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:

quoted

Hello,

On Tue, Jun 22, 2021 at 04:09:44PM +0200, Thomas Zimmermann wrote:

quoted

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@suse.de>
---
  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!  /
  ---------------
    ¯\_(ツ)_/¯

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