Re: [PATCH v2 1/1] drm/virtio: Implement device_attach

[Daniel Vetter]

On Tue, Jan 30, 2024 at 12:10:31PM +0100, Daniel Vetter wrote:
> On Mon, Jan 29, 2024 at 06:31:19PM +0800, Julia Zhang wrote:
> > As vram objects don't have backing pages and thus can't implement
> > drm_gem_object_funcs.get_sg_table callback. This removes drm dma-buf
> > callbacks in virtgpu_gem_map_dma_buf()/virtgpu_gem_unmap_dma_buf()
> > and implement virtgpu specific map/unmap/attach callbacks to support
> > both of shmem objects and vram objects.
> > 
> > Signed-off-by: Julia Zhang <julia.zhang@xxxxxxx>
> > ---
> >  drivers/gpu/drm/virtio/virtgpu_prime.c | 40 +++++++++++++++++++++++---
> >  1 file changed, 36 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/virtio/virtgpu_prime.c b/drivers/gpu/drm/virtio/virtgpu_prime.c
> > index 44425f20d91a..b490a5343b06 100644
> > --- a/drivers/gpu/drm/virtio/virtgpu_prime.c
> > +++ b/drivers/gpu/drm/virtio/virtgpu_prime.c
> > @@ -49,11 +49,26 @@ virtgpu_gem_map_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);
> > +	struct sg_table *sgt;
> > +	int ret;
> >  
> >  	if (virtio_gpu_is_vram(bo))
> >  		return virtio_gpu_vram_map_dma_buf(bo, attach->dev, dir);
> >  
> > -	return drm_gem_map_dma_buf(attach, dir);
> > +	sgt = drm_prime_pages_to_sg(obj->dev,
> > +				    to_drm_gem_shmem_obj(obj)->pages,
> > +				    obj->size >> PAGE_SHIFT);
> > +	if (IS_ERR(sgt))
> > +		return sgt;
> > +
> > +	ret = dma_map_sgtable(attach->dev, sgt, dir, DMA_ATTR_SKIP_CPU_SYNC);
> > +	if (ret) {
> > +		sg_free_table(sgt);
> > +		kfree(sgt);
> > +		return ERR_PTR(ret);
> > +	}
> > +
> > +	return sgt;
> >  }
> >  
> >  static void virtgpu_gem_unmap_dma_buf(struct dma_buf_attachment *attach,
> > @@ -63,12 +78,29 @@ static void virtgpu_gem_unmap_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 (!sgt)
> > +		return;
> > +
> >  	if (virtio_gpu_is_vram(bo)) {
> >  		virtio_gpu_vram_unmap_dma_buf(attach->dev, sgt, dir);
> > -		return;
> > +	} else {
> > +		dma_unmap_sgtable(attach->dev, sgt, dir, DMA_ATTR_SKIP_CPU_SYNC);
> > +		sg_free_table(sgt);
> > +		kfree(sgt);
> >  	}
> > +}
> > +
> > +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);
> > +	int ret = 0;
> > +
> > +	if (!virtio_gpu_is_vram(bo) && obj->funcs->pin)
> > +		ret = obj->funcs->pin(obj);
> >  
> > -	drm_gem_unmap_dma_buf(attach, sgt, dir);
> > +	return ret;
> 
> This doesn't look like what I've expected. There should be no need to
> change the map/unmap functions, especially not for the usual gem bo case.
> We should definitely keep using the exact same code for that. Instead all
> I expected is roughly
> 
> virtgpu_gem_device_attach()
> {
> 	if (virtio_gpu_is_vram(bo)) {
> 		if (can_access_virtio_vram_directly(attach->dev)
> 			return 0;
> 		else
> 			return -EBUSY;
> 	} else {
> 		return drm_gem_map_attach();
> 	}
> }
> 
> Note that I think can_access_virtio_vram_directly() needs to be
> implemented first. I'm not even sure it's possible, might be that all the
> importers need to set the attachment->peer2peer flag. Which is why this
> thing exists really. But that's a pile more work to do.
> 
> Frankly the more I look at the original patch that added vram export
> support the more this just looks like a "pls revert, this is just too
> broken".

The commit I mean is this one: ea5ea3d8a117 ("drm/virtio: support mapping
exported vram"). The commit message definitely needs to cite that one, and
also needs a cc: stable because not rejecting invalid imports is a pretty
big deal.

Also adding David.
-Sima

> 
> We should definitely not open-code any functions for the gem_bo export
> case, which your patch seems to do? Or maybe I'm just extremely confused.
> -Sima
> 
> >  
> >  static const struct virtio_dma_buf_ops virtgpu_dmabuf_ops =  {
> > @@ -83,7 +115,7 @@ 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,
> >  };
> >  
> > -- 
> > 2.34.1
> > 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

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