linux-toradex.git/Documentation/dmaengine.txt, branch v2.6.32.41 Linux kernel for Apalis and Colibri modules async_tx, dmaengine: document channel allocation and api rework 2009-01-06T01:10:19+00:00 Dan Williams dan.j.williams@intel.com 2009-01-06T00:14:31+00:00 28405d8d9ce05f5bd869ef8b48da5086f9527d73 "Wouldn't it be better if the dmaengine layer made sure it didn't pass the same channel several times to a client? I mean, you seem concerned that the memcpy() API should be transparent and easy to use, but the whole registration interface is just ridiculously complicated..." - Haavard The dmaengine and async_tx registration/allocation interface is indeed needlessly complicated. This redesign has the following goals: 1/ Simplify reference counting: dma channels are not something one would expect to be hotplugged, it should be an exceptional event handled by drivers not something clients should be mandated to handle in a callback. The common case channel removal event is 'rmmod <dma driver>', which for simplicity should be disallowed if the channel is in use. 2/ Add an interface for requesting exclusive access to a channel suitable to device-to-memory users. 3/ Convert all memory-to-memory users over to a common allocator, the goal here is to not have competing channel allocation schemes. The only competition should be between device-to-memory exclusive allocations and the memory-to-memory usage case where channels are shared between multiple "clients". Cc: Haavard Skinnemoen <haavard.skinnemoen@atmel.com> Cc: Neil Brown <neilb@suse.de> Cc: Jeff Garzik <jeff@garzik.org> Reviewed-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Dan Williams <dan.j.williams@intel.com> 
"Wouldn't it be better if the dmaengine layer made sure it didn't pass
the same channel several times to a client?

I mean, you seem concerned that the memcpy() API should be transparent
and easy to use, but the whole registration interface is just
ridiculously complicated..."
	- Haavard

The dmaengine and async_tx registration/allocation interface is indeed
needlessly complicated.  This redesign has the following goals:

1/ Simplify reference counting: dma channels are not something one would
   expect to be hotplugged, it should be an exceptional event handled by
   drivers not something clients should be mandated to handle in a
   callback.  The common case channel removal event is 'rmmod <dma driver>',
   which for simplicity should be disallowed if the channel is in use.
2/ Add an interface for requesting exclusive access to a channel
   suitable to device-to-memory users.
3/ Convert all memory-to-memory users over to a common allocator, the goal
   here is to not have competing channel allocation schemes.  The only
   competition should be between device-to-memory exclusive allocations and
   the memory-to-memory usage case where channels are shared between
   multiple "clients".

Cc: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Cc: Neil Brown <neilb@suse.de>
Cc: Jeff Garzik <jeff@garzik.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>