linux-kernel - Re: [PATCH v1] clk: Add devm_clk_{prepare,enable,prepare

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Date:   Mon, 25 Nov 2019 15:11:22 +0100
From:   Marc Gonzalez <marc.w.gonzalez@...e.fr>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Stephen Boyd <sboyd@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        linux-clk <linux-clk@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] clk: Add devm_clk_{prepare,enable,prepare_enable}

On 25/11/2019 14:37, Russell King - ARM Linux admin wrote:

> On Mon, Nov 25, 2019 at 02:10:21PM +0100, Marc Gonzalez wrote:
>
>> On 25/11/2019 13:55, Russell King - ARM Linux admin wrote:
>>
>>> It's also worth reading https://lore.kernel.org/patchwork/patch/755667/
>>> and considering whether you really are using the clk_prepare() and
>>> clk_enable() APIs correctly.  Wanting these devm functions suggests
>>> you aren't...
>>
>> In that older thread, you wrote:
>>
>>> If you take the view that trying to keep clocks disabled is a good way
>>> to save power, then you'd have the clk_prepare() or maybe
>>> clk_prepare_enable() in your run-time PM resume handler, or maybe even
>>> deeper in the driver... the original design goal of the clk API was to
>>> allow power saving and clock control.
>>>
>>> With that in mind, getting and enabling the clock together in the
>>> probe function didn't make sense.
>>>
>>> I feel that aspect has been somewhat lost, and people now regard much
>>> of the clk API as a bit of a probe-time nuisance.
>>
>> In the few drivers I've written, I call clk_prepare_enable() at probe.
> 
> Right, so the clocks are enabled as soon as the device is probed,
> in other words at boot time. It remains enabled for as long as the
> device is bound to its driver, whether or not the device is actually
> being used. Every switching edge causes heat to be generated. Every
> switching edge causes energy to be wasted.
> 
> That's fine if you have an infinite energy supply. That hasn't been
> discovered yet.
> 
> Given the prevalence of technology, don't you think we should be
> doing as much as we possibly can to reduce the energy consumption
> of the devices we use? It may be peanuts per device, but at scale
> it all adds up.

OK, I'm starting to see the bigger picture.

(To provide some rationale for the patch, I think devm is a huge
improvement for probe error-handling, and I did not understand
why every driver must do manual error-handling when dealing with
clocks in probe.)

I did envision kernel modules being loaded on an as-needed basis,
somewhat side-stepping the energy-waste issue you point out.
But I realize that such a use-case may be uncommon. (Especially
due to module auto-loading.)

A few months ago, I was discussing a similar issue with GKH:
Consider a device with a "START" register. Basically, if we write 0,
the device turns itself off; if we write 1, it runs as configured.

I was trying to start the device only when at least one user had
it "open". So I used reference counting, and started the device
on 0->1 open transitions, and stopped the device on 1->0 close
transitions. GKH told me that was the wrong way to do it, and IIRC
suggested to start the device in probe.

I probably misunderstood Greg's suggestion. Where is the right place
to start/stop a device (or gate its clocks)?

Regards.