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

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Message-ID: <20151109100804.GA24936@arm.com>
Date:	Mon, 9 Nov 2015 10:08:05 +0000
From:	Will Deacon <will.deacon@....com>
To:	sanjeev sharma <sanjeevsharmaengg@...il.com>
Cc:	"Sharma, Sanjeev" <Sanjeev_Sharma@...tor.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"m.szyprowski@...sung.com" <m.szyprowski@...sung.com>
Subject: Re: [PATCH] ARM:dma-mapping: Handle DMA_BIDIRECTIONAL in
 _dma_page_cpu_to_dev()

On Mon, Nov 09, 2015 at 11:29:17AM +0530, sanjeev sharma wrote:
>     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.
> 
> 
> 
> Please let me know what you think on above comment ?

I still don't understand the problem that you're trying to fix.

Sorry,

Will
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