Hi Thomas,
On Wed, Jun 23, 2021 at 08:43:07AM +0200, Thomas Zimmermann wrote:
> Hi Liviu
>
> Am 22.06.21 um 17:25 schrieb Liviu Dudau:
> > Hello,
> >
> > On Tue, Jun 22, 2021 at 04:09:44PM +0200, Thomas Zimmermann wrote:
> > > For KMS drivers, replace the IRQ check in VBLANK ioctls with a check for
> > > vblank support. IRQs might be enabled wthout vblanking being supported.
> > >
> > > This change also removes the DRM framework's only dependency on IRQ state
> > > for non-legacy drivers. For legacy drivers with userspace modesetting,
> > > the original test remains in drm_wait_vblank_ioctl().
> > >
> > > v2:
> > > * keep the old test for legacy drivers in
> > > drm_wait_vblank_ioctl() (Daniel)
> > >
> > > Signed-off-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
> > > ---
> > > drivers/gpu/drm/drm_irq.c | 10 +++-------
> > > drivers/gpu/drm/drm_vblank.c | 13 +++++++++----
> > > 2 files changed, 12 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> > > index c3bd664ea733..1d7785721323 100644
> > > --- a/drivers/gpu/drm/drm_irq.c
> > > +++ b/drivers/gpu/drm/drm_irq.c
> > > @@ -74,10 +74,8 @@
> > > * only supports devices with a single interrupt on the main device stored in
> > > * &drm_device.dev and set as the device paramter in drm_dev_alloc().
> > > *
> > > - * These IRQ helpers are strictly optional. Drivers which roll their own only
> > > - * need to set &drm_device.irq_enabled to signal the DRM core that vblank
> > > - * interrupts are working. Since these helpers don't automatically clean up the
> > > - * requested interrupt like e.g. devm_request_irq() they're not really
> > > + * These IRQ helpers are strictly optional. Since these helpers don't automatically
> > > + * clean up the requested interrupt like e.g. devm_request_irq() they're not really
> > > * recommended.
> > > */
> > > @@ -91,9 +89,7 @@
> > > * and after the installation.
> > > *
> > > * This is the simplified helper interface provided for drivers with no special
> > > - * needs. Drivers which need to install interrupt handlers for multiple
> > > - * interrupts must instead set &drm_device.irq_enabled to signal the DRM core
> > > - * that vblank interrupts are available.
> > > + * needs.
> > > *
> > > * @irq must match the interrupt number that would be passed to request_irq(),
> > > * if called directly instead of using this helper function.
> > > diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> > > index 3417e1ac7918..a98a4aad5037 100644
> > > --- a/drivers/gpu/drm/drm_vblank.c
> > > +++ b/drivers/gpu/drm/drm_vblank.c
> > > @@ -1748,8 +1748,13 @@ int drm_wait_vblank_ioctl(struct drm_device *dev, void *data,
> > > unsigned int pipe_index;
> > > unsigned int flags, pipe, high_pipe;
> > > - if (!dev->irq_enabled)
> > > - return -EOPNOTSUPP;
> > > + if (drm_core_check_feature(dev, DRIVER_MODESET)) {
> > > + if (!drm_dev_has_vblank(dev))
> > > + return -EOPNOTSUPP;
> > > + } else {
> > > + if (!dev->irq_enabled)
> > > + return -EOPNOTSUPP;
> > > + }
> >
> > For a system call that is used quite a lot by userspace we have increased the code size
> > in a noticeable way. Can we not cache it privately?
>
> I'm not quite sure that I understand your concern. The additionally called
> functions are trivial one-liners; probably inlined anyway.
They are inlined. However we replace the pointer dereference (which can be calculated
at compile time as offset from a base pointer) with the code in
drm_core_check_all_features() that does 3 pointer dereferences, masking and logical
AND before checking for matching value.
>
> However, irq_enabled is only relevant for legacy drivers and will eventually
> disappear behind CONFIG_DRM_LEGACY. We can rewrite the test like this:
I get the point that irq_enabled is legacy. However the IOCTL call is not and usually
is used in time critical code to wait for vblank before starting the old buffers for
a new frame. At 60Hz that's probably less of a concern, but at 120Hz refresh rate and
reduced vblank time your time slice allocation for new work matters.
Best regards,
Liviu
>
> ifdef CONFIG_DRM_LEGACY
> if (unlikely(check_feature(dev, DRIVER_LEGACY))) {
> if (!irq_enabled)
> return;
> } else
> #endif
> {
> if (!has_vblank_support(dev))
> return;
> }
>
> As CONFIG_DRM_LEGACY is most likely disabled on concurrent systems, we'd get
> a single test for the modern drivers. If DRM_LEGACYis on, the compiler at
> least knows that the else branch is preferred.
>
> Best regards
> Thomas
>
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 Nürnberg, Germany
> (HRB 36809, AG Nürnberg)
> Geschäftsführer: Felix Imendörffer
>
--
====================
| I would like to |
| fix the world, |
| but they're not |
| giving me the |
\ source code! /
---------------
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