Re: [PATCH v4 21/33] drm/msm: Add _NO_SHARE flag

["Christian König" <christian.koenig@amd.com>]

On 5/2/25 18:56, Rob Clark wrote:
> From: Rob Clark <robdclark@chromium.org>
> 
> Buffers that are not shared between contexts can share a single resv
> object.  This way drm_gpuvm will not track them as external objects, and
> submit-time validating overhead will be O(1) for all N non-shared BOs,
> instead of O(n).
> 
> Signed-off-by: Rob Clark <robdclark@chromium.org>
> ---
>  drivers/gpu/drm/msm/msm_drv.h       |  1 +
>  drivers/gpu/drm/msm/msm_gem.c       | 23 +++++++++++++++++++++++
>  drivers/gpu/drm/msm/msm_gem_prime.c | 15 +++++++++++++++
>  include/uapi/drm/msm_drm.h          | 14 ++++++++++++++
>  4 files changed, 53 insertions(+)
> 
> diff --git a/drivers/gpu/drm/msm/msm_drv.h b/drivers/gpu/drm/msm/msm_drv.h
> index b77fd2c531c3..b0add236cbb3 100644
> --- a/drivers/gpu/drm/msm/msm_drv.h
> +++ b/drivers/gpu/drm/msm/msm_drv.h
> @@ -246,6 +246,7 @@ int msm_gem_prime_vmap(struct drm_gem_object *obj, struct iosys_map *map);
>  void msm_gem_prime_vunmap(struct drm_gem_object *obj, struct iosys_map *map);
>  struct drm_gem_object *msm_gem_prime_import_sg_table(struct drm_device *dev,
>  		struct dma_buf_attachment *attach, struct sg_table *sg);
> +struct dma_buf *msm_gem_prime_export(struct drm_gem_object *obj, int flags);
>  int msm_gem_prime_pin(struct drm_gem_object *obj);
>  void msm_gem_prime_unpin(struct drm_gem_object *obj);
>  
> diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
> index 3708d4579203..d0f44c981351 100644
> --- a/drivers/gpu/drm/msm/msm_gem.c
> +++ b/drivers/gpu/drm/msm/msm_gem.c
> @@ -532,6 +532,9 @@ static int get_and_pin_iova_range_locked(struct drm_gem_object *obj,
>  
>  	msm_gem_assert_locked(obj);
>  
> +	if (to_msm_bo(obj)->flags & MSM_BO_NO_SHARE)
> +		return -EINVAL;
> +
>  	vma = get_vma_locked(obj, vm, range_start, range_end);
>  	if (IS_ERR(vma))
>  		return PTR_ERR(vma);
> @@ -1060,6 +1063,16 @@ static void msm_gem_free_object(struct drm_gem_object *obj)
>  		put_pages(obj);
>  	}
>  
> +	if (msm_obj->flags & MSM_BO_NO_SHARE) {
> +		struct drm_gem_object *r_obj =
> +			container_of(obj->resv, struct drm_gem_object, _resv);
> +
> +		BUG_ON(obj->resv == &obj->_resv);
> +
> +		/* Drop reference we hold to shared resv obj: */
> +		drm_gem_object_put(r_obj);
> +	}
> +
>  	drm_gem_object_release(obj);
>  
>  	kfree(msm_obj->metadata);
> @@ -1092,6 +1105,15 @@ int msm_gem_new_handle(struct drm_device *dev, struct drm_file *file,
>  	if (name)
>  		msm_gem_object_set_name(obj, "%s", name);
>  
> +	if (flags & MSM_BO_NO_SHARE) {
> +		struct msm_context *ctx = file->driver_priv;
> +		struct drm_gem_object *r_obj = drm_gpuvm_resv_obj(ctx->vm);
> +
> +		drm_gem_object_get(r_obj);
> +
> +		obj->resv = r_obj->resv;
> +	}
> +
>  	ret = drm_gem_handle_create(file, obj, handle);
>  
>  	/* drop reference from allocate - handle holds it now */
> @@ -1124,6 +1146,7 @@ static const struct drm_gem_object_funcs msm_gem_object_funcs = {
>  	.free = msm_gem_free_object,
>  	.open = msm_gem_open,
>  	.close = msm_gem_close,
