linux-kernel - RE: [PATCH] ARM:dma-mapping: Handle DMA_BIDIRECTIONAL in _dma_page_cpu_to

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Date:	Thu, 5 Nov 2015 05:56:10 +0000
From:	"Sharma, Sanjeev" <Sanjeev_Sharma@...tor.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Will Deacon <will.deacon@....com>
CC:	"m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] ARM:dma-mapping: Handle DMA_BIDIRECTIONAL in
 _dma_page_cpu_to_dev()

-----Original Message-----
From: Russell King - ARM Linux [mailto:linux@....linux.org.uk] 
Sent: Wednesday, November 04, 2015 4:24 PM
To: Will Deacon
Cc: Sharma, Sanjeev; m.szyprowski@...sung.com; linux-arm-kernel@...ts.infradead.org; linux-kernel@...r.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
Confusion.

(I never received the original email.)

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