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

[Tomasz Figa]

On Thu, Nov 29, 2018 at 4:23 PM Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
>
> On Thu, Nov 29, 2018 at 12:03 PM Robin Murphy <robin.murphy@xxxxxxx> wrote:
> >
> > On 29/11/2018 19:57, Tomasz Figa wrote:
> > > On Thu, Nov 29, 2018 at 11:40 AM Jordan Crouse <jcrouse@xxxxxxxxxxxxxx> 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@xxxxxx> 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?

Best regards,
Tomasz