linux-kernel - Re: [PATCH v3 03/15] soc: tegra: Add Tegra PMC clock registrations into PMC driver

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Date:   Mon, 16 Dec 2019 17:24:35 +0200
From:   Peter De Schrijver <pdeschrijver@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>
CC:     Sowjanya Komatineni <skomatineni@...dia.com>,
        <thierry.reding@...il.com>, <jonathanh@...dia.com>,
        <mperttunen@...dia.com>, <sboyd@...nel.org>,
        <gregkh@...uxfoundation.org>, <tglx@...utronix.de>,
        <robh+dt@...nel.org>, <mark.rutland@....com>,
        <allison@...utok.net>, <pgaikwad@...dia.com>,
        <mturquette@...libre.com>, <horms+renesas@...ge.net.au>,
        <Jisheng.Zhang@...aptics.com>, <krzk@...nel.org>, <arnd@...db.de>,
        <spujar@...dia.com>, <josephl@...dia.com>, <vidyas@...dia.com>,
        <daniel.lezcano@...aro.org>, <mmaddireddy@...dia.com>,
        <markz@...dia.com>, <devicetree@...r.kernel.org>,
        <linux-clk@...r.kernel.org>, <linux-tegra@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <lgirdwood@...il.com>,
        <broonie@...nel.org>, <perex@...ex.cz>, <tiwai@...e.com>,
        <alexios.zavras@...el.com>, <alsa-devel@...a-project.org>
Subject: Re: [PATCH v3 03/15] soc: tegra: Add Tegra PMC clock registrations
 into PMC driver

On Mon, Dec 16, 2019 at 05:11:32PM +0200, Peter De Schrijver wrote:
> External email: Use caution opening links or attachments
> 
> 
> On Mon, Dec 16, 2019 at 05:23:23PM +0300, Dmitry Osipenko wrote:
> > >> Could you please clarify what do you mean by the "existing users"?
> > >> AFAIK, nothing in kernel uses mux clocks.
> > >
> > > The DT clk bindings allow for parent initialization, so it's certainly
> > > possible there are some DTs which rely on this. We promised to never
> > > break the bindings, which changing to 1 clock would do.
> >
> > What about this variant:
> >
> >   1. Keep the old CaR code in place.
> >
> >   2. Make CaR driver to scan whole device-tree for the legacy PMC clocks.
> >
> >   3. If legacy clock is found, then register PMC clocks from CaR.
> >
> >   4. If legacy clocks are not found, then don't register PMC clocks from
> > CaR.
> >
> >   5. Add clocks support to the PMC driver and only register them if
> > legacy clocks are not registered by CaR.
> >
> > Now both old and new DTBs can co-exist and work, everyone happy.
> 
> That seems even more work.. Today the only upstream user is audio.
> Currently these clocks are setup by the CAR clock driver. However
> as they will move to the PMC driver, this mechanism cannot be used.
> Hence the plan is to modify the audio driver to check for the PMC clocks
> in DT and if they are not available use the CAR clocks as fallback.
> The whole reason the clocks move to the PMC driver, is that when PMC
> becomes secure, all accesses have to go via an SMC. Given that we don't
> want SMCs all over the Tegra drivers, it's a good opportunity to move
> the PMC clock handling into the PMC driver. PMC can be secure with both
> 'new' and old DTs, so just registering the PMC clocks in the CAR driver
> if legacy clocks are found in the DT, won't always work.
> 

Given the audio driver needs to change anyway, we can indeed use 1 clock
per PMC clk_out_ rather than 2 as we have today. As this models the hw
slightly better, I think we should do that as you suggested.

Peter.