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Based on kernel version 3.13. Page generated on 2014-01-20 22:02 EST.

1			     Dynamic DMA mapping Guide
2			     =========================
3	
4			 David S. Miller <davem@redhat.com>
5			 Richard Henderson <rth@cygnus.com>
6			  Jakub Jelinek <jakub@redhat.com>
7	
8	This is a guide to device driver writers on how to use the DMA API
9	with example pseudo-code.  For a concise description of the API, see
10	DMA-API.txt.
11	
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())).
22	
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.
27	
28	The following API will work of course even on platforms where no such
29	hardware exists.
30	
31	Note that the DMA API works with any bus independent of the underlying
32	microprocessor architecture. You should use the DMA API rather than
33	the bus specific DMA API (e.g. pci_dma_*).
34	
35	First of all, you should make sure
36	
37	#include <linux/dma-mapping.h>
38	
39	is in your driver. This file will obtain for you the definition of the
40	dma_addr_t (which can hold any valid DMA address for the platform)
41	type which should be used everywhere you hold a DMA (bus) address
42	returned from the DMA mapping functions.
43	
44				 What memory is DMA'able?
45	
46	The first piece of information you must know is what kernel memory can
47	be used with the DMA mapping facilities.  There has been an unwritten
48	set of rules regarding this, and this text is an attempt to finally
49	write them down.
50	
51	If you acquired your memory via the page allocator
52	(i.e. __get_free_page*()) or the generic memory allocators
53	(i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from
54	that memory using the addresses returned from those routines.
55	
56	This means specifically that you may _not_ use the memory/addresses
57	returned from vmalloc() for DMA.  It is possible to DMA to the
58	_underlying_ memory mapped into a vmalloc() area, but this requires
59	walking page tables to get the physical addresses, and then
60	translating each of those pages back to a kernel address using
61	something like __va().  [ EDIT: Update this when we integrate
62	Gerd Knorr's generic code which does this. ]
63	
64	This rule also means that you may use neither kernel image addresses
65	(items in data/text/bss segments), nor module image addresses, nor
66	stack addresses for DMA.  These could all be mapped somewhere entirely
67	different than the rest of physical memory.  Even if those classes of
68	memory could physically work with DMA, you'd need to ensure the I/O
69	buffers were cacheline-aligned.  Without that, you'd see cacheline
70	sharing problems (data corruption) on CPUs with DMA-incoherent caches.
71	(The CPU could write to one word, DMA would write to a different one
72	in the same cache line, and one of them could be overwritten.)
73	
74	Also, this means that you cannot take the return of a kmap()
75	call and DMA to/from that.  This is similar to vmalloc().
76	
77	What about block I/O and networking buffers?  The block I/O and
78	networking subsystems make sure that the buffers they use are valid
79	for you to DMA from/to.
80	
81				DMA addressing limitations
82	
83	Does your device have any DMA addressing limitations?  For example, is
84	your device only capable of driving the low order 24-bits of address?
85	If so, you need to inform the kernel of this fact.
86	
87	By default, the kernel assumes that your device can address the full
88	32-bits.  For a 64-bit capable device, this needs to be increased.
89	And for a device with limitations, as discussed in the previous
90	paragraph, it needs to be decreased.
91	
92	Special note about PCI: PCI-X specification requires PCI-X devices to
93	support 64-bit addressing (DAC) for all transactions.  And at least
94	one platform (SGI SN2) requires 64-bit consistent allocations to
95	operate correctly when the IO bus is in PCI-X mode.
96	
97	For correct operation, you must interrogate the kernel in your device
98	probe routine to see if the DMA controller on the machine can properly
99	support the DMA addressing limitation your device has.  It is good
100	style to do this even if your device holds the default setting,
101	because this shows that you did think about these issues wrt. your
102	device.
103	
104	The query is performed via a call to dma_set_mask_and_coherent():
105	
106		int dma_set_mask_and_coherent(struct device *dev, u64 mask);
107	
108	which will query the mask for both streaming and coherent APIs together.
