[Intel-gfx] [PATCH 3/7] drm/i915/psr: Make idle_frames sensible again

[Rodrigo Vivi]

On Wed, May 18, 2016 at 10:46 AM, Ville Syrjälä
<ville.syrjala at linux.intel.com> wrote:
> On Wed, May 18, 2016 at 06:47:12PM +0200, Daniel Vetter wrote:
>> This reverts
>>
>> commit dfaf37baa07513d2c37afff79978807d2d10221a
>> Author: Rodrigo Vivi <rodrigo.vivi at intel.com>
>> Date:   Mon Dec 7 14:45:20 2015 -0800
>>
>>     drm/i915: Fix idle_frames counter.
>>
>> and
>>
>> commit 97173eaf5f33b1e85efdb06d593d333480b60bf3
>> Author: Rodrigo Vivi <rodrigo.vivi at intel.com>
>> Date:   Tue Jul 7 16:28:55 2015 -0700
>>
>>     drm/i915: PSR: Increase idle_frames
>>
>> and implements
>>
>> commit d44b4dcbd1b44737462b77971d216d21a9413341
>> Author: Rodrigo Vivi <rodrigo.vivi at intel.com>
>> Date:   Fri Nov 14 08:52:31 2014 -0800
>>
>>     drm/i915: HSW/BDW PSR Set idle_frames = VBT + 1
>>
>> without the hack to use 2 idle frames when VBT says 1. We keep the + 1
>> just for safety, although I haven't really figured out why that one
>> exists.
>>
>> It's nonsense. idle_frames = number of frames where the screen is
>> entirely idle before we think about entering PSR.
>>
>> idle_patter = part of link training, and we probably totally butchered
>> link training because we told the hw to entirely skip it. No wonder
>> PSR occasionally just fell over.
>>
>> I suspect the reason we've increased idle frames is that it makes PSR
>> entry slightly less likely, and more likely to happen in a quite
>> system, which probably increased the changes the panel came back up
>> without link training. The proper fix is to implement link training
>> for PSR.
>
> Quite the mess there. At the least this makes things look a bit more
> sane.
>
> FWIW
> Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
>
>>
>> Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
>> Cc: Sonika Jindal <sonika.jindal at intel.com>
>> Cc: Durgadoss R <durgadoss.r at intel.com>
>> Cc: "Pandiyan, Dhinakaran" <dhinakaran.pandiyan at intel.com>
>> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
>> ---
>>  drivers/gpu/drm/i915/intel_psr.c | 14 +++++++-------
>>  1 file changed, 7 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/intel_psr.c b/drivers/gpu/drm/i915/intel_psr.c
>> index a788d1e9589b..0295d8dd483f 100644
>> --- a/drivers/gpu/drm/i915/intel_psr.c
>> +++ b/drivers/gpu/drm/i915/intel_psr.c
>> @@ -272,14 +272,14 @@ static void hsw_psr_enable_source(struct intel_dp *intel_dp)
>>       struct drm_i915_private *dev_priv = dev->dev_private;
>>
>>       uint32_t max_sleep_time = 0x1f;
>> -     /*
>> -      * Let's respect VBT in case VBT asks a higher idle_frame value.
>> -      * Let's use 6 as the minimum to cover all known cases including
>> -      * the off-by-one issue that HW has in some cases. Also there are
>> -      * cases where sink should be able to train
>> -      * with the 5 or 6 idle patterns.
>> +     /* Lately it was identified that depending on panel idle frame count
>> +      * calculated at HW can be off by 1. So let's use what came
>> +      * from VBT + 1.
>> +      * There are also other cases where panel demands at least 4
>> +      * but VBT is not being set. To cover these 2 cases lets use
>> +      * at least 5 when VBT isn't set to be on the safest side.
>>        */
>> -     uint32_t idle_frames = max(6, dev_priv->vbt.psr.idle_frames);
>> +     uint32_t idle_frames = dev_priv->vbt.psr.idle_frames + 1;

Hm, I believe this is dangerous. First because this comment above
explain that in the past we found cases where panel demands 4 idles
but VBT was incorrectly unset. Also because at some point HW guys
found out they had a bug on the idle frames calculation so they were
off by one, so setting it to one would mean in some cases HW would use
0 and start entering PSR when it shouldn't yet.

But yeap, I know at least nowadays our sw tracking helper could help
in this case I believed we should stat on the safest side here...

>>       uint32_t val = EDP_PSR_ENABLE;
>>
>>       val |= max_sleep_time << EDP_PSR_MAX_SLEEP_TIME_SHIFT;
>> --
>> 2.8.1
>>
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>
> --
> Ville Syrjälä
> Intel OTC
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-- 
Rodrigo Vivi
Blog: http://blog.vivi.eng.br