linux-kernel - Re: [PATCH 2/4] drm/virtio: switch virtio_gpu_wait

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Date:   Mon, 17 Jun 2019 16:15:16 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     dri-devel@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        "open list:VIRTIO GPU DRIVER" 
        <virtualization@...ts.linux-foundation.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] drm/virtio: switch virtio_gpu_wait_ioctl() to gem
 helper.

On Mon, Jun 17, 2019 at 01:14:04PM +0200, Gerd Hoffmann wrote:
> Use drm_gem_reservation_object_wait() in virtio_gpu_wait_ioctl().

Would be good to mention here that with this the wait becomes lockless, we
don't call ttm_bo_reserve/unreserve anymore.

> Signed-off-by: Gerd Hoffmann <kraxel@...hat.com>
> ---
>  drivers/gpu/drm/virtio/virtgpu_ioctl.c | 18 ++++--------------
>  1 file changed, 4 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/gpu/drm/virtio/virtgpu_ioctl.c b/drivers/gpu/drm/virtio/virtgpu_ioctl.c
> index c0ba1ead740f..e38a6bb46cc7 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_ioctl.c
> +++ b/drivers/gpu/drm/virtio/virtgpu_ioctl.c
> @@ -464,23 +464,13 @@ static int virtio_gpu_wait_ioctl(struct drm_device *dev, void *data,
>  			    struct drm_file *file)
>  {
>  	struct drm_virtgpu_3d_wait *args = data;
> -	struct drm_gem_object *gobj = NULL;
> -	struct virtio_gpu_object *qobj = NULL;
> -	int ret;
> -	bool nowait = false;
> -
> -	gobj = drm_gem_object_lookup(file, args->handle);
> -	if (gobj == NULL)
> -		return -ENOENT;
> -
> -	qobj = gem_to_virtio_gpu_obj(gobj);
> +	long timeout = 15 * HZ;
>  
>  	if (args->flags & VIRTGPU_WAIT_NOWAIT)
> -		nowait = true;
> -	ret = virtio_gpu_object_wait(qobj, nowait);
> +		timeout = 0;

timeout of 0 gets upgrade to a 1 jiffy wait, which is probably not what
you want here. You need to open-code ttm_bo_wait here and call
reservation_object_test_signaled_rcu() for this case.

With that fixed Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>

>  
> -	drm_gem_object_put_unlocked(gobj);
> -	return ret;
> +	return drm_gem_reservation_object_wait(file, args->handle,
> +					       true, timeout);
>  }
>  
>  static int virtio_gpu_get_caps_ioctl(struct drm_device *dev,
> -- 
> 2.18.1
> 

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