mainline kernel: cpufreq for bcm2835

[Stefan Wahren]

Hi Sergey,

> Sergey Suloev <ssuloev at orpaltech.com> hat am 2. April 2018 um 19:14 geschrieben:
> 
> 
> On 04/02/2018 07:09 PM, Stefan Wahren wrote:
> > Hi Sergey,
> >
> > [add linux-rpi-kernel]
> >
> >> Sergey Suloev <ssuloev at orpaltech.com> hat am 2. April 2018 um 17:45 geschrieben:
> >>
> >>
> >> Hi Stefan
> >>
> >> can any give any hint about the status of bcn2835 cpufreq in mainline
> >> kernel ?
> > in short: currently no support yet
> >
> >> Are you (or anyone else) working on it ?
> > No, this not on my TODO list [1].
> >
> > Current prio:
> > - Raspberry Pi 3+
> > - fixes and improvements for Raspberry Pi 3 DT
> > - VCHIQ data corruption
> >
> > Implementing/Porting the cpufreq driver is only half of the work. Also all drivers which depend on the VPU core clock should be able to handle a frequency change (sdhost, i2c, aux uart comes to my mind). But i will be happy if somebody takes care of it.
> >
> > Is this a critical feature for you?
> >
> > Best regards
> > Stefan
> >
> > [1] - https://github.com/lategoodbye/rpi-zero/issues
> >
> >>
> >> Thanks
> >>
> well, that seems to be a lot of work.

maybe less than you think (apart from testing).

> 
> Is the following patch an attempt of porting the driver ?
> 
> http://lists.infradead.org/pipermail/linux-rpi-kernel/2015-October/002402.html

No, this isn't a good starting point (please look at Stephen's comment, which still apply).

My suggestion would be to use or extend the cpufreq-dt. I'm not sure that we can you use clk-bcm2835 or need a separate clock driver (via mailbox) which changes the VPU clock.

@Eric: What's your opinion?

The advantages of this approach:
- no new binding required
- in good case no cpufreq driver is necessary
- chance of acceptance in mainline is very high

AFAIK there is only a need to register the cpufreq-dt within the bcm2835 plaform code and provide the operation points.

Best regards
Stefan

> 
> Thank you for details.
> 
>