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

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Message-ID: <1442317042.2917.29.camel@linaro.org>
Date:	Tue, 15 Sep 2015 12:37:22 +0100
From:	"Jon Medhurst (Tixy)" <tixy@...aro.org>
To:	Mark Rutland <mark.rutland@....com>
Cc:	Punit Agrawal <Punit.Agrawal@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Liviu Dudau <Liviu.Dudau@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lm-sensors@...sensors.org" <lm-sensors@...sensors.org>,
	"edubezval@...il.com" <edubezval@...il.com>,
	"robh+dt@...nel.org" <robh+dt@...nel.org>,
	"linux@...ck-us.net" <linux@...ck-us.net>,
	Sudeep Holla <Sudeep.Holla@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 1/5] Documentation: add DT bindings for ARM SCPI
 sensors

On Tue, 2015-09-15 at 12:03 +0100, Mark Rutland wrote:
> On Tue, Sep 15, 2015 at 11:46:02AM +0100, Jon Medhurst (Tixy) wrote:
> > 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.
> > 
> > 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 think from the PoV of the binding, this doesn't matter. The value is
> just an arbitrary opaue token written down in a spec, that the FW
> understands how to interpret.

True, it's the Linux implementation in following patches that has
assumptions, e.g. for loops iterating over id's 0..N-1
 
> 
> I only asked about how the IDs were organised because I thought there
> was additional translation in the kernel, but this is not the case.
> 
> The only potential problem is bit-width. Punit, I assume the value is
> 32-bit (or less) in the messages to the FW?

It's 16 bit.

-- 
Tixy

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