RE: [PATCH] ARM:dma-mapping: Handle DMA_BIDIRECTIONAL in _dma_page_cpu_to_dev()

["Sharma, Sanjeev" <Sanjeev_Sharma@mentor.com>]

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

(I never received the original email.)

--
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net.