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Message-ID: <CAF6AEGsNAzpzB4_bVSS9VcQwKEi_N+rdXMx3VYrL8ktf7oueZw@mail.gmail.com>
Date: Thu, 30 Sep 2021 12:04:02 -0700
From: Rob Clark <robdclark@...il.com>
To: Daniel Vetter <daniel@...ll.ch>
Cc: dri-devel <dri-devel@...ts.freedesktop.org>,
freedreno <freedreno@...ts.freedesktop.org>,
linux-arm-msm <linux-arm-msm@...r.kernel.org>,
Jordan Crouse <jordan@...micpenguin.net>,
Rob Clark <robdclark@...omium.org>,
Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
Akhil P Oommen <akhilpo@...eaurora.org>,
Jonathan Marek <jonathan@...ek.ca>,
Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
Sharat Masetty <smasetty@...eaurora.org>,
Douglas Anderson <dianders@...omium.org>,
Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
Yangtao Li <tiny.windzz@...il.com>,
open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/msm/a6xx: Track current ctx by seqno
On Thu, Sep 30, 2021 at 11:34 AM Daniel Vetter <daniel@...ll.ch> wrote:
>
> On Thu, Sep 30, 2021 at 8:20 PM Rob Clark <robdclark@...il.com> wrote:
> >
> > From: Rob Clark <robdclark@...omium.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@...omium.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
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