From mboxrd@z Thu Jan 1 00:00:00 1970 From: robherring2@gmail.com (Rob Herring) Date: Tue, 12 Nov 2013 14:22:47 -0600 Subject: [Xen-devel] [PATCH 2/2] swiotlb-xen: xen_swiotlb_map_page: do not error out if dma_capable fails In-Reply-To: References: <1384265520-6833-2-git-send-email-stefano.stabellini@eu.citrix.com> <20131112144832.GC9435@phenom.dumpdata.com> Message-ID: <52828E17.2040801@gmail.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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