PWM-based voltage management input clock

PWM-based voltage management input clock

[Martin Peres]

Hello,

We recently reverse engineered PWM-based voltage management but found an 
odity in the input clock frequency for the PWM controller. It seems like 
RM is assuming an input clock of 27.648 MHz instead of the actual 27MHz 
crystal found on the board.

I thus have the following questions:
  - Is it a fixed value hardcoded in RM? (Current way it is done in Nouveau)
  - Should we use crystal_MHz * 1.024?

I could spend some time hooking up my osciloscope to the VR to check if 
it is a bringup issue but it would not help Nouveau much as we need to 
reproduce the behaviour of RM anyway since this is the only validated 
configuration.

Thanks in advance,
Martin

PS: The registers for this PWM controller are 0x20340 and 0x20344.
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Re: PWM-based voltage management input clock

[Andy Ritger]

Thanks, Martin.  I'll try to follow up on this next week and get you an answer.  What GPUs have you observed this on?


On Fri, Sep 18, 2015 at 11:13:53PM +0300, Martin Peres wrote:
> Hello,
> 
> We recently reverse engineered PWM-based voltage management but found an
> odity in the input clock frequency for the PWM controller. It seems like RM
> is assuming an input clock of 27.648 MHz instead of the actual 27MHz crystal
> found on the board.
> 
> I thus have the following questions:
>  - Is it a fixed value hardcoded in RM? (Current way it is done in Nouveau)
>  - Should we use crystal_MHz * 1.024?
> 
> I could spend some time hooking up my osciloscope to the VR to check if it
> is a bringup issue but it would not help Nouveau much as we need to
> reproduce the behaviour of RM anyway since this is the only validated
> configuration.
> 
> Thanks in advance,
> Martin
> 
> PS: The registers for this PWM controller are 0x20340 and 0x20344.
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Re: PWM-based voltage management input clock

[Martin Peres]

On 18/09/15 23:30, Andy Ritger wrote:
> Thanks, Martin.  I'll try to follow up on this next week and get you an answer.  What GPUs have you observed this on?
>
Thanks Andy,

As far as I can tell, it is the case for all GPUs using the PWM-based 
voltage management. To be more specific, we have seen this behaviour on 
a GK106 (mobile), a GM117 and a GM204.
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Re: PWM-based voltage management input clock

[Martin Peres]

On 19/09/15 19:21, Martin Peres wrote:
> On 18/09/15 23:30, Andy Ritger wrote:
>> Thanks, Martin.  I'll try to follow up on this next week and get you 
>> an answer.  What GPUs have you observed this on?
>>
> Thanks Andy,
>
> As far as I can tell, it is the case for all GPUs using the PWM-based 
> voltage management. To be more specific, we have seen this behaviour 
> on a GK106 (mobile), a GM117 and a GM204.

I guess this has fell into the cracks :) Any updates on this?

Thanks,
Martin
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