From mboxrd@z Thu Jan  1 00:00:00 1970
From: oliver@neukum.org (Oliver Neukum)
Date: Tue, 16 Feb 2010 14:32:57 +0100
Subject: USB mass storage and ARM cache coherency
In-Reply-To: <20100216093946.GA13884@n2100.arm.linux.org.uk>
References: <20100208065519.GE1290@ucw.cz>
	<201002161007.20576.oliver@neukum.org>
	<20100216093946.GA13884@n2100.arm.linux.org.uk>
Message-ID: <201002161432.57705.oliver@neukum.org>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

Am Dienstag, 16. Februar 2010 10:39:46 schrieb Russell King - ARM Linux:
> However, because ARM CPUs can now speculatively prefetch, just leaving it
> at that results in corruption of buffers used for DMA.  So we have to
> invalidate DMA_FROM_DEVICE and DMA_BIDIRECTIONAL buffers on unmap to
> ensure coherency with DMA operations.
> 
> If the CPU writes to a DMA_FROM_DEVICE buffer between map and unmap, the
> writes can sit in the cache, and on unmap, they will be discarded.
> 
> Cleaning the cache on unmap is not an option; that too can lead to DMA
> buffer corruption in the DMA case.

I am afraid for these controllers the controller driver must be responsible
for all DMA and cache issues. Indicating the exact requirements to the
upper layer would be a battle already lost.
so the safe choice is not to set has_dma and the generic layer will leave
the issue to the lower level.

	Regards
		Oliver