109	If you have some special requirements, then the following two separate
110	queries can be used instead:
111	
112		The query for streaming mappings is performed via a call to
113		dma_set_mask():
114	
115			int dma_set_mask(struct device *dev, u64 mask);
116	
117		The query for consistent allocations is performed via a call
118		to dma_set_coherent_mask():
119	
120			int dma_set_coherent_mask(struct device *dev, u64 mask);
121	
122	Here, dev is a pointer to the device struct of your device, and mask
123	is a bit mask describing which bits of an address your device
124	supports.  It returns zero if your card can perform DMA properly on
125	the machine given the address mask you provided.  In general, the
126	device struct of your device is embedded in the bus specific device
127	struct of your device.  For example, a pointer to the device struct of
128	your PCI device is pdev->dev (pdev is a pointer to the PCI device
129	struct of your device).
130	
131	If it returns non-zero, your device cannot perform DMA properly on
132	this platform, and attempting to do so will result in undefined
133	behavior.  You must either use a different mask, or not use DMA.
134	
135	This means that in the failure case, you have three options:
136	
137	1) Use another DMA mask, if possible (see below).
138	2) Use some non-DMA mode for data transfer, if possible.
139	3) Ignore this device and do not initialize it.
140	
141	It is recommended that your driver print a kernel KERN_WARNING message
142	when you end up performing either #2 or #3.  In this manner, if a user
143	of your driver reports that performance is bad or that the device is not
144	even detected, you can ask them for the kernel messages to find out
145	exactly why.
146	
147	The standard 32-bit addressing device would do something like this:
148	
149		if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32))) {
150			printk(KERN_WARNING
151			       "mydev: No suitable DMA available.\n");
152			goto ignore_this_device;
153		}
154	
155	Another common scenario is a 64-bit capable device.  The approach here
156	is to try for 64-bit addressing, but back down to a 32-bit mask that
157	should not fail.  The kernel may fail the 64-bit mask not because the
158	platform is not capable of 64-bit addressing.  Rather, it may fail in
159	this case simply because 32-bit addressing is done more efficiently
160	than 64-bit addressing.  For example, Sparc64 PCI SAC addressing is
161	more efficient than DAC addressing.
162	
163	Here is how you would handle a 64-bit capable device which can drive
164	all 64-bits when accessing streaming DMA:
165	
166		int using_dac;
167	
168		if (!dma_set_mask(dev, DMA_BIT_MASK(64))) {
169			using_dac = 1;
170		} else if (!dma_set_mask(dev, DMA_BIT_MASK(32))) {
171			using_dac = 0;
172		} else {
173			printk(KERN_WARNING
174			       "mydev: No suitable DMA available.\n");
175			goto ignore_this_device;
176		}
177	
178	If a card is capable of using 64-bit consistent allocations as well,
179	the case would look like this:
180	
181		int using_dac, consistent_using_dac;
182	
183		if (!dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64))) {
184			using_dac = 1;
185		   	consistent_using_dac = 1;
186		} else if (!dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32))) {
187			using_dac = 0;
188			consistent_using_dac = 0;
189		} else {
190			printk(KERN_WARNING
191			       "mydev: No suitable DMA available.\n");
192			goto ignore_this_device;
193		}
194	
195	The coherent coherent mask will always be able to set the same or a
196	smaller mask as the streaming mask. However for the rare case that a
197	device driver only uses consistent allocations, one would have to
198	check the return value from dma_set_coherent_mask().
199	
200	Finally, if your device can only drive the low 24-bits of
201	address you might do something like:
202	
203		if (dma_set_mask(dev, DMA_BIT_MASK(24))) {
204			printk(KERN_WARNING
205			       "mydev: 24-bit DMA addressing not available.\n");
206			goto ignore_this_device;
207		}
208	
209	When dma_set_mask() or dma_set_mask_and_coherent() is successful, and
210	returns zero, the kernel saves away this mask you have provided.  The
211	kernel will use this information later when you make DMA mappings.
212	
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 dma_set_mask() be for the
220	most specific mask.
221	
222	Here is pseudo-code showing how this might be done:
223	
224		#define PLAYBACK_ADDRESS_BITS	DMA_BIT_MASK(32)
225		#define RECORD_ADDRESS_BITS	DMA_BIT_MASK(24)
226	
227		struct my_sound_card *card;
228		struct device *dev;
229	
230		...
