Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 13 Jun 2019 10:21:47 +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: [RESEND PATCH V3] drm/drm_vblank: Change EINVAL by the correct
 errno
Message-ID: <20190613082147.GG23020@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: <20190613021054.cdewdb3azy6zuoyw@smtp.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <20190613021054.cdewdb3azy6zuoyw@smtp.gmail.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Jun 12, 2019 at 11:10:54PM -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 V2:
>  Daniel Vetter and Chris Wilson
>  - Replace ENOTTY by EOPNOTSUPP
>  - Return EINVAL if the parameters are wrong
> 
> Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
> ---
> Update:
>   Now IGT has a way to validate if a driver has vblank support or not.
>   See: https://gitlab.freedesktop.org/drm/igt-gpu-tools/commit/2d244aed69165753f3adbbd6468db073dc1acf9A
> 
>  drivers/gpu/drm/drm_vblank.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> index 0d704bddb1a6..d76a783a7d4b 100644
> --- a/drivers/gpu/drm/drm_vblank.c
> +++ b/drivers/gpu/drm/drm_vblank.c
> @@ -1578,10 +1578,10 @@ int drm_wait_vblank_ioctl(struct drm_device *dev, void *data,
>  	unsigned int flags, pipe, high_pipe;
>  
>  	if (!dev->irq_enabled)
> -		return -EINVAL;
> +		return -EOPNOTSUPP;
>  
>  	if (vblwait->request.type & _DRM_VBLANK_SIGNAL)
> -		return -EINVAL;
> +		return -EOPNOTSUPP;

Not sure we want EINVAL here, that's kinda a "parameters are wrong"
version too. With that changed:

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

>  
>  	if (vblwait->request.type &
>  	    ~(_DRM_VBLANK_TYPES_MASK | _DRM_VBLANK_FLAGS_MASK |
> -- 
> 2.21.0


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