Re: [PATCH] drm/i915: Assert we hold the CRTC powerwell for generating vblank interrupts

[Maarten Lankhorst <maarten.lankhorst@linux.intel.com>]

Op 11-10-16 om 10:32 schreef Ville Syrjälä:
> On Tue, Oct 11, 2016 at 09:45:45AM +0200, Maarten Lankhorst wrote:
>> Op 11-10-16 om 08:55 schreef Ville Syrjälä:
>>> On Tue, Oct 11, 2016 at 08:17:22AM +0200, Maarten Lankhorst wrote:
>>>> Op 10-10-16 om 13:56 schreef Ville Syrjälä:
>>>>> On Mon, Oct 10, 2016 at 12:46:32PM +0100, Chris Wilson wrote:
>>>>>> On Mon, Oct 10, 2016 at 02:42:01PM +0300, Ville Syrjälä wrote:
>>>>>>> On Mon, Oct 10, 2016 at 12:34:54PM +0100, Chris Wilson wrote:
>>>>>>>> To enable the vblank itself, we need to have an RPM wakeref for the mmio
>>>>>>>> access, and whilst generating the vblank interrupts we continue to
>>>>>>>> require the rpm wakeref. The assumption is that the RPM wakeref is held
>>>>>>>> by the display powerwell held by the active pipe. As this chain was not
>>>>>>>> obvious to me chasing the drm_wait_vblank ioctl, document it with a WARN
>>>>>>>> during *_vblank_enable().
>>>>>>>>
>>>>>>>> v2: Check the display power well rather than digging inside the atomic
>>>>>>>> CRTC state.
>>>>>>>>
>>>>>>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>>>>>>>> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
>>>>>>>> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
>>>>>>>> ---
>>>>>>>>  drivers/gpu/drm/i915/i915_irq.c | 20 ++++++++++++++++++++
>>>>>>>>  1 file changed, 20 insertions(+)
>>>>>>>>
>>>>>>>> diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
>>>>>>>> index 1e43fe30da11..f0f17055dbb9 100644
>>>>>>>> --- a/drivers/gpu/drm/i915/i915_irq.c
>>>>>>>> +++ b/drivers/gpu/drm/i915/i915_irq.c
>>>>>>>> @@ -2715,6 +2715,14 @@ void i915_handle_error(struct drm_i915_private *dev_priv,
>>>>>>>>  	i915_reset_and_wakeup(dev_priv);
>>>>>>>>  }
>>>>>>>>  
>>>>>>>> +static void assert_pipe_is_awake(struct drm_i915_private *dev_priv,
>>>>>>>> +				 enum pipe pipe)
>>>>>>>> +{
>>>>>>>> +	WARN_ON(IS_ENABLED(CONFIG_DRM_I915_DEBUG) &&
>>>>>>>> +		!intel_display_power_is_enabled(dev_priv,
>>>>>>>> +						POWER_DOMAIN_PIPE(pipe)));
>>>>>>> Uses a mutex. And having a power well enabled doesn't mean much. It
>>>>>>> doesn't guarantee that vblanks work.
>>>>>> Impasse. :|
>>>>>>
>>>>>> There should be no point in an explicit assert_rpm_wakeref here as the
>>>>>> register access should catch an error there. Is there no safe way we can
>>>>>> assert the current state of the CRTC is correct for enabling vblanks?
>>>>> crtc->active might be the closest thing, if we just ignore any locking.
>>>>> Though it looks like that has gone a bit mad these days, what with being
>>>>> set from the .crtc_enable() hooks but getting cleared outside the
>>>>> .crtc_disable() hooks.
>>>>>
>>>> I'm trying to kill crtc->active.
>>> Because it's evil? I still don't see much problem in having a thing to
>>> track the state of each pipe fairly accurately.
>>>
>>>> Maybe you'd want to use dev_priv->active_crtcs, but that won't save you if you enable interrupts in between atomic commit and .crtc_disable
>>> Nothing atomic based will work well because the state is not flipped at
>>> the same time as the actual hardware state changes.
>>>
>>>> Safest bet is to look at the power state I think.
>>> I don't know which power state you mean, but I already shot down the
>>> power domain thing.
>>>
>>>
>> I would say use assert_pipe_enabled then.
> Nope. That one frobs with power domains too these days.
>
I don't see a nice way to do this, it probably means we shouldn't do this at all..
Maybe have a function look at dev_priv->power_domains.domain_use_count[[POWER_DOMAIN_PIPE(pipe)] >= 1?
It's potentially racy but I don't see a nice way to check, apart from hoping we handle the drm vblank on/off
calls correctly.

The only other way I see is demidlayering vblank.

~Maarten

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