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:	Tue, 12 Nov 2013 14:22:47 -0600
From:	Rob Herring <robherring2@...il.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	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 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.

Rob

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