231		if (!dma_set_mask(dev, PLAYBACK_ADDRESS_BITS)) {
232			card->playback_enabled = 1;
233		} else {
234			card->playback_enabled = 0;
235			printk(KERN_WARNING "%s: Playback disabled due to DMA limitations.\n",
236			       card->name);
237		}
238		if (!dma_set_mask(dev, RECORD_ADDRESS_BITS)) {
239			card->record_enabled = 1;
240		} else {
241			card->record_enabled = 0;
242			printk(KERN_WARNING "%s: Record disabled due to DMA limitations.\n",
243			       card->name);
244		}
245	
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.
249	
250				Types of DMA mappings
251	
252	There are two types of DMA mappings:
253	
254	- Consistent DMA mappings which are usually mapped at driver
255	  initialization, unmapped at the end and for which the hardware should
256	  guarantee that the device and the CPU can access the data
257	  in parallel and will see updates made by each other without any
258	  explicit software flushing.
259	
260	  Think of "consistent" as "synchronous" or "coherent".
261	
262	  The current default is to return consistent memory in the low 32
263	  bits of the bus space.  However, for future compatibility you should
264	  set the consistent mask even if this default is fine for your
265	  driver.
266	
267	  Good examples of what to use consistent mappings for are:
268	
269		- Network card DMA ring descriptors.
270		- SCSI adapter mailbox command data structures.
271		- Device firmware microcode executed out of
272		  main memory.
273	
274	  The invariant these examples all require is that any CPU store
275	  to memory is immediately visible to the device, and vice
276	  versa.  Consistent mappings guarantee this.
277	
278	  IMPORTANT: Consistent DMA memory does not preclude the usage of
279	             proper memory barriers.  The CPU may reorder stores to
280		     consistent memory just as it may normal memory.  Example:
281		     if it is important for the device to see the first word
282		     of a descriptor updated before the second, you must do
283		     something like:
284	
285			desc->word0 = address;
286			wmb();
287			desc->word1 = DESC_VALID;
288	
289	             in order to get correct behavior on all platforms.
290	
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).
295	
296	- Streaming DMA mappings which are usually mapped for one DMA
297	  transfer, unmapped right after it (unless you use dma_sync_* below)
298	  and for which hardware can optimize for sequential accesses.
299	
300	  This of "streaming" as "asynchronous" or "outside the coherency
301	  domain".
302	
303	  Good examples of what to use streaming mappings for are:
304	
305		- Networking buffers transmitted/received by a device.
306		- Filesystem buffers written/read by a SCSI device.
307	
308	  The interfaces for using this type of mapping were designed in
309	  such a way that an implementation can make whatever performance
310	  optimizations the hardware allows.  To this end, when using
311	  such mappings you must be explicit about what you want to happen.
312	
313	Neither type of DMA mapping has alignment restrictions that come from
314	the underlying bus, 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.
317	
318	
319			 Using Consistent DMA mappings.
320	
321	To allocate and map large (PAGE_SIZE or so) consistent DMA regions,
322	you should do:
323	
324		dma_addr_t dma_handle;
325	
326		cpu_addr = dma_alloc_coherent(dev, size, &dma_handle, gfp);
327	
328	where device is a struct device *. This may be called in interrupt
329	context with the GFP_ATOMIC flag.
330	
331	Size is the length of the region you want to allocate, in bytes.
332	
333	This routine will allocate RAM for that region, so it acts similarly to
334	__get_free_pages (but takes size instead of a page order).  If your
335	driver needs regions sized smaller than a page, you may prefer using
336	the dma_pool interface, described below.
337	
338	The consistent DMA mapping interfaces, for non-NULL dev, will by
339	default return a DMA address which is 32-bit addressable.  Even if the
340	device indicates (via DMA mask) that it may address the upper 32-bits,
341	consistent allocation will only return > 32-bit addresses for DMA if
342	the consistent DMA mask has been explicitly changed via
343	dma_set_coherent_mask().  This is true of the dma_pool interface as
344	well.
345	
346	dma_alloc_coherent returns two values: the virtual address which you
347	can use to access it from the CPU and dma_handle which you pass to the
348	card.
349	
350	The cpu return address and the DMA bus master address are both
351	guaranteed to be aligned to the smallest PAGE_SIZE order which
352	is greater than or equal to the requested size.  This invariant
353	exists (for example) to guarantee that if you allocate a chunk
354	which is smaller than or equal to 64 kilobytes, the extent of the
355	buffer you receive will not cross a 64K boundary.
