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Date:   Wed, 19 Jun 2019 09:48:56 +0200
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc:     Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <maxime.ripard@bootlin.com>,
        Sean Paul <sean@poorly.run>, David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        kernel-janitors@vger.kernel.org, intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH V4] drm/drm_vblank: Change EINVAL by the correct errno
Message-ID: <20190619074856.GJ12905@phenom.ffwll.local>
Mail-Followup-To: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <maxime.ripard@bootlin.com>,
        Sean Paul <sean@poorly.run>, David Airlie <airlied@linux.ie>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        kernel-janitors@vger.kernel.org, intel-gfx@lists.freedesktop.org
References: <20190619020750.swzerehjbvx6sbk2@smtp.gmail.com>
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On Tue, Jun 18, 2019 at 11:07:50PM -0300, Rodrigo Siqueira wrote:
> 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 EOPNOTSUPP.
> Additionally, some operations are unsupported by this function, and
> returns EINVAL; this patch also changes the return value to EOPNOTSUPP
> in this case. 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.
> 
> 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
> 

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Apologies for the confusion on the last time around. btw if someone tells
you "r-b (or a-b) with these changes", then just apply the r-b/a-b tag
next time around. Otherwise people will re-review the same thing over and
over again.
-Daniel

> Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
> ---
>  drivers/gpu/drm/drm_vblank.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> index 603ab105125d..bed233361614 100644
> --- a/drivers/gpu/drm/drm_vblank.c
> +++ b/drivers/gpu/drm/drm_vblank.c
> @@ -1582,7 +1582,7 @@ 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;
> -- 
> 2.21.0

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