linux-kernel - Re: [Xen-devel] [PATCH 2/2] swiotlb-xen: xen_swiotlb_map_page: do not error out if dma

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Date:	Wed, 13 Nov 2013 11:35:44 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Rob Herring <robherring2@...il.com>
CC:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	<konrad@...nel.org>, <xen-devel@...ts.xensource.com>,
	<linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Xen-devel] [PATCH 2/2] swiotlb-xen: xen_swiotlb_map_page: do
 not error out if dma_capable fails

On Tue, 12 Nov 2013, Rob Herring wrote:
> On 11/12/2013 11:27 AM, Stefano Stabellini wrote:
> > Russell gave a great explanation of the issue so I am just going to
> > limit myself to answering to:
> > 
> > On Tue, 12 Nov 2013, Konrad Rzeszutek Wilk wrote:
> >>> Considering that we know that the swiotlb buffer has a low address,
> >>> skip the check.
> >>
> >> I am not following that sentence. Could you please explain to me
> >> how the SWIOTLB buffer low address guarantees that we don't need
> >> the check?
> > 
> > xen_swiotlb_fixup makes sure that the swiotlb buffer is lower than 4GB,
> > probably lower than 3GB, by passing dma_bits to
> > xen_create_contiguous_region.
> > This meets the requirements of most devices out there.
> > In fact we are not even running this check under the same conditions in
> > swiotlb_map_sg_attrs.
> > I admit that it is possible to come up with a scenario where the check
> > would be useful, but it is far easier to come up with scenarios where
> > not only is unneeded but it is even harmful.
> > 
> > Alternatively (without Rob's "of: set dma_mask to point to
> > coherent_dma_mask") Linux 3.13 is going to fail to get the network
> > running on Midway. It is going to avoid fs mounting failures just
> > because we don't do the same check in swiotlb_map_sg_attrs.
> > 
> > FYI given that Rob's patch is probably going upstream soon anyway, I
> > don't feel so strongly about this.
> 
> It is in Linus' tree now.
 
OK,  I'll drop this patch then.
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