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Date: Wed, 08 Jul 2009 12:30:46 +0200
From: Sebastian Huber <sebastian.huber@embedded-brains.de>
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Cc: Rafal Jaworowski <raj@semihalf.com>, freebsd-arm@freebsd.org,
	freebsd-usb@freebsd.org, thompsa@freebsd.org
Subject: Re: CPU Cache and busdma usage in USB
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Hi!

The general approach for systems with no hardware cache coherence protocol
should be:

Memory to device DMA transfer of a buffer:

1. write all modified cache lines of the buffer back to memory (cache flush for
buffer)
2. start DMA transfer

Device to memory DMA transfer of a buffer:

1. invalidate the cache of the buffer, here you have to make sure that this
buffer does not share a cache line with other buffers (!)
2. start DMA transfer
3. all accesses to the buffer via the cache must wait until the DMA has finished

CU

-- 
Sebastian Huber, Embedded Brains GmbH

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