Re: [PATCH] drm/i915: Drop pipe_enable checks in vblank funcs

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Daniel,

Thank you for the patch.

On Monday 26 January 2015 07:41:59 Daniel Vetter wrote:
> With Ville's rework to use drm_crtc_vblank_on/off the core will take
> care of rejecting drm_vblank_get calls when the pipe is off.

After debugging a related issue in the omapdrm driver I think this is only 
partially true. drm_vblank_off() will increase the vblank refcount, resulting 
in drm_vblank_get() returning an error instead of calling the driver's 
.enable_vblank operation. However, this only works if drm_vblank_off() has 
been called, not when the pipe is off because it hasn't been turned on yet.

Consider this case. The driver has just been loaded, all pipes are off. An 
application opens the DRM device node and closes it without enabling the pipe. 
The lastclose operation will be called, and it will call 
drm_fb_helper_restore_fbdev_mode_unlocked() to restore the CRTC. This triggers 
the timeout warning in drm_wait_one_vblank with omapdrm when using the atomic 
update transitional helpers. The callstack is as follows.

[<bf3b46f8>] (drm_wait_one_vblank [drm])
[<bf429e50>] (drm_plane_helper_commit [drm_kms_helper])
[<bf427ab0>] (drm_crtc_helper_set_mode [drm_kms_helper])
[<bf428570>] (drm_crtc_helper_set_config [drm_kms_helper])
[<bf3baa34>] (drm_mode_set_config_internal [drm])
[<bf431374>] (restore_fbdev_mode [drm_kms_helper])
[<bf432978>] (drm_fb_helper_restore_fbdev_mode_unlocked [drm_kms_helper])
[<bf45d4f4>] (dev_lastclose [omapdrm])
[<bf3b1218>] (drm_lastclose [drm])
[<bf3b15e0>] (drm_release [drm])
[<c0149104>] (__fput)
[<c00571d4>] (task_work_run)
[<c00114f8>] (do_work_pending)

The drm_crtc_vblank_get() call in drm_plane_helper_commit() will not return an 
error as drm_vblank_off() has never been called so far.

The exact same issue obviously doesn't affect the i915 driver as it doesn't 
use the transitional helpers, but a different issue could be present with the 
same root cause.

Where do you think this should be fixed ?

