[Nouveau] Addressing the problem of noisy GPUs under Nouveau

[John Hubbard]

On 11/23/2017 02:48 PM, Martin Peres wrote:
> On 23/11/17 10:06, John Hubbard wrote:
>> On 11/22/2017 05:07 PM, Martin Peres wrote:
>>> Hey,
>>>
>>> Thanks for your answer, Andy!
>>>
>>> On 22/11/17 04:06, Ilia Mirkin wrote:
>>>> On Tue, Nov 21, 2017 at 8:29 PM, Andy Ritger <aritger at nvidia.com> wrote:
>>>> Martin's question was very long, but it boils down to this:
>>>>
>>>> How do we compute the correct values to write into the e114/e118 pwm
>>>> registers based on the VBIOS contents and current state of the board
>>>> (like temperature).
>>>
>>> Unfortunately, it can also be the e11c/e120 couple, or 0x200d8/dc on
>>> GF119+, or 0x200cd/d0 on Kepler+.
>>>
>>> At least, it looks like we know which PWM controler we need to drive, so
>>> I did not want to muddy the water even more by giving register
>>> addresses, rather concentrating on the problem at hand: How to compute
>>> the duty value for the PWM controler.
>>>
>>>>
>>>> We generally do this right, but appear to get it extra-wrong for certain GPUs.
>>>
>>> Yes... So far, we are always safe, but users tend to mind when their
>>> computer sound like a jumbo jet at take off... Who would have thought? :D
>>>
>>> Anyway, looking forward to your answer!
>>>
>>> Cheers,
>>> Martin
>>
>>
>> Hi Martin,
>>
>> One of our firmware engineers thinks that this looks a lot like PWM inversion.
>> For some SKUs, the interpretation of the PWM duty cycle is inverted. That 
>> would probably make it *very* difficult to find a sensible algorithm that 
>> covered all the SKUs, given that some are inverted and others are not.
>>
>> For the noisy GPUs, a very useful experiment would be to try inverting it, 
>> like this:
>>
>> 	pwmDutyCycle = pwmPeriod - pwmDutyCycle;
>>
>> ...and then see if fan control starts behaving closer to how you've actually 
>> programmed it.
>>
>> Would that be easy enough to try out? It should help narrow down the
>> problem at least.
>>
> 
> Hey John,
> 
> Unfortunately, we know about PWM inversion, and one can know which mode
> to use based on the GPIO entry associated to the fan (inverted). We have
> had support for this in Nouveau for a long time. At the very least, this
> is not the problem on my GF108.
> 
> I am certain that the problem I am seeing is related to this vbios table
> I wrote about (BIT P, offset 0x18). It is used to compute what PWM duty
> I should use for both 0 and 100% of the fan speed.
> 
> Computing the value for 0% fan speed is difficult because of
> non-continuous nature of some of the functions[1], but I can always
> over-approximate. However, I failed to accurately compute the duty I
> need to write to get the 100% fan speed (I have cases where I greatly
> over-estimate it...).
> 
> Could you please check out the vbios table I am pointing at? I am quite
> sure that your documentation will be clearer than my babbling :D

Yes. We will check on this. There has been some productive discussion 
internally, but it will take some more investigation.

thanks,
John Hubbard


> 
> Thanks,
> Martin
> 
> [1] http://fs.mupuf.org/nvidia/fan_calib/pwm_offset.png
>