linux-kernel - Re: [PATCH v8 13/19] swiotlb-xen: use xen_dma_map/unmap_page, xen_dma_sync_single_for

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Date:	Thu, 24 Oct 2013 11:44:50 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	<xen-devel@...ts.xensource.com>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <Ian.Campbell@...rix.com>
Subject: Re: [PATCH v8 13/19] swiotlb-xen: use xen_dma_map/unmap_page,
 xen_dma_sync_single_for_cpu/device

On Wed, 23 Oct 2013, Konrad Rzeszutek Wilk wrote:
> On Wed, Oct 23, 2013 at 06:20:25PM +0100, Stefano Stabellini wrote:
> > On Wed, 23 Oct 2013, Konrad Rzeszutek Wilk wrote:
> > > On Thu, Oct 17, 2013 at 06:43:28PM +0100, Stefano Stabellini wrote:
> > > > Call xen_dma_map_page, xen_dma_unmap_page, xen_dma_sync_single_for_cpu,
> > > > xen_dma_sync_single_for_device from swiotlb-xen to ensure cpu/device
> > > > coherency of the pages used for DMA, including the ones belonging to the
> > > > swiotlb buffer.
> > > 
> > > You lost me.
> > > 
> > > Isn't it the driver's responsibility to do this?
> > > 
> > > Looking at what 'xen_dma_map_page()' does for x86 it looks to add an extra
> > > call - page_to_phys - and we ignore it here.
> > 
> > map_page on arm calls the right cache flushes needed to communicate with
> > the device. Same with unmap_page.
> 
> If this is flushing the cache then I think it makes more sense to do
> that without this fancy 'dma_map_page'.
> 
> Just call it 'xen_flush_dma_page' and make it a nop on all platforms
> except ARM.

I am OK with making it a nop on x86, it makes sense.
However I would like to keep it called xen_dma_map_page: after all it
corresponds exactly to the native map_page dma_op. It is part of the same
"contract".


> > On x86 they are basically nop.
> 
> It calls page_to_phys in your patch. That is hardly nop.

I see. It is certainly worth optimizing it out on x86.
Of course if one day the x86 map_page dma_op starts doing something
useful, we can go back to call it from xen_dma_map_page.
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