356	
357	To unmap and free such a DMA region, you call:
358	
359		dma_free_coherent(dev, size, cpu_addr, dma_handle);
360	
361	where dev, size are the same as in the above call and cpu_addr and
362	dma_handle are the values dma_alloc_coherent returned to you.
363	This function may not be called in interrupt context.
364	
365	If your driver needs lots of smaller memory regions, you can write
366	custom code to subdivide pages returned by dma_alloc_coherent,
367	or you can use the dma_pool API to do that.  A dma_pool is like
368	a kmem_cache, but it uses dma_alloc_coherent not __get_free_pages.
369	Also, it understands common hardware constraints for alignment,
370	like queue heads needing to be aligned on N byte boundaries.
371	
372	Create a dma_pool like this:
373	
374		struct dma_pool *pool;
375	
376		pool = dma_pool_create(name, dev, size, align, alloc);
377	
378	The "name" is for diagnostics (like a kmem_cache name); dev and size
379	are as above.  The device's hardware alignment requirement for this
380	type of data is "align" (which is expressed in bytes, and must be a
381	power of two).  If your device has no boundary crossing restrictions,
382	pass 0 for alloc; passing 4096 says memory allocated from this pool
383	must not cross 4KByte boundaries (but at that time it may be better to
384	go for dma_alloc_coherent directly instead).
385	
386	Allocate memory from a dma pool like this:
387	
388		cpu_addr = dma_pool_alloc(pool, flags, &dma_handle);
389	
390	flags are SLAB_KERNEL if blocking is permitted (not in_interrupt nor
391	holding SMP locks), SLAB_ATOMIC otherwise.  Like dma_alloc_coherent,
392	this returns two values, cpu_addr and dma_handle.
393	
394	Free memory that was allocated from a dma_pool like this:
395	
396		dma_pool_free(pool, cpu_addr, dma_handle);
397	
398	where pool is what you passed to dma_pool_alloc, and cpu_addr and
399	dma_handle are the values dma_pool_alloc returned. This function
400	may be called in interrupt context.
401	
402	Destroy a dma_pool by calling:
403	
404		dma_pool_destroy(pool);
405	
406	Make sure you've called dma_pool_free for all memory allocated
407	from a pool before you destroy the pool. This function may not
408	be called in interrupt context.
409	
410				DMA Direction
411	
412	The interfaces described in subsequent portions of this document
413	take a DMA direction argument, which is an integer and takes on
414	one of the following values:
415	
416	 DMA_BIDIRECTIONAL
417	 DMA_TO_DEVICE
418	 DMA_FROM_DEVICE
419	 DMA_NONE
420	
421	One should provide the exact DMA direction if you know it.
422	
423	DMA_TO_DEVICE means "from main memory to the device"
424	DMA_FROM_DEVICE means "from the device to main memory"
425	It is the direction in which the data moves during the DMA
426	transfer.
427	
428	You are _strongly_ encouraged to specify this as precisely
429	as you possibly can.
430	
431	If you absolutely cannot know the direction of the DMA transfer,
432	specify DMA_BIDIRECTIONAL.  It means that the DMA can go in
433	either direction.  The platform guarantees that you may legally
434	specify this, and that it will work, but this may be at the
435	cost of performance for example.
436	
437	The value DMA_NONE is to be used for debugging.  One can
438	hold this in a data structure before you come to know the
439	precise direction, and this will help catch cases where your
440	direction tracking logic has failed to set things up properly.
441	
442	Another advantage of specifying this value precisely (outside of
443	potential platform-specific optimizations of such) is for debugging.
444	Some platforms actually have a write permission boolean which DMA
445	mappings can be marked with, much like page protections in the user
446	program address space.  Such platforms can and do report errors in the
447	kernel logs when the DMA controller hardware detects violation of the
448	permission setting.
449	
450	Only streaming mappings specify a direction, consistent mappings
451	implicitly have a direction attribute setting of
452	DMA_BIDIRECTIONAL.
453	
454	The SCSI subsystem tells you the direction to use in the
455	'sc_data_direction' member of the SCSI command your driver is
456	working on.
