Date: Fri, 15 Dec 2017 11:32:42 +0000
From: Mark Rutland <mark.rutland@arm.com>
To: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: Rob Herring <robh+dt@kernel.org>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Nicolas Ferre <nicolas.ferre@microchip.com>,
        Thomas Gleixner <tglx@linutronix.de>, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        marc.zyngier@arm.com
Subject: Re: [PATCH 1/3] dt-bindings: chosen: Add clocksource and clockevent
 selection
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 <20171213185313.20017-2-alexandre.belloni@free-electrons.com>
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Hi,

On Wed, Dec 13, 2017 at 07:53:11PM +0100, Alexandre Belloni wrote:
> The clocksource and clockevent timer are probed early in the boot process.
> At that time it is difficult for linux to know whether a particular timer
> can be used as the clocksource or the clockevent or by another driver,
> especially when they are all identical or have similar features.

I think that to solve this problem, we need to stop treating
clocksources and clockevent devices as completely separate device types,
and instead treat them as particular cases of a more general clock
device.

That way, a driver can register a single device, with flags saying
whether it is:

* a clocksource only
* a clockevent device only
* both a clocksource and clockevent device
* both, but mutually exclusive at runtime

... and thus drivers don't have to make an impossible decision up-front
as to how the device should be used.

As more devices get registered, the core timekeeping code can improve on
its choices and re-assign devices.

That doesn't solve the case where a clock device may use resources we
want for something else, but I think we can solve that separately.

Thanks,
Mark.

> Until now, multiple strategies have been used to solve that:
>  - use Kconfig option as MXC_USE_EPIT or ATMEL_TCB_CLKSRC_BLOCK
>  - use a kernel parameter as the "clocksource" early_param in mach-omap2
>  - registering the first seen timer as a clockevent and the second one as
>  a clocksource as in rk_timer_init or dw_apb_timer_init
> 
> Add a linux,clocksource and a linux,clockevent node in chosen with a timer
> property pointing to the timer to use. Other properties, like the targeted
> precision may be added later.
> 
> Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
> ---
>  Documentation/devicetree/bindings/chosen.txt | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/chosen.txt b/Documentation/devicetree/bindings/chosen.txt
> index e3b13ea7d2ae..c7ee3ecb5276 100644
> --- a/Documentation/devicetree/bindings/chosen.txt
> +++ b/Documentation/devicetree/bindings/chosen.txt
> @@ -120,3 +120,23 @@ e.g.
>  While this property does not represent a real hardware, the address
>  and the size are expressed in #address-cells and #size-cells,
>  respectively, of the root node.
> +
> +linux,clocksource and linux,clockevent
> +--------------------------------------
> +
> +Those nodes have a timer property. This property is a phandle to the timer to be
> +chosen as the clocksource or clockevent. This is only useful when the platform
> +has multiple identical timers and it is not possible to let linux make the
> +correct choice.
> +
> +/ {
> +	chosen {
> +		linux,clocksource {
> +			timer = <&timer0>;
> +		};
> +
> +		linux,clockevent {
> +			timer = <&timer1>;
> +		};
> +	};
> +};
> -- 
> 2.15.1
> 
> --
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