linux-kernel - Re: [PATCH] drm/drm_vblank: Change EINVAL by the correct errno

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Date:   Mon, 08 Oct 2018 16:00:02 +0100
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...ux.ie>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>,
        Sean Paul <sean@...rly.run>
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm/drm_vblank: Change EINVAL by the correct errno

Quoting Rodrigo Siqueira (2018-10-08 15:52:20)
> For historical reason, the function drm_wait_vblank_ioctl always return
> -EINVAL if something gets wrong. This scenario limits the flexibility
> for the userspace make detailed verification of the 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 ENOTTY
> (Inappropriate ioctl for device). Additionally, some operations are
> unsupported by this function, and returns EINVAL; this patch changes the
> return value to EOPNOTSUPP (Operation not supported). Lastly, the
> function drm_wait_vblank_ioctl is 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-core
> * Weston
> * Xorg (67 different drivers)
> 
> For each repository the verification happened in three steps:
> 
> * Update the main branch
> * Look for any occurrence "drmWaitVBlank" with the command:
>   git grep -n "drmWaitVBlank"
> * Look in the git history of the project with the command:
>   git log -SdrmWaitVBlank
> 
> Finally, none of the above projects validate the use of EINVAL which
> make safe, at least for these projects, to change the return values.
> 
> Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@...il.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 98e091175921..88ec6fb49afb 100644
> --- a/drivers/gpu/drm/drm_vblank.c
> +++ b/drivers/gpu/drm/drm_vblank.c
> @@ -1533,10 +1533,10 @@ int drm_wait_vblank_ioctl(struct drm_device *dev, void *data,
>         unsigned int flags, pipe, high_pipe;
>  
>         if (!dev->irq_enabled)
> -               return -EINVAL;
> +               return -ENOTTY;

Arguable.

>  
>         if (vblwait->request.type & _DRM_VBLANK_SIGNAL)
> -               return -EINVAL;

User error -> einval.

> +               return -EOPNOTSUPP;
>  
>         if (vblwait->request.type &
>             ~(_DRM_VBLANK_TYPES_MASK | _DRM_VBLANK_FLAGS_MASK |
> @@ -1545,7 +1545,7 @@ int drm_wait_vblank_ioctl(struct drm_device *dev, void *data,
>                           vblwait->request.type,
>                           (_DRM_VBLANK_TYPES_MASK | _DRM_VBLANK_FLAGS_MASK |
>                            _DRM_VBLANK_HIGH_CRTC_MASK));
> -               return -EINVAL;
> +               return -EOPNOTSUPP;

User error -> einval.
-Chris