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

[Daniel Vetter]

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".

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