linux-kernel - Re: [RFC v2 3/5] clk: bcm2835: use firmware interface to update pllb

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Message-ID: <c756c54d5e924d61b8ec6eccecf9bfdce7c2b543.camel@suse.de>
Date:   Thu, 23 May 2019 10:51:07 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Stefan Wahren <stefan.wahren@...e.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        bcm-kernel-feedback-list@...adcom.com,
        Eric Anholt <eric@...olt.net>
Cc:     linux-arm-kernel@...ts.infradead.org, ptesarik@...e.com,
        sboyd@...nel.org, viresh.kumar@...aro.org, mturquette@...libre.com,
        linux-pm@...r.kernel.org, rjw@...ysocki.net,
        linux-kernel@...r.kernel.org, linux-rpi-kernel@...ts.infradead.org,
        linux-clk@...r.kernel.org, mbrugger@...e.de, ssuloev@...altech.com
Subject: Re: [RFC v2 3/5] clk: bcm2835: use firmware interface to update pllb

On Tue, 2019-05-21 at 23:43 +0200, Stefan Wahren wrote:
> > Nicolas Saenz Julienne <nsaenzjulienne@...e.de> hat am 21. Mai 2019 um 17:47
> > geschrieben:
> > 
> > 
> > Hi Stefan, thanks for your comments!
> > 
> > On Tue, 2019-05-21 at 14:40 +0200, Stefan Wahren wrote:
> > > Hi Nicolas,
> > > 
> > > On 20.05.19 14:11, Stefan Wahren wrote:
> > > > Hi Nicolas,
> > > > 
> > > > the following comments applies only in case Eric is fine with the whole
> > > > approach.
> > > > 
> > > > On 20.05.19 12:47, Nicolas Saenz Julienne wrote:
> > > > > Raspberry Pi's firmware, which runs in a dedicated processor, keeps
> > > > maybe we should clarify that the firmware is running in the VPU
> > > > > track of the board's temperature and voltage. It's resposible for
> > > > > scaling the CPU frequency whenever it deems the device reached an
> > > > > unsafe
> > > > > state. On top of that the firmware provides an interface which allows
> > > > > Linux to to query the clock's state or change it's frequency.
> > > > I think this requires a separate update of the devicetree binding.
> > > > > Being the sole user of the bcm2835 clock driver, this integrates the
> > > > > firmware interface into the clock driver and adds a first user: the
> > > > > CPU
> > > > > pll, also known as 'pllb'.
> > > > Please verify that the kernel still works (and this clock driver probe)
> > > > under the following conditions:
> > > > 
> > > > - CONFIG_RASPBERRYPI_FIRMWARE=n
> > > > - CONFIG_RASPBERRYPI_FIRMWARE=m
> > > > - older DTBs without patch #1
> > > i thought about this and the case this driver would return
> > > -EPROBE_DEFER. The clock driver is too essential for doing such a thing.
> > > So i think the best solution would be to move these changes into a
> > > separate driver which should be register by the clock driver (similiar
> > > to vchiq). This also avoid the need of a new device tree binding.
> > 
> > I understand your concerns.
> > 
> > Wouldn't you prefer registering the device trough the device tree? I'd go
> > with
> > the same approach as the firmware touchscreen driver, which is registered
> > after
> > the firmware's probe trough dt's 'simple-bus'. That said, it's not a
> > strongly
> > held opinion, I'm happy with whatever solution as long as it works.
> 
> A devicetree binding always introduce some kind of inflexibility. In case
> someone finds a better solution later things can get really messy. A recent
> example is the clock handling for i2c-bcm2835.

Fair enough.

> > I get from your comments that you'd like the register based version of
> > 'pllb'
> > and 'pllb_arm' to be loaded if for some reason the firmware isn't available.
> > Is
> > that right? 
> 
> This wasn't my intention. I would prefer a simple approch here (no handover). 
> 
> > The main problem I see with this is the duplication of 'pllb' and
> > 'pllb_arm'. Both drivers will create the same clock device through different
> > interfaces. Any suggestions on how to deal with that? If not I can simply
> > remove 'pllb' and 'pllb_arm' from clk-bcm2835.c.
> 
> Yes. So even if this driver is disabled, there shouldn't be a regression. Or
> did i miss something?

No, there shoudn't be any regressions as these clocks are not being used at the
moment.

I'll send a follow-up series soon :)

Regrads,
Nicolas


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