linux-kernel - Re: [PATCH V3 2/4] of: Add binding document for MIPS GIC

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Date:	Wed, 29 Oct 2014 09:55:56 -0700
From:	Andrew Bresticker <abrestic@...omium.org>
To:	James Hogan <james.hogan@...tec.com>
Cc:	Ralf Baechle <ralf@...ux-mips.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	John Crispin <blogic@...nwrt.org>,
	David Daney <ddaney.cavm@...il.com>,
	Qais Yousef <qais.yousef@...tec.com>,
	Linux-MIPS <linux-mips@...ux-mips.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V3 2/4] of: Add binding document for MIPS GIC

Hi James,

On Wed, Oct 29, 2014 at 2:21 AM, James Hogan <james.hogan@...tec.com> wrote:
> Hi Andrew,
>
> On 29/10/14 00:12, Andrew Bresticker wrote:
>>  - changed compatible string to include CPU version
>
>> +Required properties:
>> +- compatible : Should be "mti,<cpu>-gic".  Supported variants:
>> +  - "mti,interaptiv-gic"
>
>> +Required properties for timer sub-node:
>> +- compatible : Should be "mti,<cpu>-gic-timer".  Supported variants:
>> +  - "mti,interaptiv-gic-timer"
>
> Erm, I'm a bit confused...
> Why do you include the core name in the compatible string?
>
> You seem to be suggesting that:
>
> 1) The GIC/timer drivers need to know what core they're running on.
>
> Is that really true?

They don't now, but it's possible that a future CPU has a newer
revision of the GIC which has some differences that need to be
accounted for in the driver.

> 2) It isn't possible to probe the core type.
>
> But the kernel already knows this, so what's wrong with using
> current_cpu_type() like everything else that needs to know?
>
> 3) Every new core should require a new compatible string to be added
> before the GIC will work. You don't even have a generic compatible
> string that DT can specify after the core specific one as a fallback.

Yes, adding a generic compatible string would be a good idea.

> Please lets not do this unless it's actually necessary (which AFAICT it
> really isn't).

The point of this was to future-proof these bindings and I though that
CPU type was the best way to indicate version in the compatible
string.  This is also how it's done for the ARM GIC and arch timers.
Perhaps the best thing to do is to require both a core-specific
("mti,interaptiv-gic") and generic ("mti,gic") compatible string and
just match on the generic one for now until there's a need to use the
core-specific one.  Thoughts?
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