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

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Date: Wed, 10 Jan 2024 11:19:37 +0100
From: Christian König <christian.koenig@....com>
To: 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
Cc: 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

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.

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,
>   };
>