Re: [PATCH] drm/i915: Improve PSR activation timing

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> On Feb 8, 2018, at 4:39 PM, Pandiyan, Dhinakaran <dhinakaran.pandiyan@xxxxxxxxx> wrote:
> 
> 
>> On Thu, 2018-02-08 at 14:48 -0800, Rodrigo Vivi wrote:
>> Hi Andy,
>> 
>> thanks for getting involved with PSR and sorry for not replying sooner.
>> 
>> I first saw this patch on that bugzilla entry but only now I stop to
>> really think why I have written the code that way.
>> 
>> So some clarity below.
>> 
>>> On Mon, Feb 05, 2018 at 10:07:09PM +0000, Andy Lutomirski wrote:
>>> The current PSR code has a two call sites that each schedule delayed
>>> work to activate PSR.  As far as I can tell, each call site intends
>>> to keep PSR inactive for the given amount of time and then allow it
>>> to be activated.
>>> 
>>> The call sites are:
>>> 
>>> - intel_psr_enable(), which explicitly states in a comment that
>>>   it's trying to keep PSR off a short time after the dispay is
>>>   initialized as a workaround.
>> 
>> First of all I really want to kill this call here and remove the
>> FIXME. It was an ugly hack that I added to solve a corner case
>> that was leaving me with blank screens when activating so sooner.
>> 
>>> 
>>> - intel_psr_flush().  There isn't an explcit explanation, but the
>>>   intent is presumably to keep PSR off until the display has been
>>>   idle for 100ms.
>> 
>> The reason for 100 is kind of ugly-nonsense-empirical value
>> I concluded from VLV/CHV experience.
>> On platforms with HW tracking HW waits few identical frames
>> until really activating PSR. VLV/CHV activation is immediate.
>> But HW is also different and there it seemed that hw needed a
>> few more time before starting the transitions.
>> Furthermore I didn't want to add that so quickly because I didn't
>> want to take the risk of killing battery with software tracking
>> when doing transitions so quickly using software tracking.
>> 
>>> 
>>> The current code doesn't actually accomplish either of these goals.
>>> Rather than keeping PSR inactive for the given amount of time, it
>>> will schedule PSR for activation after the given time, with the
>>> earliest target time in such a request winning.
>> 
>> Putting that way I was asking myself how that hack had ever fixed
>> my issue. Because the way you explained here seems obvious that it
>> wouldn't ever fix my bug or any other.
>> 
>> So I applied your patch and it made even more sense (without considering
>> the fact I want to kill the first call anyways).
>> 
>> So I came back, removed your patch and tried to understand how did
>> it ever worked.
>> 
>> So, the thing is that intel_psr_flush will never be really executed
>> if intel_psr_enable wasn't executed. That is guaranteed by:
>> 
>> mutex_lock(&dev_priv->psr.lock);
>>    if (!dev_priv->psr.enabled) {
>> 
>> So, intel_psr_enable will be for sure the first one to schedule the
>> work delayed to the ugly higher delay.
>> 
>>> 
>>> In other words, if intel_psr_enable() is immediately followed by
>>> intel_psr_flush(), then PSR will be activated after 100ms even if
>>> intel_psr_enable() wanted a longer delay.  And, if the screen is
>>> being constantly updated so that intel_psr_flush() is called once
>>> per frame at 60Hz, PSR will still be activated once every 100ms.
>> 
>> During this time you are right, many calls of intel_psr_exit
>> coming from flush functions can be called... But none of
>> them will schedule the work with 100 delay.
>> 
>> they will skip on
>> if (!work_busy(&dev_priv->psr.work.work))

As below, the first call will.  Then, 100ms later, the work will fire.  Then the next flush will schedule it again, etc.

> 
> Wouldn't work_busy() return false until the work is actually queued
> which is 100ms after calling schedule_delayed_work()?
> 
> For e.g, flushes at 0, 16, 32...96 will have work_busy() returning false
> until 100ms.
> 
> The first psr_work will end up getting scheduled at 100ms, which I
> believe is not what we want. 

Indeed.  I stuck some printks in and this seems to be what happens.

> 
> 
> However, I think 
> 
>    if (dev_priv->psr.busy_frontbuffer_bits)
>        goto unlock;
> 
>    intel_psr_activate(intel_dp);
> 
> in psr_work might prevent activate being called at 100ms if an
> invalidate happened to be called before that.
> 

On my system, invalidate is never called.  Even if it were called, that check would only help if we got lucky and the work fired after invalidate and before the subsequent flush.

> 
> 
> 
>> 
>> So, the higher delayed *hack* will be respected and PSR won't get
>> activated before that.
>> 
>> On the other hand you might ask what if,
>> for some strange reason,
>> (intel_dp->panel_power_cycle_delay * 5) is lesser than 100.
>> Well, on this case this would be ok, because it happens only
>> once and only on gen > 9 where hw tracking will wait the minimal
>> number of frames before the actual transition to PSR.
>> 
>> In either cases I believe we are safe.
>> 
>> Thanks,
>> Rodrigo.
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