[PATCH] ISA DMA API documentation

                        DMA with ISA and LPC devices

                        ============================

                      Pierre Ossman <drzeus@drzeus.cx>

This document describes how to do DMA transfers using the old ISA DMA

controller. Even though ISA is more or less dead today the LPC bus

uses the same DMA system so it will be around for quite some time.

Part I - Headers and dependencies

---------------------------------

To do ISA style DMA you need to include two headers:

#include <linux/dma-mapping.h>

#include <asm/dma.h>

The first is the generic DMA API used to convert virtual addresses to

physical addresses (see Documentation/DMA-API.txt for details).

The second contains the routines specific to ISA DMA transfers. Since

this is not present on all platforms make sure you construct your

Kconfig to be dependent on ISA_DMA_API (not ISA) so that nobody tries

to build your driver on unsupported platforms.

Part II - Buffer allocation

---------------------------

The ISA DMA controller has some very strict requirements on which

memory it can access so extra care must be taken when allocating

buffers.

(You usually need a special buffer for DMA transfers instead of

transferring directly to and from your normal data structures.)

The DMA-able address space is the lowest 16 MB of _physical_ memory.

Also the transfer block may not cross page boundaries (which are 64

or 128 KiB depending on which channel you use).

In order to allocate a piece of memory that satisfies all these

requirements you pass the flag GFP_DMA to kmalloc.

Unfortunately the memory available for ISA DMA is scarce so unless you

allocate the memory during boot-up it's a good idea to also pass

__GFP_REPEAT and __GFP_NOWARN to make the allocater try a bit harder.

(This scarcity also means that you should allocate the buffer as

early as possible and not release it until the driver is unloaded.)

Part III - Address translation

------------------------------

To translate the virtual address to a physical use the normal DMA

API. Do _not_ use isa_virt_to_phys() even though it does the same

thing. The reason for this is that the function isa_virt_to_phys()

will require a Kconfig dependency to ISA, not just ISA_DMA_API which

is really all you need. Remember that even though the DMA controller

has its origins in ISA it is used elsewhere.

Note: x86_64 had a broken DMA API when it came to ISA but has since

been fixed. If your arch has problems then fix the DMA API instead of

reverting to the ISA functions.

Part IV - Channels

------------------

A normal ISA DMA controller has 8 channels. The lower four are for

8-bit transfers and the upper four are for 16-bit transfers.

(Actually the DMA controller is really two separate controllers where

channel 4 is used to give DMA access for the second controller (0-3).

This means that of the four 16-bits channels only three are usable.)

You allocate these in a similar fashion as all basic resources:

extern int request_dma(unsigned int dmanr, const char * device_id);

extern void free_dma(unsigned int dmanr);

The ability to use 16-bit or 8-bit transfers is _not_ up to you as a

driver author but depends on what the hardware supports. Check your

specs or test different channels.

Part V - Transfer data

----------------------

Now for the good stuff, the actual DMA transfer. :)

Before you use any ISA DMA routines you need to claim the DMA lock

using claim_dma_lock(). The reason is that some DMA operations are

not atomic so only one driver may fiddle with the registers at a

time.

The first time you use the DMA controller you should call

clear_dma_ff(). This clears an internal register in the DMA

controller that is used for the non-atomic operations. As long as you

(and everyone else) uses the locking functions then you only need to

reset this once.

Next, you tell the controller in which direction you intend to do the

transfer using set_dma_mode(). Currently you have the options

DMA_MODE_READ and DMA_MODE_WRITE.

Set the address from where the transfer should start (this needs to

be 16-bit aligned for 16-bit transfers) and how many bytes to

transfer. Note that it's _bytes_. The DMA routines will do all the

required translation to values that the DMA controller understands.

The final step is enabling the DMA channel and releasing the DMA

lock.

Once the DMA transfer is finished (or timed out) you should disable

the channel again. You should also check get_dma_residue() to make

sure that all data has been transfered.

Example:

int flags, residue;

flags = claim_dma_lock();

clear_dma_ff();

set_dma_mode(channel, DMA_MODE_WRITE);

set_dma_addr(channel, phys_addr);

set_dma_count(channel, num_bytes);

dma_enable(channel);

release_dma_lock(flags);

while (!device_done());

flags = claim_dma_lock();

dma_disable(channel);

residue = dma_get_residue(channel);

if (residue != 0)

	printk(KERN_ERR "driver: Incomplete DMA transfer!"

		" %d bytes left!\n", residue);

release_dma_lock(flags);

Part VI - Suspend/resume

------------------------

It is the driver's responsibility to make sure that the machine isn't

suspended while a DMA transfer is in progress. Also, all DMA settings

are lost when the system suspends so if your driver relies on the DMA

controller being in a certain state then you have to restore these

registers upon resume.