[Nouveau] Addressing the problem of noisy GPUs under Nouveau

[Martin Peres]

On 28/11/17 07:32, John Hubbard wrote:
> 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

Have the productive discussions panned out?

Thanks in advance,
Martin