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From:   Rob Clark <robdclark@gmail.com>
Date:   Thu, 30 Sep 2021 12:04:02 -0700
Message-ID: <CAF6AEGsNAzpzB4_bVSS9VcQwKEi_N+rdXMx3VYrL8ktf7oueZw@mail.gmail.com>
Subject: Re: [PATCH] drm/msm/a6xx: Track current ctx by seqno
To:     Daniel Vetter <daniel@ffwll.ch>
Cc:     dri-devel <dri-devel@lists.freedesktop.org>,
        freedreno <freedreno@lists.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        Jordan Crouse <jordan@cosmicpenguin.net>,
        Rob Clark <robdclark@chromium.org>,
        Sean Paul <sean@poorly.run>, David Airlie <airlied@linux.ie>,
        Akhil P Oommen <akhilpo@codeaurora.org>,
        Jonathan Marek <jonathan@marek.ca>,
        Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
        Sharat Masetty <smasetty@codeaurora.org>,
        Douglas Anderson <dianders@chromium.org>,
        Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>,
        Yangtao Li <tiny.windzz@gmail.com>,
        open list <linux-kernel@vger.kernel.org>
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On Thu, Sep 30, 2021 at 11:34 AM Daniel Vetter <daniel@ffwll.ch> wrote:
>
> On Thu, Sep 30, 2021 at 8:20 PM Rob Clark <robdclark@gmail.com> wrote:
> >
> > From: Rob Clark <robdclark@chromium.org>
> >
> > In theory a context can be destroyed and a new one allocated at the same
> > address, making the pointer comparision to detect when we don't need to
> > update the current pagetables invalid.  Instead assign a sequence number
> > to each context on creation, and use this for the check.
> >
> > Fixes: 84c31ee16f90 ("drm/msm/a6xx: Add support for per-instance pagetables")
> > Signed-off-by: Rob Clark <robdclark@chromium.org>
>
> Usually the weak reference pattern is to wrap a spinlock or something
> around it and clear it out on destruction. Or hold a full reference
> (e.g. on intel hw the hw goes unhappy if there's no context/pagetables
> at all, so we always need some). But I guess this works too.

yeah, the seqno approach was so that gen agnostic code didn't have to
go fishing around in a6xx_gpu..

I guess the downside is userspace could open/close the dev file 2^32
times without submitting any rendering to trigger the same sort of
issue.  But there are plenty of easier ways for userspace to trigger
faults if it wanted.

BR,
-R

> -Daniel
>
> > ---
> >  drivers/gpu/drm/msm/adreno/a6xx_gpu.c |  6 +++---
> >  drivers/gpu/drm/msm/adreno/a6xx_gpu.h | 11 ++++++++++-
> >  drivers/gpu/drm/msm/msm_drv.c         |  3 +++
> >  drivers/gpu/drm/msm/msm_drv.h         |  1 +
> >  4 files changed, 17 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> > index 5e1ae3df42ba..e0a8b2fd1ff0 100644
> > --- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> > +++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> > @@ -106,7 +106,7 @@ static void a6xx_set_pagetable(struct a6xx_gpu *a6xx_gpu,
> >         u32 asid;
> >         u64 memptr = rbmemptr(ring, ttbr0);
> >
> > -       if (ctx == a6xx_gpu->cur_ctx)
> > +       if (ctx->seqno == a6xx_gpu->cur_ctx_seqno)
> >                 return;
> >
> >         if (msm_iommu_pagetable_params(ctx->aspace->mmu, &ttbr, &asid))
> > @@ -139,7 +139,7 @@ static void a6xx_set_pagetable(struct a6xx_gpu *a6xx_gpu,
> >         OUT_PKT7(ring, CP_EVENT_WRITE, 1);
> >         OUT_RING(ring, 0x31);
> >
> > -       a6xx_gpu->cur_ctx = ctx;
> > +       a6xx_gpu->cur_ctx_seqno = ctx->seqno;
> >  }
> >
> >  static void a6xx_submit(struct msm_gpu *gpu, struct msm_gem_submit *submit)
> > @@ -1081,7 +1081,7 @@ static int hw_init(struct msm_gpu *gpu)
> >         /* Always come up on rb 0 */
> >         a6xx_gpu->cur_ring = gpu->rb[0];
> >
> > -       a6xx_gpu->cur_ctx = NULL;
> > +       a6xx_gpu->cur_ctx_seqno = 0;
> >
> >         /* Enable the SQE_to start the CP engine */
> >         gpu_write(gpu, REG_A6XX_CP_SQE_CNTL, 1);
> > diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.h b/drivers/gpu/drm/msm/adreno/a6xx_gpu.h
> > index 0bc2d062f54a..8e5527c881b1 100644
> > --- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.h
> > +++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.h
> > @@ -19,7 +19,16 @@ struct a6xx_gpu {
> >         uint64_t sqe_iova;
> >
> >         struct msm_ringbuffer *cur_ring;
> > -       struct msm_file_private *cur_ctx;
> > +
> > +       /**
> > +        * cur_ctx_seqno:
> > +        *
> > +        * The ctx->seqno value of the context with current pgtables
> > +        * installed.  Tracked by seqno rather than pointer value to
> > +        * avoid dangling pointers, and cases where a ctx can be freed
> > +        * and a new one created with the same address.
> > +        */
> > +       int cur_ctx_seqno;
> >
> >         struct a6xx_gmu gmu;
> >
> > diff --git a/drivers/gpu/drm/msm/msm_drv.c b/drivers/gpu/drm/msm/msm_drv.c
> > index 624078b3adf2..30c1efc3d8a0 100644
> > --- a/drivers/gpu/drm/msm/msm_drv.c
> > +++ b/drivers/gpu/drm/msm/msm_drv.c
> > @@ -711,6 +711,7 @@ static void load_gpu(struct drm_device *dev)
> >
> >  static int context_init(struct drm_device *dev, struct drm_file *file)
> >  {
> > +       static atomic_t ident = ATOMIC_INIT(0);
> >         struct msm_drm_private *priv = dev->dev_private;
> >         struct msm_file_private *ctx;
> >
> > @@ -727,6 +728,8 @@ static int context_init(struct drm_device *dev, struct drm_file *file)
> >         ctx->aspace = msm_gpu_create_private_address_space(priv->gpu, current);
> >         file->driver_priv = ctx;
> >
> > +       ctx->seqno = atomic_inc_return(&ident);
> > +
> >         return 0;
> >  }
> >
> > diff --git a/drivers/gpu/drm/msm/msm_drv.h b/drivers/gpu/drm/msm/msm_drv.h
> > index de062450add4..8633d0059a3e 100644
> > --- a/drivers/gpu/drm/msm/msm_drv.h
> > +++ b/drivers/gpu/drm/msm/msm_drv.h
> > @@ -59,6 +59,7 @@ struct msm_file_private {
> >         int queueid;
> >         struct msm_gem_address_space *aspace;
> >         struct kref ref;
> > +       int seqno;
> >  };
> >
> >  enum msm_mdp_plane_property {
> > --
> > 2.31.1
> >
>
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch