linux-kernel - Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync

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Date:   Thu, 29 Nov 2018 16:23:59 -0800
From:   Tomasz Figa <tfiga@...omium.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Rob Clark <robdclark@...il.com>, Christoph Hellwig <hch@....de>,
        Vivek Gautam <vivek.gautam@...eaurora.org>,
        David Airlie <airlied@...ux.ie>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Archit Taneja <architt@...eaurora.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Sean Paul <seanpaul@...omium.org>
Subject: Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*

On Thu, Nov 29, 2018 at 12:03 PM Robin Murphy <robin.murphy@....com> wrote:
>
> On 29/11/2018 19:57, Tomasz Figa wrote:
> > On Thu, Nov 29, 2018 at 11:40 AM Jordan Crouse <jcrouse@...eaurora.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@....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.

Best regards,
Tomasz