[PATCH] drm/msm: Disable frequency clamping on a630

[Akhil P Oommen]

On 9/10/2021 11:04 PM, Caleb Connolly wrote:
> 
> 
> On 10/09/2021 18:18, Rob Clark wrote:
>> On Tue, Sep 7, 2021 at 7:20 PM Bjorn Andersson
>> <bjorn.andersson at linaro.org> wrote:
>>>
>>> On Mon 09 Aug 10:26 PDT 2021, Akhil P Oommen wrote:
>>>
>>>> On 8/9/2021 9:48 PM, Caleb Connolly wrote:
>>>>>
>>>>>
>>>>> On 09/08/2021 17:12, Rob Clark wrote:
>>>>>> On Mon, Aug 9, 2021 at 7:52 AM Akhil P Oommen
>>>>>> <akhilpo at codeaurora.org> wrote:
>>> [..]
>>>>>>> I am a bit confused. We don't define a power domain for gpu in dt,
>>>>>>> correct? Then what exactly set_opp do here? Do you think this 
>>>>>>> usleep is
>>>>>>> what is helping here somehow to mask the issue?
>>>>> The power domains (for cx and gx) are defined in the GMU DT, the 
>>>>> OPPs in
>>>>> the GPU DT. For the sake of simplicity I'll refer to the lowest
>>>>> frequency (257000000) and OPP level (RPMH_REGULATOR_LEVEL_LOW_SVS) as
>>>>> the "min" state, and the highest frequency (710000000) and OPP level
>>>>> (RPMH_REGULATOR_LEVEL_TURBO_L1) as the "max" state. These are 
>>>>> defined in
>>>>> sdm845.dtsi under the gpu node.
>>>>>
>>>>> The new devfreq behaviour unmasks what I think is a driver bug, it
>>>>> inadvertently puts much more strain on the GPU regulators than they
>>>>> usually get. With the new behaviour the GPU jumps from it's min 
>>>>> state to
>>>>> the max state and back again extremely rapidly under workloads as 
>>>>> small
>>>>> as refreshing UI. Where previously the GPU would rarely if ever go 
>>>>> above
>>>>> 342MHz when interacting with the device, it now jumps between min and
>>>>> max many times per second.
>>>>>
>>>>> If my understanding is correct, the current implementation of the GMU
>>>>> set freq is the following:
>>>>>    - Get OPP for frequency to set
>>>>>    - Push the frequency to the GMU - immediately updating the core 
>>>>> clock
>>>>>    - Call dev_pm_opp_set_opp() which triggers a notify chain, this 
>>>>> winds
>>>>> up somewhere in power management code and causes the gx regulator 
>>>>> level
>>>>> to be updated
>>>>
>>>> Nope. dev_pm_opp_set_opp() sets the bandwidth for gpu and nothing 
>>>> else. We
>>>> were using a different api earlier which got deprecated -
>>>> dev_pm_opp_set_bw().
>>>>
>>>
>>> On the Lenovo Yoga C630 this is reproduced by starting alacritty and if
>>> I'm lucky I managed to hit a few keys before it crashes, so I spent a
>>> few hours looking into this as well...
>>>
>>> As you say, the dev_pm_opp_set_opp() will only cast a interconnect vote.
>>> The opp-level is just there for show and isn't used by anything, at
>>> least not on 845.
>>>
>>> Further more, I'm missing something in my tree, so the interconnect
>>> doesn't hit sync_state, and as such we're not actually scaling the
>>> buses. So the problem is not that Linux doesn't turn on the buses in
>>> time.
>>>
>>> So I suspect that the "AHB bus error" isn't saying that we turned off
>>> the bus, but rather that the GPU becomes unstable or something of that
>>> sort.
>>>
>>>
>>> Lastly, I reverted 9bc95570175a ("drm/msm: Devfreq tuning") and ran
>>> Aquarium for 20 minutes without a problem. I then switched the gpu
>>> devfreq governor to "userspace" and ran the following:
>>>
>>> while true; do
>>>    echo 257000000 > /sys/class/devfreq/5000000.gpu/userspace/set_freq
>>>    echo 710000000 > /sys/class/devfreq/5000000.gpu/userspace/set_freq
>>> done
>>>
>>> It took 19 iterations of this loop to crash the GPU.
>>
>> I assume you still had aquarium running, to keep the gpu awake while
>> you ran that loop?
>>
>> Fwiw, I modified this slightly to match sc7180's min/max gpu freq and
>> could not trigger any issue.. interestingly sc7180 has a lower min
>> freq (180) and higher max freq (800) so it was toggling over a wider
>> freq range.  I also tried on a device that  had the higher 825MHz opp
>> (since I noticed that was the only opp that used
>> RPMH_REGULATOR_LEVEL_TURBO_L1 and wanted to rule that out), but could
>> not reproduce.
>>
>> I guess a630 (sdm845) should have higher power draw (it is 2x # of
>> shader cores and 2x GMEM size, but lower max freq).. the question is,
>> is this the reason we see this on sdm845 and not sc7180?  Or is there
>> some other difference.  On the gpu side of this, they are both closely
>> related (ie. the same "sub-generation" of a6xx, same gmu fw, etc)..
>> I'm less sure about the other parts (icc, rpmh, etc)
> 
> My guess would be power draw, nobody has mentioned this yet but I've 
> realised that the vdd_gfx rail is powered by a buck converter, which 
> could explain a lot of the symptoms.
> 
> Buck converters depend on high frequency switching and inductors to 
> work, this inherently leads to some lag time when changing voltages, and 
> also means that the behaviour of the regulator is defined in part by how 
> much current is being drawn. Wikipedia has a pretty good explanation: 
> https://en.wikipedia.org/wiki/Buck_converter
> 
> At the best of times these regulators have a known voltage ripple, when 
> under load and when rapidly switching voltages this will get a lot worse.
> 
> Someone with an oscilloscope and schematics could probe the rail and 
> probably see exactly what's going on when the GPU crashes. Because of 
> the lag time in the regulator changing voltage, it might be 
> undershooting whilst the GPU is trying to clock up and draw more current 
> - causing instability and crashes.

Both of you are correct. The GPU is very similar including the GMU (we 
have same fw for both), except the GBIF block. As far as I am aware, the 
non-gpu blocks within SoC should be similar except the configs.

And yes, for these sort of issues where we suspect a power issue, gx 
rail should be probed for droops using a very high resolution 
oscilloscopes (these droops might last less than 1us).

I am aware of only Dragonboard that is still alive from QC perspective. 
Can someone report this issue to DB support team as it is fairly easy to 
reproduce?

-Akhil.

>>
>> BR,
>> -R
>>
>>> So the problem doesn't seem to be Rob's change, it's just that prior to
>>> it the chance to hitting it is way lower. Question is still what it is
>>> that we're triggering.
>>>
>>> Regards,
>>> Bjorn
>