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Documentation / DMA-ISA-LPC.txt




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

1	                        DMA with ISA and LPC devices
2	                        ============================
3	
4	                      Pierre Ossman <drzeus@drzeus.cx>
5	
6	This document describes how to do DMA transfers using the old ISA DMA
7	controller. Even though ISA is more or less dead today the LPC bus
8	uses the same DMA system so it will be around for quite some time.
9	
10	Part I - Headers and dependencies
11	---------------------------------
12	
13	To do ISA style DMA you need to include two headers:
14	
15	#include <linux/dma-mapping.h>
16	#include <asm/dma.h>
17	
18	The first is the generic DMA API used to convert virtual addresses to
19	physical addresses (see Documentation/DMA-API.txt for details).
20	
21	The second contains the routines specific to ISA DMA transfers. Since
22	this is not present on all platforms make sure you construct your
23	Kconfig to be dependent on ISA_DMA_API (not ISA) so that nobody tries
24	to build your driver on unsupported platforms.
25	
26	Part II - Buffer allocation
27	---------------------------
28	
29	The ISA DMA controller has some very strict requirements on which
30	memory it can access so extra care must be taken when allocating
31	buffers.
32	
33	(You usually need a special buffer for DMA transfers instead of
34	transferring directly to and from your normal data structures.)
35	
36	The DMA-able address space is the lowest 16 MB of _physical_ memory.
37	Also the transfer block may not cross page boundaries (which are 64
38	or 128 KiB depending on which channel you use).
39	
40	In order to allocate a piece of memory that satisfies all these
41	requirements you pass the flag GFP_DMA to kmalloc.
42	
43	Unfortunately the memory available for ISA DMA is scarce so unless you
44	allocate the memory during boot-up it's a good idea to also pass
45	__GFP_REPEAT and __GFP_NOWARN to make the allocater try a bit harder.
46	
47	(This scarcity also means that you should allocate the buffer as
48	early as possible and not release it until the driver is unloaded.)
49	
50	Part III - Address translation
51	------------------------------
52	
53	To translate the virtual address to a physical use the normal DMA
54	API. Do _not_ use isa_virt_to_phys() even though it does the same
55	thing. The reason for this is that the function isa_virt_to_phys()
56	will require a Kconfig dependency to ISA, not just ISA_DMA_API which
57	is really all you need. Remember that even though the DMA controller
58	has its origins in ISA it is used elsewhere.
59	
60	Note: x86_64 had a broken DMA API when it came to ISA but has since
61	been fixed. If your arch has problems then fix the DMA API instead of
62	reverting to the ISA functions.
63	
64	Part IV - Channels
65	------------------
66	
67	A normal ISA DMA controller has 8 channels. The lower four are for
68	8-bit transfers and the upper four are for 16-bit transfers.
69	
70	(Actually the DMA controller is really two separate controllers where
71	channel 4 is used to give DMA access for the second controller (0-3).
72	This means that of the four 16-bits channels only three are usable.)
73	
74	You allocate these in a similar fashion as all basic resources:
75	
76	extern int request_dma(unsigned int dmanr, const char * device_id);
77	extern void free_dma(unsigned int dmanr);
78	
79	The ability to use 16-bit or 8-bit transfers is _not_ up to you as a
80	driver author but depends on what the hardware supports. Check your
81	specs or test different channels.
82	
83	Part V - Transfer data
84	----------------------
85	
86	Now for the good stuff, the actual DMA transfer. :)
87	
88	Before you use any ISA DMA routines you need to claim the DMA lock
89	using claim_dma_lock(). The reason is that some DMA operations are
90	not atomic so only one driver may fiddle with the registers at a
91	time.
92	
93	The first time you use the DMA controller you should call
94	clear_dma_ff(). This clears an internal register in the DMA
95	controller that is used for the non-atomic operations. As long as you
96	(and everyone else) uses the locking functions then you only need to
97	reset this once.
98	
99	Next, you tell the controller in which direction you intend to do the
100	transfer using set_dma_mode(). Currently you have the options
101	DMA_MODE_READ and DMA_MODE_WRITE.
102	
103	Set the address from where the transfer should start (this needs to
104	be 16-bit aligned for 16-bit transfers) and how many bytes to
105	transfer. Note that it's _bytes_. The DMA routines will do all the
106	required translation to values that the DMA controller understands.
107	
108	The final step is enabling the DMA channel and releasing the DMA
109	lock.
110	
111	Once the DMA transfer is finished (or timed out) you should disable
112	the channel again. You should also check get_dma_residue() to make
113	sure that all data has been transferred.
114	
115	Example:
116	
117	int flags, residue;
118	
119	flags = claim_dma_lock();
120	
121	clear_dma_ff();
122	
123	set_dma_mode(channel, DMA_MODE_WRITE);
124	set_dma_addr(channel, phys_addr);
125	set_dma_count(channel, num_bytes);
126	
127	dma_enable(channel);
128	
129	release_dma_lock(flags);
130	
131	while (!device_done());
132	
133	flags = claim_dma_lock();
134	
135	dma_disable(channel);
136	
137	residue = dma_get_residue(channel);
138	if (residue != 0)
139		printk(KERN_ERR "driver: Incomplete DMA transfer!"
140			" %d bytes left!\n", residue);
141	
142	release_dma_lock(flags);
143	
144	Part VI - Suspend/resume
145	------------------------
146	
147	It is the driver's responsibility to make sure that the machine isn't
148	suspended while a DMA transfer is in progress. Also, all DMA settings
149	are lost when the system suspends so if your driver relies on the DMA
150	controller being in a certain state then you have to restore these
151	registers upon resume.
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