[Nouveau] Addressing the problem of noisy GPUs under Nouveau

[Martin Peres]

On 07/02/18 05:31, John Hubbard wrote:
> On 01/28/2018 04:05 PM, Martin Peres wrote:
>> On 29/01/18 01:24, Martin Peres wrote:
>>> 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,
> 
> I strongly suspect you are seeing a special behavior, which is: on
> some GF108 boards we use only a very limited range of PWM,
> 0.4 to 2.5%, due to the particular type of DC power conversion
> circuit on those boards. However, it could also just be difficulties
> in interpreting the fixed-point variables in the tables. In either
> case, the answer is to explain those formats, so I'll do that now.
> 
> I am attaching the fan cooler table, in HTML format. We have also
> published the BIT (BIOS Information Table) format, separately:
> 
>     http://download.nvidia.com/open-gpu-doc/BIOS-Information-Table/1/BIOS-Information-Table.html
> 
> , but I don't think it has any surprises for you, in this regard. You
> can check it, to be sure you're looking at the right subtable, though,
> just in case.
> 
> The interesting parts of that table are:
> 
> PWM Scale Slope (16 bits):
> 
>   Slope to scale effective PWM to actual PWM (1/4096, F4.12, signed).
>   For backwards compatibility, a value of 0.0 (0x0000) is interpreted as 1.0 (0x1000).
>   This value is used to scale the effective PWM duty cycle, a conceptual fraction
>   of full speed (0% to 100%), to the actual electrical PWM duty cycle.
>   PWM(actual) = Slope × PWM(effective) + Offset
> 
> PWM Scale Offset (16 bits):
> 
>   Offset to scale effective PWM to actual PWM (1/4096, F4.12, signed).
>   This value is used to scale the effective PWM duty cycle, a conceptual fraction
>   of full speed (0% to 100%), to the actual electrical PWM duty cycle.
>   PWM(actual) = Slope × PWM(effective) + Offset
> 
> 
> However, the calculations are hard to get right, and the table stores
> values in fixed-point format, so I'm showing a few simplified code excerpts
> that use these. The various fixed point macro definitions are found as part of
> our normal driver package, in nvmisc.h and nvtypes.h. Any other definitions
> that you need are included right here (I ran a quick compiler check to be sure.)

Wow John, thanks a lot! Sorry for the delay, I was on vacation when you
posted this, but this definitely is what I was looking for!

Thanks a lot for the code example, I will try to make use of it soon and
come back to you if I still have issues!

Martin