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

From: Will Deacon
Date: Mon Nov 09 2015 - 05:08:17 EST

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.


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