro.
677
6782) Use pci_unmap_{addr,len}_set to set these values.
679   Example, before:
680
681    ringp->mapping = FOO;
682    ringp->len = BAR;
683
684   after:
685
686    pci_unmap_addr_set(ringp, mapping, FOO);
687    pci_unmap_len_set(ringp, len, BAR);
688
6893) Use pci_unmap_{addr,len} to access these values.
690   Example, before:
691
692    pci_unmap_single(pdev, ringp->mapping, ringp->len,
693             PCI_DMA_FROMDEVICE);
694
695   after:
696
697    pci_unmap_single(pdev,
698             pci_unmap_addr(ringp, mapping),
699             pci_unmap_len(ringp, len),
700             PCI_DMA_FROMDEVICE);
701
702It really should be self-explanatory. We treat the ADDR and LEN
703separately, because it is possible for an implementation to only
704need the address in order to perform the unmap operation.
705
706            Platform Issues
707
708If you are just writing drivers for Linux and do not maintain
709an architecture port for the kernel, you can safely skip down
710to "Closing".
711
7121) Struct scatterlist requirements.
713
714   Struct scatterlist must contain, at a minimum, the following
715   members:
716
717    struct page *page;
718    unsigned int offset;
719    unsigned int length;
720
721   The base address is specified by a "page+offset" pair.
722
723   Previous versions of struct scatterlist contained a "void *address"
724   field that was sometimes used instead of page+offset. As of Linux
725   2.5., page+offset is always used, and the "address" field has been
726   deleted.
727
7282) More to come...
729
730            Handling Errors
731
732DMA address space is limited on some architectures and an allocation
733failure can be determined by:
734
735- checking if pci_alloc_consistent returns NULL or pci_map_sg returns 0
736
737- checking the returned dma_addr_t of pci_map_single and pci_map_page
738  by using pci_dma_mapping_error():
739
740    dma_addr_t dma_handle;
741
742    dma_handle = pci_map_single(pdev, addr, size, direction);
743    if (pci_dma_mapping_error(pdev, dma_handle)) {
744        /*
745         * reduce current DMA mapping usage,
746         * delay and try again later or
747         * reset driver.
748         */
749    }
750
751               Closing
752
753This document, and the API itself, would not be in it's current
754form without the feedback and suggestions from numerous individuals.
755We would like to specifically mention, in no particular order, the
756following people:
757
758    Russell King <rmk@arm.linux.org.uk>
759    Leo Dagum <dagum@barrel.engr.sgi.com>
760    Ralf Baechle <ralf@oss.sgi.com>
761    Grant Grundler <grundler@cup.hp.com>
762    Jay Estabrook <Jay.Estabrook@compaq.com>
763    Thomas Sailer <sailer@ife.ee.ethz.ch>
764    Andrea Arcangeli <andrea@suse.de>
765    Jens Axboe <jens.axboe@oracle.com>
766    David Mosberger-Tang <davidm@hpl.hp.com>
767

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