457	
458	For Networking drivers, it's a rather simple affair.  For transmit
459	packets, map/unmap them with the DMA_TO_DEVICE direction
460	specifier.  For receive packets, just the opposite, map/unmap them
461	with the DMA_FROM_DEVICE direction specifier.
462	
463			  Using Streaming DMA mappings
464	
465	The streaming DMA mapping routines can be called from interrupt
466	context.  There are two versions of each map/unmap, one which will
467	map/unmap a single memory region, and one which will map/unmap a
468	scatterlist.
469	
470	To map a single region, you do:
471	
472		struct device *dev = &my_dev->dev;
473		dma_addr_t dma_handle;
474		void *addr = buffer->ptr;
475		size_t size = buffer->len;
476	
477		dma_handle = dma_map_single(dev, addr, size, direction);
478		if (dma_mapping_error(dma_handle)) {
479			/*
480			 * reduce current DMA mapping usage,
481			 * delay and try again later or
482			 * reset driver.
483			 */
484			goto map_error_handling;
485		}
486	
487	and to unmap it:
488	
489		dma_unmap_single(dev, dma_handle, size, direction);
490	
491	You should call dma_mapping_error() as dma_map_single() could fail and return
492	error. Not all dma implementations support dma_mapping_error() interface.
493	However, it is a good practice to call dma_mapping_error() interface, which
494	will invoke the generic mapping error check interface. Doing so will ensure
495	that the mapping code will work correctly on all dma implementations without
496	any dependency on the specifics of the underlying implementation. Using the
497	returned address without checking for errors could result in failures ranging
498	from panics to silent data corruption. A couple of examples of incorrect ways
499	to check for errors that make assumptions about the underlying dma
500	implementation are as follows and these are applicable to dma_map_page() as
501	well.
502	
503	Incorrect example 1:
504		dma_addr_t dma_handle;
505	
506		dma_handle = dma_map_single(dev, addr, size, direction);
507		if ((dma_handle & 0xffff != 0) || (dma_handle >= 0x1000000)) {
508			goto map_error;
509		}
510	
511	Incorrect example 2:
512		dma_addr_t dma_handle;
513	
514		dma_handle = dma_map_single(dev, addr, size, direction);
515		if (dma_handle == DMA_ERROR_CODE) {
516			goto map_error;
517		}
518	
519	You should call dma_unmap_single when the DMA activity is finished, e.g.
520	from the interrupt which told you that the DMA transfer is done.
521	
522	Using cpu pointers like this for single mappings has a disadvantage,
523	you cannot reference HIGHMEM memory in this way.  Thus, there is a
524	map/unmap interface pair akin to dma_{map,unmap}_single.  These
525	interfaces deal with page/offset pairs instead of cpu pointers.
526	Specifically:
527	
528		struct device *dev = &my_dev->dev;
529		dma_addr_t dma_handle;
530		struct page *page = buffer->page;
531		unsigned long offset = buffer->offset;
532		size_t size = buffer->len;
533	
534		dma_handle = dma_map_page(dev, page, offset, size, direction);
535		if (dma_mapping_error(dma_handle)) {
536			/*
537			 * reduce current DMA mapping usage,
538			 * delay and try again later or
539			 * reset driver.
540			 */
541			goto map_error_handling;
542		}
543	
544		...
545	
546		dma_unmap_page(dev, dma_handle, size, direction);
547	
548	Here, "offset" means byte offset within the given page.
549	
550	You should call dma_mapping_error() as dma_map_page() could fail and return
551	error as outlined under the dma_map_single() discussion.
552	
553	You should call dma_unmap_page when the DMA activity is finished, e.g.
554	from the interrupt which told you that the DMA transfer is done.
555	
556	With scatterlists, you map a region gathered from several regions by:
557	
558		int i, count = dma_map_sg(dev, sglist, nents, direction);
559		struct scatterlist *sg;
560	
561		for_each_sg(sglist, sg, count, i) {
562			hw_address[i] = sg_dma_address(sg);
563			hw_len[i] = sg_dma_len(sg);
564		}
565	
566	where nents is the number of entries in the sglist.
567	
568	The implementation is free to merge several consecutive sglist entries
569	into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
570	consecutive sglist entries can be merged into one provided the first one
571	ends and the second one starts on a page boundary - in fact this is a huge
572	advantage for cards which either cannot do scatter-gather or have very
573	limited number of scatter-gather entries) and returns the actual number
574	of sg entries it mapped them to. On failure 0 is returned.
