Re: [PATCH] drm/i915: Wait for PP cycle delay only if panel is in power off sequence

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Dec 09, 2015 at 08:59:26PM +0530, Shobhit Kumar wrote:
> On Wed, Dec 9, 2015 at 8:34 PM, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> > On Wed, Dec 09, 2015 at 08:07:10PM +0530, Shobhit Kumar wrote:
> >> On Wed, Dec 9, 2015 at 7:27 PM, Ville Syrjälä
> >> <ville.syrjala@linux.intel.com> wrote:
> >> > On Wed, Dec 09, 2015 at 06:51:48PM +0530, Shobhit Kumar wrote:
> >> >> During resume, while turning the EDP panel power on, we need not wait
> >> >> blindly for panel_power_cycle_delay. Check if panel power down sequence
> >> >> in progress and then only wait. This improves our resume time significantly.
> >> >>
> >> >> Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
> >> >> ---
> >> >>  drivers/gpu/drm/i915/intel_dp.c | 17 ++++++++++++++++-
> >> >>  1 file changed, 16 insertions(+), 1 deletion(-)
> >> >>
> >> >> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> >> >> index f335c92..10ec669 100644
> >> >> --- a/drivers/gpu/drm/i915/intel_dp.c
> >> >> +++ b/drivers/gpu/drm/i915/intel_dp.c
> >> >> @@ -617,6 +617,20 @@ static bool edp_have_panel_power(struct intel_dp *intel_dp)
> >> >>       return (I915_READ(_pp_stat_reg(intel_dp)) & PP_ON) != 0;
> >> >>  }
> >> >>
> >> >> +static bool edp_panel_off_seq(struct intel_dp *intel_dp)
> >> >> +{
> >> >> +     struct drm_device *dev = intel_dp_to_dev(intel_dp);
> >> >> +     struct drm_i915_private *dev_priv = dev->dev_private;
> >> >> +
> >> >> +     lockdep_assert_held(&dev_priv->pps_mutex);
> >> >> +
> >> >> +     if (IS_VALLEYVIEW(dev) &&
> >> >> +         intel_dp->pps_pipe == INVALID_PIPE)
> >> >> +             return false;
> >> >> +
> >> >> +     return (I915_READ(_pp_stat_reg(intel_dp)) & PP_SEQUENCE_POWER_DOWN) != 0;
> >> >> +}
> >> >
> >> > This doens't make sense to me. The power down cycle may have
> >> > completed just before, and so this would claim we don't have to
> >> > wait for the power_cycle_delay.
> >>
> >> Not sure I understand your concern correctly. You are right, power
> >> down cycle may have completed just before and if it has then we don't
> >> need to wait. But in case the power down cycle is in progress as per
> >> internal state, then we need to wait for it to complete. This will
> >> happen for example in non-suspend disable path and will be handled
> >> correctly. In case of actual suspend/resume, this would have
> >> successfully completed and will skip the wait as it is not needed
> >> before enabling panel power.
> >>
> >> >
> >> >> +
> >> >>  static bool edp_have_panel_vdd(struct intel_dp *intel_dp)
> >> >>  {
> >> >>       struct drm_device *dev = intel_dp_to_dev(intel_dp);
> >> >> @@ -2025,7 +2039,8 @@ static void edp_panel_on(struct intel_dp *intel_dp)
> >> >>                port_name(dp_to_dig_port(intel_dp)->port)))
> >> >>               return;
> >> >>
> >> >> -     wait_panel_power_cycle(intel_dp);
> >> >> +     if (edp_panel_off_seq(intel_dp))
> >> >> +             wait_panel_power_cycle(intel_dp);
> >
> > Looking in from the side, I have no idea what this is meant to do. At
> > the very least you need your explanatory paragraph here which would
> > include what exactly you are waiting for at the start of edp_panel_on
> > (and please try and find a better name for edp_panel_off_seq()).
> 
> I will add a comment. Basically I am not additionally waiting, but
> converting the wait which was already there to a conditional wait. The
> edp_panel_off_seq, checks if panel power down sequence is in progress.
> In that case we need to wait for the panel power cycle delay. If it is
> not in that sequence, there is no need to wait. I will make an attempt
> again on the naming in next patch update.

As far I remeber you need to wait for power_cycle_delay between power
down cycle and power up cycle. You're trying to throw that wait away
entirely, unless the function happens get called while the power down
cycle is still in progress. We should already optimize away redundant
waits by tracking the end of the power down cycle with the jiffies
tracking.

Actually looking at the code the power_cycle_delay gets counted from
the start of the last power down cycle, so supposedly it's always at
least as long as the power down cycle, and typically it's quite a bit
longer that that. But that doesn't change the fact that you can't
just skip it because the power down cycle delay happened to end
already.

So what we do now is:
1. initiate power down cycle
2. last_power_cycle=jiffies
3. wait for power down (I suppose this actually waits
   until the power down delay has passed since that's
   programmes into the PPS).
4. wait for power_cycle_delay from last_power_cycle
5. initiate power up cycle

I think with your patch step 4 would always be skipped since the
power down cycle has already ended, and then we fail to honor the
power cycle delay.

Actually the power_cycle delay also gets programmed into the PPS so I
supose it would enforce the wait anyway when you initiate the power
up cycle (unless the PPS got totally reset due to power wells etc.,
which does seem like a real concern. The even bigger concern is the
vdd force bit for which the PPS does no enforcement.

The power_down_delay handling seems a bit wonky. We only wait for it
when turning off the port. I guess I would need to go re-read the spec
to figure out what it's meant to protect anyway.

-- 
Ville Syrjälä
Intel OTC
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