linux-kernel - Re: [RFC 0/5] drivers: Add boot constraints core

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Date:   Fri, 30 Jun 2017 10:42:03 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Chen-Yu Tsai <wens@...e.org>
Cc:     "Enrico Weigelt, metux IT consult" <enrico.weigelt@...3.net>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Rob Herring <robh@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Stephen Boyd <sboyd@...eaurora.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>, rnayak@...eaurora.org,
        Shiraz Hashim <shashim@...eaurora.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC 0/5] drivers: Add boot constraints core

On 30-06-17, 12:22, Chen-Yu Tsai wrote:
> On Fri, Jun 30, 2017 at 12:12 PM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > On 30-06-17, 12:05, Chen-Yu Tsai wrote:

> >> I also want to mention that for DT based platforms, this constraint
> >> should already be set in the device tree for the regulator, so the
> >> scenario where DMA comes up and sets a voltage level that LCD cannot
> >> use should not even be possible.
> 
> What I'm saying is for the DT case, the constraints are already limited
> to the intersection of all users, regardless of whether they are turned
> on or not.

Right, but someone needs to get the regulator first to have that
considered by the regulator core while deciding the final range.

Both DMA and LCD driver do regulator_get() for their devices but if
only DMA driver is probed until now, then the regulator core wouldn't
consider LCD as regulator_get() is never called for LCD.

> I think what you mean is that the DT constraints are the union of all
> consumer constraints (1.8 - 3.0 V in this case), then each consumer
> comes in and adds its own constraints. And for such a design, the kernel
> needs to know which and what constraints to apply.

Sorry, I am confused with what you just said and not sure if I
understand it completely.

Each consumer DT node will have its own set of constraints for the
regulator device. The kernel will do regulator_get() for them one by
one, based on when their drivers get probed. And an intersection of
those constraints (which already did regulator_get()) will be used by
the regulator core.

Now this series is saying that even if the driver didn't come up (for
LCD) and haven't done its regulator_get() yet, consider that device's
constraint while calculating the target voltage for the regulator.

> Either way regulators already support constraints, so they are easier
> to deal with. Clocks on the other hand, while the core does support
> clock rate constraints, AFAIK no one really uses or supports them.

Yeah, so I started with just regulators and that's when Mark suggested
to do something generic which can be reused by other resource types.
We may end up covering clk for sure I believe. Not sure yet about
other resource types though.

-- 
viresh