[RESEND PATCH V3] drm/drm_vblank: Change EINVAL by the correct errno

Rodrigo Siqueira
2019-06-13T02:10:54Z

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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 |
--
2.21.0

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