linux-kernel - Re: [PATCH v2 4/7] cpufreq: add driver for Raspbery Pi

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Date:   Thu, 06 Jun 2019 20:31:47 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Stephen Boyd <sboyd@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>, stefan.wahren@...e.com
Cc:     f.fainelli@...il.com, linux-pm@...r.kernel.org,
        mturquette@...libre.com, ptesarik@...e.com,
        linux-kernel@...r.kernel.org, mbrugger@...e.de, eric@...olt.net,
        bcm-kernel-feedback-list@...adcom.com,
        linux-rpi-kernel@...ts.infradead.org, linux-clk@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, ssuloev@...altech.com
Subject: Re: [PATCH v2 4/7] cpufreq: add driver for Raspbery Pi

On Thu, 2019-06-06 at 11:23 -0700, Stephen Boyd wrote:
> Quoting Nicolas Saenz Julienne (2019-06-06 11:10:04)
> > On Thu, 2019-06-06 at 10:36 -0700, Stephen Boyd wrote:
> > > Quoting Nicolas Saenz Julienne (2019-06-06 10:22:16)
> > > > Hi Stephen,
> > > > Thanks for the review.
> > > > 
> > > > On Thu, 2019-06-06 at 10:09 -0700, Stephen Boyd wrote:
> > > > > Quoting Nicolas Saenz Julienne (2019-06-06 07:22:56)
> > > > > > diff --git a/drivers/cpufreq/raspberrypi-cpufreq.c
> > > > > > b/drivers/cpufreq/raspberrypi-cpufreq.c
> > > > > > new file mode 100644
> > > > > > index 000000000000..99b59d5a50aa
> > > > > > --- /dev/null
> > > > > > +++ b/drivers/cpufreq/raspberrypi-cpufreq.c
> > > > > [...]
> > > > > > +
> > > > > > +/*
> > > > > > + * Since the driver depends on clk-raspberrypi, which may return
> > > > > > EPROBE_DEFER,
> > > > > > + * all the activity is performed in the probe, which may be defered
> > > > > > as
> > > > > > well.
> > > > > > + */
> > > > > > +static struct platform_driver raspberrypi_cpufreq_driver = {
> > > > > > +       .driver = {
> > > > > > +               .name = "raspberrypi-cpufreq",
> > > > > > +       },
> > > > > > +       .probe          = raspberrypi_cpufreq_probe,
> > > > > > +       .remove         = raspberrypi_cpufreq_remove,
> > > > > > +};
> > > > > > +module_platform_driver(raspberrypi_cpufreq_driver);
> > > > > 
> > > > > How does this driver probe? Do you have a node in DT named
> > > > > raspberrypi-cpufreq that matches and probes this? I would think this
> > > > > would follow the drivers/cpufreq/cpufreq-dt-platdev.c design where
> > > > > it's
> > > > > an initcall that probes the board compatible string.
> > > > > 
> > > > > Or, if it depends on clk-raspberrypi probing, maybe it could create
> > > > > the
> > > > > platform device in that drivers probe function.
> > > > 
> > > > Well you just reviewed that patch :)
> > > 
> > > Ok. So what's your plan?
> > 
> > So as discussed previously with the RPi mantainers, they preferred for the
> > platform device for raspberrypi-clk to be created by the firmware interface
> > driver. IIRC Stefan said it was more flexible and the approach used with
> > RPi's
> > hwmon driver already. Also, it's not really clear whether this driver really
> > fits the device tree as it wouldn't be describing hardware.
> > 
> > As far as raspberrypi-cpufreq is concerned the max and min frequencies are
> > configurable in the firmware. So we can't really integrate cpufreq into the
> > device tree as we need to create the opp table dynamically. Hence the
> > dedicated
> > driver. On top of that the CPU might not have a clock during the init
> > process,
> > as both the firmware interface and raspberrypi-clk can be compiled as
> > modules.
> > So I decided the simplest solution was to create the raspberrypi-cpufreq
> > platform device at the end of raspberrypi-clk's probe.
> > 
> > Once raspberrypi-cpufreq is loaded it queries the min/max frequencies,
> > populates the CPU's opp table and creates an instance of cpufreq-dt. Which
> > finally can operate, without the need of any dt info, as opp tables are
> > populated and CPUs have a clock.
> > 
> > I hope this makes it a little more clear :).
> > 
> 
> Yes, thanks. I see that largely follows the commit description so it
> looks OK to me.
> 

Thanks!


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