Re: [PATCH v3] drm/gem: Internally test import_attach for imported objects

[Simona Vetter <simona.vetter@ffwll.ch>]

On Wed, Apr 16, 2025 at 11:38:03AM +0200, Simona Vetter wrote:
> On Wed, Apr 16, 2025 at 08:57:45AM +0200, Thomas Zimmermann wrote:
> > Test struct drm_gem_object.import_attach to detect imported objects.
> > 
> > During object clenanup, the dma_buf field might be NULL. Testing it in
> > an object's free callback then incorrectly does a cleanup as for native
> > objects. Happens for calls to drm_mode_destroy_dumb_ioctl() that
> > clears the dma_buf field in drm_gem_object_exported_dma_buf_free().
> > 
> > v3:
> > - only test for import_attach (Boris)
> > v2:
> > - use import_attach.dmabuf instead of dma_buf (Christian)
> > 
> > Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
> > Fixes: b57aa47d39e9 ("drm/gem: Test for imported GEM buffers with helper")
> > Reported-by: Andy Yan <andyshrk@163.com>
> > Closes: https://lore.kernel.org/dri-devel/38d09d34.4354.196379aa560.Coremail.andyshrk@163.com/
> > Tested-by: Andy Yan <andyshrk@163.com>
> > Cc: Thomas Zimmermann <tzimmermann@suse.de>
> > Cc: Anusha Srivatsa <asrivats@redhat.com>
> > Cc: Christian König <christian.koenig@amd.com>
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Cc: Maxime Ripard <mripard@kernel.org>
> > Cc: David Airlie <airlied@gmail.com>
> > Cc: Simona Vetter <simona@ffwll.ch>
> > Cc: Sumit Semwal <sumit.semwal@linaro.org>
> > Cc: "Christian König" <christian.koenig@amd.com>
> > Cc: dri-devel@lists.freedesktop.org
> > Cc: linux-media@vger.kernel.org
> > Cc: linaro-mm-sig@lists.linaro.org
> 
> Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>

Also quick doc request: We do have a bit of overview documentation for
prime here about specifically this lifetime fun, and why there's a chain
of references and hence a distinction between imported foreign dma-buf and
re-imported native dma-buf:

https://dri.freedesktop.org/docs/drm/gpu/drm-mm.html#reference-counting-for-gem-drivers

I think it would be good to augment this with more links to functions
(like this one recently added and fixed in this patch here) and struct
members to that overview. And maybe also link from key function and struct
functions back to that overview doc. Otherwise I think the next person
will get confused by this rather tricky code again and break a corner
cases.

Thanks, Sima

> 
> > ---
> >  include/drm/drm_gem.h | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> > index 9b71f7a9f3f8..a3133a08267c 100644
> > --- a/include/drm/drm_gem.h
> > +++ b/include/drm/drm_gem.h
> > @@ -588,8 +588,7 @@ static inline bool drm_gem_object_is_shared_for_memory_stats(struct drm_gem_obje
> >   */
> >  static inline bool drm_gem_is_imported(const struct drm_gem_object *obj)
> >  {
> > -	/* The dma-buf's priv field points to the original GEM object. */
> > -	return obj->dma_buf && (obj->dma_buf->priv != obj);
> > +	return !!obj->import_attach;
> >  }
> >  
> >  #ifdef CONFIG_LOCKDEP
> > -- 
> > 2.49.0
> > 
> 
> -- 
> Simona Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch