[PATCH v2] drm: Copy drm_wait_vblank to user before returning

[Sean Paul]

On Fri, Aug 13, 2021 at 11:48 AM Mark Yacoub <markyacoub at chromium.org> wrote:
>
> Thanks for your review Michel!
> @MAINTAINER, could you please strip the Change-Id when applying.
> Thanks!

Applied to drm-misc-fixes with the Change-Id removed.

Thank you for your patch!

Sean

>
> On Fri, Aug 13, 2021 at 3:33 AM Michel Dänzer <michel at daenzer.net> wrote:
> >
> > On 2021-08-12 9:49 p.m., Mark Yacoub wrote:
> > > From: Mark Yacoub <markyacoub at google.com>
> > >
> > > [Why]
> > > Userspace should get back a copy of drm_wait_vblank that's been modified
> > > even when drm_wait_vblank_ioctl returns a failure.
> > >
> > > Rationale:
> > > drm_wait_vblank_ioctl modifies the request and expects the user to read
> > > it back. When the type is RELATIVE, it modifies it to ABSOLUTE and updates
> > > the sequence to become current_vblank_count + sequence (which was
> > > RELATIVE), but now it became ABSOLUTE.
> > > drmWaitVBlank (in libdrm) expects this to be the case as it modifies
> > > the request to be Absolute so it expects the sequence to would have been
> > > updated.
> > >
> > > The change is in compat_drm_wait_vblank, which is called by
> > > drm_compat_ioctl. This change of copying the data back regardless of the
> > > return number makes it en par with drm_ioctl, which always copies the
> > > data before returning.
> > >
> > > [How]
> > > Return from the function after everything has been copied to user.
> > >
> > > Fixes: IGT:kms_flip::modeset-vs-vblank-race-interruptible
> > > Tested on ChromeOS Trogdor(msm)
> > >
> > > Signed-off-by: Mark Yacoub <markyacoub at chromium.org>
> > > Change-Id: I98da279a5f1329c66a9d1e06b88d40b247b51313
> >
> > With the Gerrit Change-Id removed,
> >
> > Reviewed-by: Michel Dänzer <mdaenzer at redhat.com>
> >
> >
> > --
> > Earthling Michel Dänzer               |               https://redhat.com
> > Libre software enthusiast             |             Mesa and X developer