Re: Drop use of DRM_WAIT_ON() [Was: drm/drm_vblank: Change EINVAL by the correct errno]

[Sam Ravnborg <sam@ravnborg.org>]

Hi Rodrigo et al.

On Thu, Jun 13, 2019 at 07:04:03AM +0200, Sam Ravnborg wrote:
> Hi Rodrigo.
> 
> 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;
> >  
> >  	if (vblwait->request.type &
> >  	    ~(_DRM_VBLANK_TYPES_MASK | _DRM_VBLANK_FLAGS_MASK |
> 
> When touching this function, could I ask you to take a look at
> eliminating the use of DRM_WAIT_ON()?
> It comes from the deprecated drm_os_linux.h header, and it is only of
> the few remaining users of DRM_WAIT_ON().
> 
> Below you can find my untested first try - where I did an attempt not to
> change behaviour.

intel-gfx did not like the patch - so no need to spend time looking at
the patch until I have that fixed.

	Sam