575	
576	Then you should loop count times (note: this can be less than nents times)
577	and use sg_dma_address() and sg_dma_len() macros where you previously
578	accessed sg->address and sg->length as shown above.
579	
580	To unmap a scatterlist, just call:
581	
582		dma_unmap_sg(dev, sglist, nents, direction);
583	
584	Again, make sure DMA activity has already finished.
585	
586	PLEASE NOTE:  The 'nents' argument to the dma_unmap_sg call must be
587	              the _same_ one you passed into the dma_map_sg call,
588		      it should _NOT_ be the 'count' value _returned_ from the
589	              dma_map_sg call.
590	
591	Every dma_map_{single,sg} call should have its dma_unmap_{single,sg}
592	counterpart, because the bus address space is a shared resource (although
593	in some ports the mapping is per each BUS so less devices contend for the
594	same bus address space) and you could render the machine unusable by eating
595	all bus addresses.
596	
597	If you need to use the same streaming DMA region multiple times and touch
598	the data in between the DMA transfers, the buffer needs to be synced
599	properly in order for the cpu and device to see the most uptodate and
600	correct copy of the DMA buffer.
601	
602	So, firstly, just map it with dma_map_{single,sg}, and after each DMA
603	transfer call either:
604	
605		dma_sync_single_for_cpu(dev, dma_handle, size, direction);
606	
607	or:
608	
609		dma_sync_sg_for_cpu(dev, sglist, nents, direction);
610	
611	as appropriate.
612	
613	Then, if you wish to let the device get at the DMA area again,
614	finish accessing the data with the cpu, and then before actually
615	giving the buffer to the hardware call either:
616	
617		dma_sync_single_for_device(dev, dma_handle, size, direction);
618	
619	or:
620	
621		dma_sync_sg_for_device(dev, sglist, nents, direction);
622	
623	as appropriate.
624	
625	After the last DMA transfer call one of the DMA unmap routines
626	dma_unmap_{single,sg}. If you don't touch the data from the first dma_map_*
627	call till dma_unmap_*, then you don't have to call the dma_sync_*
628	routines at all.
629	
630	Here is pseudo code which shows a situation in which you would need
631	to use the dma_sync_*() interfaces.
632	
633		my_card_setup_receive_buffer(struct my_card *cp, char *buffer, int len)
634		{
635			dma_addr_t mapping;
636	
637			mapping = dma_map_single(cp->dev, buffer, len, DMA_FROM_DEVICE);
638			if (dma_mapping_error(dma_handle)) {
639				/*
640				 * reduce current DMA mapping usage,
641				 * delay and try again later or
642				 * reset driver.
643				 */
644				goto map_error_handling;
645			}
646	
647			cp->rx_buf = buffer;
648			cp->rx_len = len;
649			cp->rx_dma = mapping;
650	
651			give_rx_buf_to_card(cp);
652		}
653	
654		...
655	
656		my_card_interrupt_handler(int irq, void *devid, struct pt_regs *regs)
657		{
658			struct my_card *cp = devid;
659	
660			...
661			if (read_card_status(cp) == RX_BUF_TRANSFERRED) {
662				struct my_card_header *hp;
663	
664				/* Examine the header to see if we wish
665				 * to accept the data.  But synchronize
666				 * the DMA transfer with the CPU first
667				 * so that we see updated contents.
668				 */
669				dma_sync_single_for_cpu(&cp->dev, cp->rx_dma,
670							cp->rx_len,
671							DMA_FROM_DEVICE);
672	
673				/* Now it is safe to examine the buffer. */
674				hp = (struct my_card_header *) cp->rx_buf;
675				if (header_is_ok(hp)) {
676					dma_unmap_single(&cp->dev, cp->rx_dma, cp->rx_len,
677							 DMA_FROM_DEVICE);
678					pass_to_upper_layers(cp->rx_buf);
679					make_and_setup_new_rx_buf(cp);
680				} else {
681					/* CPU should not write to
682					 * DMA_FROM_DEVICE-mapped area,
683					 * so dma_sync_single_for_device() is
684					 * not needed here. It would be required
685					 * for DMA_BIDIRECTIONAL mapping if
686					 * the memory was modified.
