linux-kernel - Re: [PATCH][RFC] asm-generic:remove calling flush_write_buffers() in dma_sync_*_for

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Date:	Tue, 7 Jul 2009 18:36:05 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Ming Lei <tom.leiming@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Joerg Roedel <joerg.roedel@....com>,
	fujita.tomonori@....ntt.co.jp, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org,
	linux-arm-kernel <linux-arm-kernel@...ts.arm.linux.org.uk>
Subject: Re: [PATCH][RFC] asm-generic:remove calling flush_write_buffers()
	in dma_sync_*_for_cpu

On Tue, Jul 07, 2009 at 04:06:48PM +0200, Arnd Bergmann wrote:
> Note that actually you need to do writeback+invalidate in DMA_TO_DEVICE

Rubbish.  DMA _to_ the device just needs any data held in the cache to
be pushed out into the hardware RAM.  There's absolutely no point in
invalidating it - that's a complete waste of time.

> and at least an invalidate in DMA_FROM_DEVICE during dma_map_*.
> For the unmap, I don't think you ever need to invalidate the cache.

As I've already explained, you do if you have speculative prefetching
which can bring in data into the cache from _any_ memory location at
_any_ time.  We _are_ going to have to do this, no questions asked.
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