[Intel-gfx] [PATCH] drm/i915/pps: Reuse POWER_DOMAIN_DISPLAY_CORE in pps_{lock, unlock}

[Jani Nikula]

On Thu, 07 Jan 2021, Anshuman Gupta <anshuman.gupta at intel.com> wrote:
> We need a power_domain wakeref in pps_{lock,unlock} to prevent
> a race while resetting pps state in intel_power_sequencer_reset().
>
> intel_power_sequencer_reset() need a pps_mutex to access pps_pipe
> but it can't grab pps_mutex due to deadlock with power_well
> functions are called while holding pps_mutex.
> intel_power_sequencer_reset() is called by power_well function
> associated with legacy platforms like vlv and chv therefore re-use
> the POWER_DOMAIN_DISPLAY_CORE power domain, which only used
> by vlv and chv display power domain.
>
> This will avoids the unnecessary noise of unrelated power wells
> in pps_{lock,unlock}.
>
> Cc: Jani Nikula <jani.nikula at intel.com>
> Cc: Imre Deak <imre.deak at intel.com>
> Signed-off-by: Anshuman Gupta <anshuman.gupta at intel.com>

Imre convinced me yesterday on irc that this should work.

Reviewed-by: Jani Nikula <jani.nikula at intel.com>

On the surface, this reduces the need to enable/disable the aux power so
much. It's unnecessary, so it stands to reason to optimize it. We should
only grab the domain references we actually need.

However, this *also* papers over an issue we've been seeing [1]. We need
to be aware the root cause for that remains unknown, and needs to be
figured out.

I presume simply doing aux transfers won't reproduce the problem,
because that disables the power asynchronously since commit f39194a7a8b9
("drm/i915: Disable power asynchronously during DP AUX
transfers"). Perhaps we wouldn't have seen this at all if pps_unlock()
also did that as suggested in the commit message.

Anyway, I'd like to get acks or rb's from Imre and Ville before merging
this.


BR,
Jani.


[1] http://lore.kernel.org/r/20201204081845.26528-1-anshuman.gupta@intel.com


> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index 8a00e609085f..4f190a82d4ad 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -895,9 +895,7 @@ pps_lock(struct intel_dp *intel_dp)
>  	 * See intel_power_sequencer_reset() why we need
>  	 * a power domain reference here.
>  	 */
> -	wakeref = intel_display_power_get(dev_priv,
> -					  intel_aux_power_domain(dp_to_dig_port(intel_dp)));
> -
> +	wakeref = intel_display_power_get(dev_priv, POWER_DOMAIN_DISPLAY_CORE);
>  	mutex_lock(&dev_priv->pps_mutex);
>  
>  	return wakeref;
> @@ -909,9 +907,7 @@ pps_unlock(struct intel_dp *intel_dp, intel_wakeref_t wakeref)
>  	struct drm_i915_private *dev_priv = dp_to_i915(intel_dp);
>  
>  	mutex_unlock(&dev_priv->pps_mutex);
> -	intel_display_power_put(dev_priv,
> -				intel_aux_power_domain(dp_to_dig_port(intel_dp)),
> -				wakeref);
> +	intel_display_power_put(dev_priv, POWER_DOMAIN_DISPLAY_CORE, wakeref);
>  	return 0;
>  }

-- 
Jani Nikula, Intel Open Source Graphics Center