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

From: Sharma, Sanjeev
Date: Thu Nov 05 2015 - 00:57:39 EST

-----Original Message-----
From: Russell King - ARM Linux [mailto:linux@xxxxxxxxxxxxxxxx]
Sent: Wednesday, November 04, 2015 4:24 PM
To: Will Deacon
Cc: Sharma, Sanjeev; m.szyprowski@xxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
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.)

FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at