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

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Date:   Thu, 06 Jun 2019 11:23:34 -0700
From:   Stephen Boyd <sboyd@...nel.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Viresh Kumar <viresh.kumar@...aro.org>, stefan.wahren@...e.com
Cc:     linux-arm-kernel@...ts.infradead.org, f.fainelli@...il.com,
        ptesarik@...e.com, mturquette@...libre.com,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        eric@...olt.net, bcm-kernel-feedback-list@...adcom.com,
        linux-rpi-kernel@...ts.infradead.org, linux-clk@...r.kernel.org,
        mbrugger@...e.de, ssuloev@...altech.com
Subject: Re: [PATCH v2 4/7] cpufreq: add driver for Raspbery Pi

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.