Re: [PATCH] drm/msm/a6xx: Track current ctx by seqno

[Daniel Vetter]

On Thu, Sep 30, 2021 at 8:20 PM Rob Clark <robdclark@xxxxxxxxx> wrote:
>
> From: Rob Clark <robdclark@xxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxx>

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.
-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