linux-kernel - Re: [PATCH v2 1/2] devicetree: add binding for generic mmio clocksource

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Date:	Wed, 7 Oct 2015 16:47:28 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Mans Rullgard <mans@...sr.com>
Cc:	Rob Herring <robh+dt@...nel.org>, Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] devicetree: add binding for generic mmio
 clocksource

On Wed, Oct 07, 2015 at 04:37:13PM +0100, Mans Rullgard wrote:
> This adds a DT binding for a generic mmio clocksource as implemented
> by clocksource_mmio_init().
> 
> Signed-off-by: Mans Rullgard <mans@...sr.com>
> ---
> Changed in v2:
> - added sched_clock support
> ---
>  .../devicetree/bindings/timer/clocksource-mmio.txt | 28 ++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/timer/clocksource-mmio.txt
> 
> diff --git a/Documentation/devicetree/bindings/timer/clocksource-mmio.txt b/Documentation/devicetree/bindings/timer/clocksource-mmio.txt
> new file mode 100644
> index 0000000..cfb3601
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/timer/clocksource-mmio.txt
> @@ -0,0 +1,28 @@
> +Generic MMIO clocksource
> +
> +Required properties:
> +
> +- compatible: should be "clocksource-mmio"
> +- reg: the physical address of the counter register
> +- reg-io-width: size of counter register in bytes, should be 2 or 4

Can this not be inferred from the reg?

What about 8 byte counters?

> +- clocks: phandle to the source clock

Is the frequency expected to be exactly the source clock frequency? I
imagine it's possible for there to be a divisor.

We can add properties for that later, but we should be explcit as to
what we currently expect the relationship between the clock and the
clocksource to be.

> +- clocksource-bits: number of valid bits
> +- clocksource-rating: rating of the clocksource

NAK. This has no meaning w.r.t. the hardware. This should not be in the
DT. If there are criteria that bias this (e.g. frequency, latency), they
should eitehr be describedi nteh DT or determined dynamically.

> +Optional properties:
> +
> +- clocksource-counts-down: indicates that counter counts down
> +- label: name of the clocksource
> +- linux,sched-clock: boolean, register clocksource as sched_clock

Likewise, this property doesn't belong in the DT for the same reasons as
clocksource-rating.

Mark.

> +
> +Example:
> +
> +clocksource {
> +	compatible = "clocksource-mmio";
> +	reg = <0x10000 4>;
> +	reg-io-width = <4>;
> +	clocksource-bits = <32>;
> +	clocksource-rating = <300>;
> +	clocks = <&clk>;
> +	linux,sched_clock;
> +}
> -- 
> 2.5.3
> 
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