linux-kernel - Re: [PATCH v2] drm/drm_vblank: Change EINVAL by the correct errno

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Date:   Tue, 16 Oct 2018 18:38:31 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
Cc:     Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH v2] drm/drm_vblank: Change EINVAL by the correct errno

On Tue, Oct 16, 2018 at 03:36:20PM +0200, Maarten Lankhorst wrote:
> Op 15-10-18 om 19:05 schreef Rodrigo Siqueira:
> > 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 V1:
> >  Daniel Vetter and Chris Wilson
> >  - Replace ENOTTY by EOPNOTSUPP
> >  - Return EINVAL if the parameters are wrong
> >
> > Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>
> > ---
> >  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 98e091175921..80f5a3bb427e 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 -EOPNOTSUPP;
> Change to -EIO?
> 
> If userspace would ever print this out, it would print the following
> confusing message to userspace:
> "Operation not supported on transport endpoint"

You're a bit late, EOPNOTSUPP is not established already in upstream for
this. And -EIO is taken already for "the gpu is dead".

> >  
> >  	if (vblwait->request.type & _DRM_VBLANK_SIGNAL)
> > -		return -EINVAL;
> > +		return -EOPNOTSUPP;
> I would keep this -EINVAL, tbh and making it part of the below if statement..

We discussed this, it's different: This here is an ioctl flag that's no
longer supported, the below is just an invalid request. Hence different
errno.

I think you missed a bit with your bikeshed :-)
-Daniel

> >  	if (vblwait->request.type &
> >  	    ~(_DRM_VBLANK_TYPES_MASK | _DRM_VBLANK_FLAGS_MASK |
> 
> Cheers,
> 
> Maarten
> 

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