> Also the core won't call the get_vblank_counter hooks in that case either.
> And since we've dropped ums support recently we can now remove these
> hacks, yay!
> 
> Noticed while trying to answer questions Laurent had about how the new
> atomic helpers work.
>
> Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_irq.c | 52 --------------------------------------
>  1 file changed, 52 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_irq.c
> b/drivers/gpu/drm/i915/i915_irq.c index 2399eaed2ca3..4761f7fe6fb1 100644
> --- a/drivers/gpu/drm/i915/i915_irq.c
> +++ b/drivers/gpu/drm/i915/i915_irq.c
> @@ -492,31 +492,6 @@ static void i915_enable_asle_pipestat(struct drm_device
> *dev) spin_unlock_irq(&dev_priv->irq_lock);
>  }
> 
> -/**
> - * i915_pipe_enabled - check if a pipe is enabled
> - * @dev: DRM device
> - * @pipe: pipe to check
> - *
> - * Reading certain registers when the pipe is disabled can hang the chip.
> - * Use this routine to make sure the PLL is running and the pipe is active
> - * before reading such registers if unsure.
> - */
> -static int
> -i915_pipe_enabled(struct drm_device *dev, int pipe)
> -{
> -	struct drm_i915_private *dev_priv = dev->dev_private;
> -
> -	if (drm_core_check_feature(dev, DRIVER_MODESET)) {
> -		/* Locking is horribly broken here, but whatever. */
> -		struct drm_crtc *crtc = dev_priv->pipe_to_crtc_mapping[pipe];
> -		struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
> -
> -		return intel_crtc->active;
> -	} else {
> -		return I915_READ(PIPECONF(pipe)) & PIPECONF_ENABLE;
> -	}
> -}
> -
>  /*
>   * This timing diagram depicts the video signal in and
>   * around the vertical blanking period.
> @@ -583,12 +558,6 @@ static u32 i915_get_vblank_counter(struct drm_device
> *dev, int pipe) unsigned long low_frame;
>  	u32 high1, high2, low, pixel, vbl_start, hsync_start, htotal;
> 
> -	if (!i915_pipe_enabled(dev, pipe)) {
> -		DRM_DEBUG_DRIVER("trying to get vblank count for disabled "
> -				"pipe %c\n", pipe_name(pipe));
> -		return 0;
> -	}
> -
>  	if (drm_core_check_feature(dev, DRIVER_MODESET)) {
>  		struct intel_crtc *intel_crtc =
>  			to_intel_crtc(dev_priv->pipe_to_crtc_mapping[pipe]);
> @@ -648,12 +617,6 @@ static u32 gm45_get_vblank_counter(struct drm_device
> *dev, int pipe) struct drm_i915_private *dev_priv = dev->dev_private;
>  	int reg = PIPE_FRMCOUNT_GM45(pipe);
> 
> -	if (!i915_pipe_enabled(dev, pipe)) {
> -		DRM_DEBUG_DRIVER("trying to get vblank count for disabled "
> -				 "pipe %c\n", pipe_name(pipe));
> -		return 0;
> -	}
> -
>  	return I915_READ(reg);
>  }
> 
> @@ -2660,9 +2623,6 @@ static int i915_enable_vblank(struct drm_device *dev,
> int pipe) struct drm_i915_private *dev_priv = dev->dev_private;
>  	unsigned long irqflags;
> 
> -	if (!i915_pipe_enabled(dev, pipe))
> -		return -EINVAL;
> -
>  	spin_lock_irqsave(&dev_priv->irq_lock, irqflags);
>  	if (INTEL_INFO(dev)->gen >= 4)
>  		i915_enable_pipestat(dev_priv, pipe,
> @@ -2682,9 +2642,6 @@ static int ironlake_enable_vblank(struct drm_device
> *dev, int pipe) uint32_t bit = (INTEL_INFO(dev)->gen >= 7) ?
> DE_PIPE_VBLANK_IVB(pipe) : DE_PIPE_VBLANK(pipe);
> 
> -	if (!i915_pipe_enabled(dev, pipe))
> -		return -EINVAL;
> -
>  	spin_lock_irqsave(&dev_priv->irq_lock, irqflags);
>  	ironlake_enable_display_irq(dev_priv, bit);
>  	spin_unlock_irqrestore(&dev_priv->irq_lock, irqflags);
> @@ -2697,9 +2654,6 @@ static int valleyview_enable_vblank(struct drm_device
> *dev, int pipe) struct drm_i915_private *dev_priv = dev->dev_private;
>  	unsigned long irqflags;
> 
> -	if (!i915_pipe_enabled(dev, pipe))
> -		return -EINVAL;
> -
>  	spin_lock_irqsave(&dev_priv->irq_lock, irqflags);
>  	i915_enable_pipestat(dev_priv, pipe,
>  			     PIPE_START_VBLANK_INTERRUPT_STATUS);
> @@ -2713,9 +2667,6 @@ static int gen8_enable_vblank(struct drm_device *dev,
> int pipe) struct drm_i915_private *dev_priv = dev->dev_private;
>  	unsigned long irqflags;
> 
> -	if (!i915_pipe_enabled(dev, pipe))
> -		return -EINVAL;
> -
>  	spin_lock_irqsave(&dev_priv->irq_lock, irqflags);
>  	dev_priv->de_irq_mask[pipe] &= ~GEN8_PIPE_VBLANK;
>  	I915_WRITE(GEN8_DE_PIPE_IMR(pipe), dev_priv->de_irq_mask[pipe]);
> @@ -2767,9 +2718,6 @@ static void gen8_disable_vblank(struct drm_device
> *dev, int pipe) struct drm_i915_private *dev_priv = dev->dev_private;
>  	unsigned long irqflags;
> 
> -	if (!i915_pipe_enabled(dev, pipe))
> -		return;
> -
>  	spin_lock_irqsave(&dev_priv->irq_lock, irqflags);
>  	dev_priv->de_irq_mask[pipe] |= GEN8_PIPE_VBLANK;
>  	I915_WRITE(GEN8_DE_PIPE_IMR(pipe), dev_priv->de_irq_mask[pipe]);

-- 
Regards,

Laurent Pinchart

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