linux-kernel - Re: [RFC v2 0/5] cpufreq support for the Raspberry Pi

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Message-ID: <a54bd83c219b328153352f0d2c96badbac042353.camel@suse.de>
Date:   Tue, 21 May 2019 14:02:20 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     stefan.wahren@...e.com, devicetree@...r.kernel.org,
        linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-pm@...r.kernel.org,
        mbrugger@...e.de, rjw@...ysocki.net, sboyd@...nel.org,
        eric@...olt.net, f.fainelli@...il.com,
        bcm-kernel-feedback-list@...adcom.com, ptesarik@...e.com,
        ssuloev@...altech.com, linux-clk@...r.kernel.org,
        mturquette@...libre.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 0/5] cpufreq support for the Raspberry Pi

Hi Viresh, thanks for the comments.

On Mon, 2019-05-20 at 16:21 +0530, Viresh Kumar wrote:
> On 20-05-19, 12:47, Nicolas Saenz Julienne wrote:
> > Hi all,
> > as some of you may recall I've been spending some time looking into
> > providing 'cpufreq' support for the Raspberry Pi platform[1]. I think
> > I'm close to something workable, so I'd love for you to comment on it.
> > 
> > There has been some design changes since the last version. Namely the
> > fact that I now make sure *only* the CPU frequency is updated. The
> > firmware API we use has two modes, with or without turbo. Enabling turbo
> > implies not only scaling the CPU clock but also the VPU and other
> > peripheral related clocks.  This is problematic as some of them are not
> > prepared for this kind frequency changes. I spent some time adapting the
> > peripheral drivers, but the result was disappointing as they poorly
> > support live frequency changes (which most other chips accept, think for
> > instance I2C and clock stretching) but also turned out hard to integrate
> > into the kernel. As we were planning to use 'clk_notifiers' which turns
> > out not to be such a good idea as it's prone to deadlocks and not
> > recommended by the clock maintainers[2]. It's also worth mentioning that
> > the foundation kernel doesn't support VPU frequency scaling either.
> > 
> > With this in mind, and as suggested by clock maintainers[2], I've
> > decided to integrate the firmware clock interface into the bcm2835 clock
> > driver. This, in my opinion, provides the least friction with the
> > firmware and lets us write very simple and portable higher level
> > drivers. As I did with the 'cpufreq' driver which simply queries the max
> > and min frequencies available, which are configurable in the firmware,
> > to then trigger the generic 'cpufreq-dt'.
> > 
> > In the future we could further integrate other firmware dependent clocks
> > into the main driver. For instance to be able to scale the VPU clock,
> > which should be operated through a 'devfreq' driver.
> > 
> > This was tested on a RPi3b+ and if the series is well received I'll test
> > it further on all platforms I own.
> 
> Please always supply version history on what has changed from V1.

Will do

> And why do you keep sending it as RFC ?

Well it's because of patch #3 which integrates the firmware interface into the
clock driver. I want some approval from the maintainers before cleaning it up
testing it on all RPi versions.

> Just keep the default PATCH thing,the patches are in good shape I would say.

Thanks :)

Regards,
Nicolas


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