Re: [PATCH] drm/i915: Avoid vblank counter for gen9+

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Feb 11, 2016 at 09:00:47AM -0800, Rodrigo Vivi wrote:
> Framecounter register is read-only so DMC cannot restore it
> after exiting DC5 and DC6.
> 
> Easiest way to go is to avoid the counter and use vblank
> interruptions for this platform and for all the following
> ones since DMC came to stay. At least while we can't change
> this register to read-write.
> 
> Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

Now my comments also in public:
- Do we still get reasonable dc5 residency with this - it means we'll keep
  vblank irq running forever.

- I'm a bit unclear on what exactly this fixes - have you tested that
  long-lasting vblank waits are still accurate? Just want to make sure we
  don't just paper over the issue and desktops can still get stuck waiting
  for a vblank.

Just a bit suprised that the only problem is the framecounter, and not
that vblanks stop happening too.

We need to also know these details for the proper fix, which will involve
grabbing power well references (might need a new one for vblank
interrupts) to make sure.

Cheers, Daniel

> ---
>  drivers/gpu/drm/i915/i915_irq.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
> index 25a8937..c294a4b 100644
> --- a/drivers/gpu/drm/i915/i915_irq.c
> +++ b/drivers/gpu/drm/i915/i915_irq.c
> @@ -4556,7 +4556,10 @@ void intel_irq_init(struct drm_i915_private *dev_priv)
>  
>  	pm_qos_add_request(&dev_priv->pm_qos, PM_QOS_CPU_DMA_LATENCY, PM_QOS_DEFAULT_VALUE);
>  
> -	if (IS_GEN2(dev_priv)) {
> +	if (INTEL_INFO(dev_priv)->gen >= 9) {
> +		dev->max_vblank_count = 0;
> +		dev->driver->get_vblank_counter = g4x_get_vblank_counter;
> +	} else if (IS_GEN2(dev_priv)) {
>  		dev->max_vblank_count = 0;
>  		dev->driver->get_vblank_counter = i8xx_get_vblank_counter;
>  	} else if (IS_G4X(dev_priv) || INTEL_INFO(dev_priv)->gen >= 5) {
> @@ -4572,7 +4575,7 @@ void intel_irq_init(struct drm_i915_private *dev_priv)
>  	 * Gen2 doesn't have a hardware frame counter and so depends on
>  	 * vblank interrupts to produce sane vblank seuquence numbers.
>  	 */
> -	if (!IS_GEN2(dev_priv))
> +	if (!IS_GEN2(dev_priv) && !INTEL_INFO(dev_priv)->gen >= 9)
>  		dev->vblank_disable_immediate = true;
>  
>  	dev->driver->get_vblank_timestamp = i915_get_vblank_timestamp;
> -- 
> 2.4.3
> 
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> Intel-gfx@lists.freedesktop.org
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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