> +	.export = msm_gem_prime_export,
>  	.pin = msm_gem_prime_pin,
>  	.unpin = msm_gem_prime_unpin,
>  	.get_sg_table = msm_gem_prime_get_sg_table,
> diff --git a/drivers/gpu/drm/msm/msm_gem_prime.c b/drivers/gpu/drm/msm/msm_gem_prime.c
> index ee267490c935..1a6d8099196a 100644
> --- a/drivers/gpu/drm/msm/msm_gem_prime.c
> +++ b/drivers/gpu/drm/msm/msm_gem_prime.c
> @@ -16,6 +16,9 @@ struct sg_table *msm_gem_prime_get_sg_table(struct drm_gem_object *obj)
>  	struct msm_gem_object *msm_obj = to_msm_bo(obj);
>  	int npages = obj->size >> PAGE_SHIFT;
>  
> +	if (msm_obj->flags & MSM_BO_NO_SHARE)
> +		return ERR_PTR(-EINVAL);
> +
>  	if (WARN_ON(!msm_obj->pages))  /* should have already pinned! */
>  		return ERR_PTR(-ENOMEM);
>  
> @@ -45,6 +48,15 @@ struct drm_gem_object *msm_gem_prime_import_sg_table(struct drm_device *dev,
>  	return msm_gem_import(dev, attach->dmabuf, sg);
>  }
>  
> +
> +struct dma_buf *msm_gem_prime_export(struct drm_gem_object *obj, int flags)
> +{
> +	if (to_msm_bo(obj)->flags & MSM_BO_NO_SHARE)
> +		return ERR_PTR(-EPERM);
> +
> +	return drm_gem_prime_export(obj, flags);
> +}
> +
>  int msm_gem_prime_pin(struct drm_gem_object *obj)
>  {
>  	struct page **pages;
> @@ -53,6 +65,9 @@ int msm_gem_prime_pin(struct drm_gem_object *obj)
>  	if (obj->import_attach)
>  		return 0;
>  
> +	if (to_msm_bo(obj)->flags & MSM_BO_NO_SHARE)
> +		return -EINVAL;
> +
>  	pages = msm_gem_pin_pages_locked(obj);
>  	if (IS_ERR(pages))
>  		ret = PTR_ERR(pages);
> diff --git a/include/uapi/drm/msm_drm.h b/include/uapi/drm/msm_drm.h
> index b974f5a24dbc..1bccc347945c 100644
> --- a/include/uapi/drm/msm_drm.h
> +++ b/include/uapi/drm/msm_drm.h
> @@ -140,6 +140,19 @@ struct drm_msm_param {
>  
>  #define MSM_BO_SCANOUT       0x00000001     /* scanout capable */
>  #define MSM_BO_GPU_READONLY  0x00000002
> +/* Private buffers do not need to be explicitly listed in the SUBMIT
> + * ioctl, unless referenced by a drm_msm_gem_submit_cmd.  Private
> + * buffers may NOT be imported/exported or used for scanout (or any
> + * other situation where buffers can be indefinitely pinned, but
> + * cases other than scanout are all kernel owned BOs which are not
> + * visible to userspace).

Why is pinning for scanout a problem with those?

Maybe I missed something but for other drivers that doesn't seem to be a problem.

Regards,
Christian.


> + *
> + * In exchange for those constraints, all private BOs associated with
> + * a single context (drm_file) share a single dma_resv, and if there
> + * has been no eviction since the last submit, there are no per-BO
> + * bookeeping to do, significantly cutting the SUBMIT overhead.
> + */
> +#define MSM_BO_NO_SHARE      0x00000004
>  #define MSM_BO_CACHE_MASK    0x000f0000
>  /* cache modes */
>  #define MSM_BO_CACHED        0x00010000
> @@ -149,6 +162,7 @@ struct drm_msm_param {
>  
>  #define MSM_BO_FLAGS         (MSM_BO_SCANOUT | \
>                                MSM_BO_GPU_READONLY | \
> +                              MSM_BO_NO_SHARE | \
>                                MSM_BO_CACHE_MASK)
>  
>  struct drm_msm_gem_new {