687					 */
688					give_rx_buf_to_card(cp);
689				}
690			}
691		}
692	
693	Drivers converted fully to this interface should not use virt_to_bus any
694	longer, nor should they use bus_to_virt. Some drivers have to be changed a
695	little bit, because there is no longer an equivalent to bus_to_virt in the
696	dynamic DMA mapping scheme - you have to always store the DMA addresses
697	returned by the dma_alloc_coherent, dma_pool_alloc, and dma_map_single
698	calls (dma_map_sg stores them in the scatterlist itself if the platform
699	supports dynamic DMA mapping in hardware) in your driver structures and/or
700	in the card registers.
701	
702	All drivers should be using these interfaces with no exceptions.  It
703	is planned to completely remove virt_to_bus() and bus_to_virt() as
704	they are entirely deprecated.  Some ports already do not provide these
705	as it is impossible to correctly support them.
706	
707				Handling Errors
708	
709	DMA address space is limited on some architectures and an allocation
710	failure can be determined by:
711	
712	- checking if dma_alloc_coherent returns NULL or dma_map_sg returns 0
713	
714	- checking the returned dma_addr_t of dma_map_single and dma_map_page
715	  by using dma_mapping_error():
716	
717		dma_addr_t dma_handle;
718	
719		dma_handle = dma_map_single(dev, addr, size, direction);
720		if (dma_mapping_error(dev, dma_handle)) {
721			/*
722			 * reduce current DMA mapping usage,
723			 * delay and try again later or
724			 * reset driver.
725			 */
726			goto map_error_handling;
727		}
728	
729	- unmap pages that are already mapped, when mapping error occurs in the middle
730	  of a multiple page mapping attempt. These example are applicable to
731	  dma_map_page() as well.
732	
733	Example 1:
734		dma_addr_t dma_handle1;
735		dma_addr_t dma_handle2;
736	
737		dma_handle1 = dma_map_single(dev, addr, size, direction);
738		if (dma_mapping_error(dev, dma_handle1)) {
739			/*
740			 * reduce current DMA mapping usage,
741			 * delay and try again later or
742			 * reset driver.
743			 */
744			goto map_error_handling1;
745		}
746		dma_handle2 = dma_map_single(dev, addr, size, direction);
747		if (dma_mapping_error(dev, dma_handle2)) {
748			/*
749			 * reduce current DMA mapping usage,
750			 * delay and try again later or
751			 * reset driver.
752			 */
753			goto map_error_handling2;
754		}
755	
756		...
757	
758		map_error_handling2:
759			dma_unmap_single(dma_handle1);
760		map_error_handling1:
761	
762	Example 2: (if buffers are allocated in a loop, unmap all mapped buffers when
763		    mapping error is detected in the middle)
764	
765		dma_addr_t dma_addr;
766		dma_addr_t array[DMA_BUFFERS];
767		int save_index = 0;
768	
769		for (i = 0; i < DMA_BUFFERS; i++) {
770	
771			...
772	
773			dma_addr = dma_map_single(dev, addr, size, direction);
774			if (dma_mapping_error(dev, dma_addr)) {
775				/*
776				 * reduce current DMA mapping usage,
777				 * delay and try again later or
778				 * reset driver.
779				 */
780				goto map_error_handling;
781			}
782			array[i].dma_addr = dma_addr;
783			save_index++;
784		}
785	
786		...
787	
788		map_error_handling:
789	
790		for (i = 0; i < save_index; i++) {
791	
792			...
793	
794			dma_unmap_single(array[i].dma_addr);
795		}
796	
797	Networking drivers must call dev_kfree_skb to free the socket buffer
798	and return NETDEV_TX_OK if the DMA mapping fails on the transmit hook
799	(ndo_start_xmit). This means that the socket buffer is just dropped in
800	the failure case.
801	
802	SCSI drivers must return SCSI_MLQUEUE_HOST_BUSY if the DMA mapping
803	fails in the queuecommand hook. This means that the SCSI subsystem
804	passes the command to the driver again later.
805	
806			Optimizing Unmap State Space Consumption
807	
808	On many platforms, dma_unmap_{single,page}() is simply a nop.
809	Therefore, keeping track of the mapping address and length is a waste
810	of space.  Instead of filling your drivers up with ifdefs and the like
811	to "work around" this (which would defeat the whole purpose of a
812	portable API) the following facilities are provided.
