linux-kernel - Re: [PATCH v3 1/5] Documentation: add DT bindings for ARM SCPI sensors

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Message-ID: <9hheghzvfbz.fsf@e105922-lin.cambridge.arm.com>
Date:	Tue, 15 Sep 2015 17:04:48 +0100
From:	Punit Agrawal <punit.agrawal@....com>
To:	"Jon Medhurst \(Tixy\)" <tixy@...aro.org>
Cc:	Mark Rutland <mark.rutland@....com>,
	"devicetree\@vger.kernel.org" <devicetree@...r.kernel.org>,
	"linux-pm\@vger.kernel.org" <linux-pm@...r.kernel.org>,
	Liviu Dudau <Liviu.Dudau@....com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"lm-sensors\@lm-sensors.org" <lm-sensors@...sensors.org>,
	"edubezval\@gmail.com" <edubezval@...il.com>,
	"robh+dt\@kernel.org" <robh+dt@...nel.org>,
	"linux\@roeck-us.net" <linux@...ck-us.net>,
	Sudeep Holla <Sudeep.Holla@....com>,
	"linux-arm-kernel\@lists.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 1/5] Documentation: add DT bindings for ARM SCPI sensors

"Jon Medhurst (Tixy)" <tixy@...aro.org> writes:

> On Tue, 2015-09-15 at 10:37 +0100, Punit Agrawal wrote:
>> "Jon Medhurst (Tixy)" <tixy@...aro.org> writes:
>> 
>> > On Mon, 2015-09-14 at 15:38 +0100, Punit Agrawal wrote:
>> >> Mark Rutland <mark.rutland@....com> writes:
>> >> 
> [...]
>> >> The way the SCP interface is defined, the sensor identifiers are
>> >> contiguous,
>> >
>> > Is there any documentation other than DUI0922A? [1] From what I can seen
>> > that just says it's a 16-bit value and doesn't put any particular
>> > constraints on its value.
>> 
>> Although not explicitly stated, if you look at the Get Sensor Capability
>> [2] and Get Sensor Info [3] commands you can indirectly infer that the
>> Sensor IDs are contiguous.
>
> I personally wouldn't even indirectly infer they are contiguous from
> what the document says. If I were implementing the firmware I would feel
> quite in my rights to, for example, use the top 8 bits of the ID for a
> sensor type and the bottom 8 for an index, if that made dispatching of
> requests more efficient. Or if some optional hardware was detected as
> missing, leaving some holes in ID space.

True. And without a command to convey the list of valid IDs, the
consumer of the API would have to iterate over the entire 16bit space to
locate valid IDs.

>
> As a specification of a 'standard' the document seems to be rather
> lacking. So, Sensor ID should be documented as being "an unsigned
> integer less than then number of sensors returned by the Get Sensor
> Capability command", or something like that. I guess clocks and other
> devices suffer from similar lack of specificity.

I was thinking of suggesting something similar as an update.

>
>>  Not the strongest guarantee I know.
>> 
>> All platforms currently using SCP (Juno R0 and R1) do indeed expose
>> contiguous identifiers.
>
> IMO, Linux drivers should be coded to the standard or written
> specification (where they are available) not the particular
> implementations available.
>
>> >
>> > [1] http://community.arm.com/servlet/JiveServlet/download/8401-40-18262/DUI0922A_scp_message_interface.pdf
>> [2] http://arminfo.emea.arm.com/help/index.jsp?topic=/com.arm.doc.dui0922b/ch03s02s21.html
>> [3] http://arminfo.emea.arm.com/help/index.jsp?topic=/com.arm.doc.dui0922b/BABCCCJJ.html
>
> I think those links are on ARM's intranet, they return NXDOMAIN for me.

Apologies, the below should work.

[2] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0922b/ch03s02s21.html
[3] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0922b/BABCCCJJ.html
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