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 13:48:15 -0500
From:   Rob Clark <robdclark@...il.com>
To:     hch@....de
Cc:     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>,
        Tomasz Figa <tfiga@...omium.org>,
        Archit Taneja <architt@...eaurora.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jordan Crouse <jcrouse@...eaurora.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 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.)

BR,
-R