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Documentation / DMA-ISA-LPC.txt

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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

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