Re: [PATCH V6] drm/drm_vblank: Change EINVAL by the correct errno

[Pekka Paalanen <pekka.paalanen@collabora.com>]

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On Wed, 2 Oct 2019 11:05:16 -0300
Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com> wrote:

> For historical reasons, the function drm_wait_vblank_ioctl always return
> -EINVAL if something gets wrong. This scenario limits the flexibility
> for the userspace to make detailed verification of any problem and take
> some action. In particular, the validation of “if (!dev->irq_enabled)”
> in the drm_wait_vblank_ioctl is responsible for checking if the driver
> support vblank or not. If the driver does not support VBlank, the
> function drm_wait_vblank_ioctl returns EINVAL, which does not represent
> the real issue; this patch changes this behavior by return EOPNOTSUPP.
> Additionally, drm_crtc_get_sequence_ioctl and
> drm_crtc_queue_sequence_ioctl, also returns EINVAL if vblank is not
> supported; this patch also changes the return value to EOPNOTSUPP in
> these functions. Lastly, these functions are invoked by libdrm, which is
> used by many compositors; because of this, it is important to check if
> this change breaks any compositor. In this sense, the following projects
> were examined:
> 
> * Drm-hwcomposer
> * Kwin
> * Sway
> * Wlroots
> * Wayland
> * Weston
> * Mutter
> * Xorg (67 different drivers)
> 
> For each repository the verification happened in three steps:
> 
> * Update the main branch
> * Look for any occurrence of "drmCrtcQueueSequence",
>   "drmCrtcGetSequence", and "drmWaitVBlank" with the command git grep -n
>   "STRING".
> * Look in the git history of the project with the command
> git log -S<STRING>
> 
> None of the above projects validate the use of EINVAL when using
> drmWaitVBlank(), which make safe, at least for these projects, to change
> the return values. On the other hand, mesa and xserver project uses
> drmCrtcQueueSequence() and drmCrtcGetSequence(); this change is harmless
> for both projects.
> 
> Change since V5 (Pekka Paalanen):
>  - Check if the change also affects Mutter
> 
> Change since V4 (Daniel):
>  - Also return EOPNOTSUPP in drm_crtc_[get|queue]_sequence_ioctl
> 
> Change since V3:
>  - Return EINVAL for _DRM_VBLANK_SIGNAL (Daniel)
> 
> Change since V2:
>  Daniel Vetter and Chris Wilson
>  - Replace ENOTTY by EOPNOTSUPP
>  - Return EINVAL if the parameters are wrong
> 
> Cc: Keith Packard <keithp@keithp.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
> Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
> ---
>  drivers/gpu/drm/drm_vblank.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> index 9c6899758bc9..71cf2633ac58 100644
> --- a/drivers/gpu/drm/drm_vblank.c
> +++ b/drivers/gpu/drm/drm_vblank.c
> @@ -1610,7 +1610,7 @@ int drm_wait_vblank_ioctl(struct drm_device *dev, void *data,
>  	unsigned int flags, pipe, high_pipe;
>  
>  	if (!dev->irq_enabled)
> -		return -EINVAL;
> +		return -EOPNOTSUPP;
>  
>  	if (vblwait->request.type & _DRM_VBLANK_SIGNAL)
>  		return -EINVAL;

Hi,

this part looks safe for Weston indeed, so this part:

Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>

There is no need to check the "Wayland" project, it will never do
anything with DRM.

(What looks extremely suspicious though is the libdrm implementation of
drmWaitVblank() in case the ioctl returns EINTR, but that's unrelated.)


Thanks,
pq

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