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Date: Wed, 10 Jan 2024 11:46:35 +0100
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Subject: Re: [PATCH 1/1] drm/virtio: Implement device_attach
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To: Julia Zhang <julia.zhang@amd.com>,
 Gurchetan Singh <gurchetansingh@chromium.org>, Chia-I Wu
 <olvaffe@gmail.com>, David Airlie <airlied@redhat.com>,
 Gerd Hoffmann <kraxel@redhat.com>, linux-kernel@vger.kernel.org,
 dri-devel@lists.freedesktop.org, amd-gfx@lists.freedesktop.org,
 virtualization@lists.linux-foundation.org,
 Alex Deucher <alexander.deucher@amd.com>, David Airlie <airlied@gmail.com>,
 Erik Faye-Lund <kusmabite@gmail.com>, =?UTF-8?B?TWFyZWsgT2zFocOhaw==?=
 <marek.olsak@amd.com>,
 Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>,
 Honglei Huang <honglei1.huang@amd.com>, Chen Jiqian <Jiqian.Chen@amd.com>,
 Huang Rui <ray.huang@amd.com>
References: <20240110095627.227454-1-julia.zhang@amd.com>
 <20240110095627.227454-2-julia.zhang@amd.com>
 <8ce0d90d-c751-4250-8656-fcab27aec6c2@amd.com>
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From: =?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>
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Am 10.01.24 um 11:22 schrieb Daniel Vetter:
> 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
> :-)

Ah, yes that makes much more sense!

So basically just the "return 0;" needs to be "return -EBUSY;".

Regards,
Christian.

> -Sima
>
>> Regards,
>> Christian.
>>
>>> Signed-off-by: Julia Zhang <julia.zhang@amd.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,
>>>    };