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

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Date:	Wed, 07 Oct 2015 17:47:12 +0100
From:	Måns Rullgård <mans@...sr.com>
To:	Mark Rutland <mark.rutland@....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

Mark Rutland <mark.rutland@....com> writes:

> 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?

You're right, it can.

> What about 8 byte counters?

The existing code only supports 2 or 4, but that of course doesn't
matter here.

>> +- 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.

There could of course be, though there isn't in the hardware I'm dealing
with.  Is specifying it here preferable using a fixed-factor-clock?

> 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 either be described in the DT or determined dynamically.

I had a bad feeling about this.  How would you suggest determining a
suitable value from actual hardware parameters?

>> +- 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.

What would be a proper way to select a sched_clock source?  I realise
it's a Linux-specific thing and DT is supposed to be generic, but the
information must be provided somehow.

-- 
Måns Rullgård
mans@...sr.com
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