linux-kernel - Re: [PATCH 1/1] drm/virtio: Implement device

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Date: Wed, 10 Jan 2024 11:22:36 +0100
From: Daniel Vetter <daniel@...ll.ch>
To: Christian König <christian.koenig@....com>
Cc: Julia Zhang <julia.zhang@....com>,
	Gurchetan Singh <gurchetansingh@...omium.org>,
	Chia-I Wu <olvaffe@...il.com>, David Airlie <airlied@...hat.com>,
	Gerd Hoffmann <kraxel@...hat.com>, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, amd-gfx@...ts.freedesktop.org,
	virtualization@...ts.linux-foundation.org,
	Alex Deucher <alexander.deucher@....com>,
	Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...il.com>,
	Erik Faye-Lund <kusmabite@...il.com>,
	Marek Olšák <marek.olsak@....com>,
	Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@....com>,
	Honglei Huang <honglei1.huang@....com>,
	Chen Jiqian <Jiqian.Chen@....com>, Huang Rui <ray.huang@....com>
Subject: Re: [PATCH 1/1] drm/virtio: Implement device_attach

On Wed, Jan 10, 2024 at 11:19:37AM +0100, Christian König wrote:
> Am 10.01.24 um 10:56 schrieb Julia Zhang:
> > drm_gem_map_attach() requires drm_gem_object_funcs.get_sg_table to be
> > implemented, or else return ENOSYS. Virtio has no get_sg_table
> > implemented for vram object. To fix this, add a new device_attach to
> > call drm_gem_map_attach() for shmem object and return 0 for vram object
> > instead of calling drm_gem_map_attach for both of these two kinds of
> > object.
> 
> Well as far as I can see this is nonsense from the DMA-buf side of things.
> 
> SG tables are always needed as long as you don't re-import the same object
> into your driver and then you shouldn't end up in this function in the first
> place.
> 
> So that drm_gem_map_attach() requires get_sg_table to be implemented is
> intentional and should never be overridden like this.

See my reply, tldr; you're allowed to reject ->attach with -EBUSY to
handle exactly this case of non-shareable buffer types. But definitely
don't silently fail, that's a "we'll oops on map_attachment" kind of bug
:-)
-Sima

> 
> Regards,
> Christian.
> 
> > 
> > Signed-off-by: Julia Zhang <julia.zhang@....com>
> > ---
> >   drivers/gpu/drm/virtio/virtgpu_prime.c | 14 +++++++++++++-
> >   1 file changed, 13 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/virtio/virtgpu_prime.c b/drivers/gpu/drm/virtio/virtgpu_prime.c
> > index 44425f20d91a..f0b0ff6f3813 100644
> > --- a/drivers/gpu/drm/virtio/virtgpu_prime.c
> > +++ b/drivers/gpu/drm/virtio/virtgpu_prime.c
> > @@ -71,6 +71,18 @@ static void virtgpu_gem_unmap_dma_buf(struct dma_buf_attachment *attach,
> >   	drm_gem_unmap_dma_buf(attach, sgt, dir);
> >   }
> > +static int virtgpu_gem_device_attach(struct dma_buf *dma_buf,
> > +				     struct dma_buf_attachment *attach)
> > +{
> > +	struct drm_gem_object *obj = attach->dmabuf->priv;
> > +	struct virtio_gpu_object *bo = gem_to_virtio_gpu_obj(obj);
> > +
> > +	if (virtio_gpu_is_vram(bo))
> > +		return 0;
> > +
> > +	return drm_gem_map_attach(dma_buf, attach);
> > +}
> > +
> >   static const struct virtio_dma_buf_ops virtgpu_dmabuf_ops =  {
> >   	.ops = {
> >   		.cache_sgt_mapping = true,
> > @@ -83,7 +95,7 @@ static const struct virtio_dma_buf_ops virtgpu_dmabuf_ops =  {
> >   		.vmap = drm_gem_dmabuf_vmap,
> >   		.vunmap = drm_gem_dmabuf_vunmap,
> >   	},
> > -	.device_attach = drm_gem_map_attach,
> > +	.device_attach = virtgpu_gem_device_attach,
> >   	.get_uuid = virtgpu_virtio_get_uuid,
> >   };
> 

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