813	
814	Actually, instead of describing the macros one by one, we'll
815	transform some example code.
816	
817	1) Use DEFINE_DMA_UNMAP_{ADDR,LEN} in state saving structures.
818	   Example, before:
819	
820		struct ring_state {
821			struct sk_buff *skb;
822			dma_addr_t mapping;
823			__u32 len;
824		};
825	
826	   after:
827	
828		struct ring_state {
829			struct sk_buff *skb;
830			DEFINE_DMA_UNMAP_ADDR(mapping);
831			DEFINE_DMA_UNMAP_LEN(len);
832		};
833	
834	2) Use dma_unmap_{addr,len}_set to set these values.
835	   Example, before:
836	
837		ringp->mapping = FOO;
838		ringp->len = BAR;
839	
840	   after:
841	
842		dma_unmap_addr_set(ringp, mapping, FOO);
843		dma_unmap_len_set(ringp, len, BAR);
844	
845	3) Use dma_unmap_{addr,len} to access these values.
846	   Example, before:
847	
848		dma_unmap_single(dev, ringp->mapping, ringp->len,
849				 DMA_FROM_DEVICE);
850	
851	   after:
852	
853		dma_unmap_single(dev,
854				 dma_unmap_addr(ringp, mapping),
855				 dma_unmap_len(ringp, len),
856				 DMA_FROM_DEVICE);
857	
858	It really should be self-explanatory.  We treat the ADDR and LEN
859	separately, because it is possible for an implementation to only
860	need the address in order to perform the unmap operation.
861	
862				Platform Issues
863	
864	If you are just writing drivers for Linux and do not maintain
865	an architecture port for the kernel, you can safely skip down
866	to "Closing".
867	
868	1) Struct scatterlist requirements.
869	
870	   Don't invent the architecture specific struct scatterlist; just use
871	   <asm-generic/scatterlist.h>. You need to enable
872	   CONFIG_NEED_SG_DMA_LENGTH if the architecture supports IOMMUs
873	   (including software IOMMU).
874	
875	2) ARCH_DMA_MINALIGN
876	
877	   Architectures must ensure that kmalloc'ed buffer is
878	   DMA-safe. Drivers and subsystems depend on it. If an architecture
879	   isn't fully DMA-coherent (i.e. hardware doesn't ensure that data in
880	   the CPU cache is identical to data in main memory),
881	   ARCH_DMA_MINALIGN must be set so that the memory allocator
882	   makes sure that kmalloc'ed buffer doesn't share a cache line with
883	   the others. See arch/arm/include/asm/cache.h as an example.
884	
885	   Note that ARCH_DMA_MINALIGN is about DMA memory alignment
886	   constraints. You don't need to worry about the architecture data
887	   alignment constraints (e.g. the alignment constraints about 64-bit
888	   objects).
889	
890	3) Supporting multiple types of IOMMUs
891	
892	   If your architecture needs to support multiple types of IOMMUs, you
893	   can use include/linux/asm-generic/dma-mapping-common.h. It's a
894	   library to support the DMA API with multiple types of IOMMUs. Lots
895	   of architectures (x86, powerpc, sh, alpha, ia64, microblaze and
896	   sparc) use it. Choose one to see how it can be used. If you need to
897	   support multiple types of IOMMUs in a single system, the example of
898	   x86 or powerpc helps.
899	
900				   Closing
901	
902	This document, and the API itself, would not be in its current
903	form without the feedback and suggestions from numerous individuals.
904	We would like to specifically mention, in no particular order, the
905	following people:
906	
907		Russell King <rmk@arm.linux.org.uk>
908		Leo Dagum <dagum@barrel.engr.sgi.com>
909		Ralf Baechle <ralf@oss.sgi.com>
910		Grant Grundler <grundler@cup.hp.com>
911		Jay Estabrook <Jay.Estabrook@compaq.com>
912		Thomas Sailer <sailer@ife.ee.ethz.ch>
913		Andrea Arcangeli <andrea@suse.de>
914		Jens Axboe <jens.axboe@oracle.com>
915		David Mosberger-Tang <davidm@hpl.hp.com>
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