Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*

[Tomasz Figa <tfiga@chromium.org>]

On Sat, Dec 1, 2018 at 3:47 AM Rob Clark <robdclark@gmail.com> wrote:
>
> On Fri, Nov 30, 2018 at 9:05 PM Tomasz Figa <tfiga@chromium.org> wrote:
> >
> > On Thu, Nov 29, 2018 at 4:23 PM Tomasz Figa <tfiga@chromium.org> wrote:
> > >
> > > On Thu, Nov 29, 2018 at 12:03 PM Robin Murphy <robin.murphy@arm.com> wrote:
> > > >
> > > > On 29/11/2018 19:57, Tomasz Figa wrote:
> > > > > On Thu, Nov 29, 2018 at 11:40 AM Jordan Crouse <jcrouse@codeaurora.org> wrote:
> > > > >>
> > > > >> On Thu, Nov 29, 2018 at 01:48:15PM -0500, Rob Clark wrote:
> > > > >>> On Thu, Nov 29, 2018 at 10:54 AM Christoph Hellwig <hch@lst.de> wrote:
> > > > >>>>
> > > > >>>> On Thu, Nov 29, 2018 at 09:42:50AM -0500, Rob Clark wrote:
> > > > >>>>> Maybe the thing we need to do is just implement a blacklist of
> > > > >>>>> compatible strings for devices which should skip the automatic
> > > > >>>>> iommu/dma hookup.  Maybe a bit ugly, but it would also solve a problem
> > > > >>>>> preventing us from enabling per-process pagetables for a5xx (where we
> > > > >>>>> need to control the domain/context-bank that is allocated by the dma
> > > > >>>>> api).
> > > > >>>>
> > > > >>>> You can detach from the dma map attachment using arm_iommu_detach_device,
> > > > >>>> which a few drm drivers do, but I don't think this is the problem.
> > > > >>>
> > > > >>> I think even with detach, we wouldn't end up with the context-bank
> > > > >>> that the gpu firmware was hard-coded to expect, and so it would
> > > > >>> overwrite the incorrect page table address register.  (I could be
> > > > >>> mis-remembering that, Jordan spent more time looking at that.  But it
> > > > >>> was something along those lines.)
> > > > >>
> > > > >> Right - basically the DMA domain steals context bank 0 and the GPU is hard coded
> > > > >> to use that context bank for pagetable switching.
> > > > >>
> > > > >> I believe the Tegra guys also had a similar problem with a hard coded context
> > > > >> bank.
> > > >
> > > > AIUI, they don't need a specific hardware context, they just need to
> > > > know which one they're actually using, which the domain abstraction hides.
> > > >
> > > > > Wait, if we detach the GPU/display struct device from the default
> > > > > domain and attach it to a newly allocated domain, wouldn't the newly
> > > > > allocated domain use the context bank we need? Note that we're already
> > > >
> > > > The arm-smmu driver doesn't, but there's no fundamental reason it
> > > > couldn't. That should just need code to refcount domain users and
> > > > release hardware contexts for domains with no devices currently attached.
> > > >
> > > > Robin.
> > > >
> > > > > doing that, except that we're doing it behind the back of the DMA
> > > > > mapping subsystem, so that it keeps using the IOMMU version of the DMA
> > > > > ops for the device and doing any mapping operations on the default
> > > > > domain. If we ask the DMA mapping to detach, wouldn't it essentially
> > > > > solve the problem?
> > >
> > > Thanks Robin.
> > >
> > > Still, my point is that the MSM DRM driver attaches the GPU struct
> > > device to a new domain it allocates using iommu_domain_alloc() and it
> > > seems to work fine, so I believe it's not the problem we're looking
> > > into with this patch.
> >
> > Could we just make the MSM DRM call arch_teardown_dma_ops() and then
> > arch_setup_dma_ops() with the `iommu` argument set to NULL and be done
> > with it?
>
> I don't think those are exported to modules?
>

Indeed, if we compile MSM DRM as modules, it wouldn't work...

> I have actually a simpler patch, that adds a small blacklist to check
> in of_dma_configure() before calling arch_setup_dma_ops(), which can
> replace this patch.  It also solves the problem of dma api allocating
> the context bank that he gpu wants to use for context-switching, and
> should be a simple thing to backport to stable branches.
>
> I was just spending some time trying to figure out what changed
> recently to start causing dma_map_sg() to opps on boot for us, so I
> could write a more detailed commit msg.

Yeah, that sounds much better, thanks. Reviewed that patch.

Best regards,
Tomasz