[PATCH] ARM:dma-mapping: Handle DMA_BIDIRECTIONAL in _dma_page_cpu_to_dev()
Sanjeev_Sharma at mentor.com
Wed Nov 4 21:56:10 PST 2015
From: Russell King - ARM Linux [mailto:linux at arm.linux.org.uk]
Sent: Wednesday, November 04, 2015 4:24 PM
To: Will Deacon
Cc: Sharma, Sanjeev; m.szyprowski at samsung.com; linux-arm-kernel at lists.infradead.org; linux-kernel at vger.kernel.org
Subject: Re: [PATCH] ARM:dma-mapping: Handle DMA_BIDIRECTIONAL in _dma_page_cpu_to_dev()
On Wed, Nov 04, 2015 at 10:39:13AM +0000, Will Deacon wrote:
> On Wed, Nov 04, 2015 at 03:26:48PM +0530, Sanjeev Sharma wrote:
> > _dma_page_cpu_to_dev() treat DMA_BIDIRECTIONAL similar to
> > DMA_TO_DEVICE which means that destination buffer is device
> > memory,means cpu may have written some data to source buffer and
> > data may be in cache line.For cleaner operation we need to call
> > outer_flush_range() which will clean and invalidate outer cache lines.
> Why isn't the clean sufficient in this case? We're mapping the buffer
> to the device, so we clean the dirty lines in the CPU caches and make
> them visible to the device. If the CPU later wants to read the buffer
> (i.e. after the device has DMA'd into it), you'll need to map the
> buffer to the CPU, which will perform the invalidation of the CPU caches.
Indeed. bidirectional mode is already handled prefectly well by this code. No patches are required.
Thanks Russell & Will for providing input.
Let's assume , CPU don't read the buffer then there could be the problem correct ? IMO, to handle every use case outer_flush_range can be used ?
If still it doesn't make sense to use flush on bidirectional mappings, then FIXME comment should be removed from the function to avoid any
(I never received the original email.)
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