linux-kernel - Re: [PATCH v4 03/10] pinctrl: mvebu: kirkwood pinctrl driver

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Message-ID: <505C47C6.3070907@codethink.co.uk>
Date:	Fri, 21 Sep 2012 11:56:06 +0100
From:	Ben Dooks <ben.dooks@...ethink.co.uk>
To:	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
CC:	Linus Walleij <linus.walleij@...aro.org>,
	Arnd Bergmann <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org,
	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
	Andrew Lunn <andrew@...n.ch>,
	Russell King <linux@....linux.org.uk>,
	Jason Cooper <jason@...edaemon.net>,
	Stephen Warren <swarren@...dotorg.org>,
	devicetree-discuss@...ts.ozlabs.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Rob Herring <rob.herring@...xeda.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Lior Amsalem <alior@...vell.com>,
	Rob Landley <rob@...dley.net>,
	Gregory CLEMENT <gregory.clement@...e-electrons.com>
Subject: Re: [PATCH v4 03/10] pinctrl: mvebu: kirkwood pinctrl driver

On 20/09/12 20:36, Thomas Petazzoni wrote:
> Dear Linus Walleij,
>
> On Thu, 20 Sep 2012 21:28:20 +0200, Linus Walleij wrote:
>
>> So what I'm after is whether in this case statically encoding this
>> onto the .dtsi files is the right thing to do, or whether the boot loader
>> or kernel should runtime-modify the device tree, patching in
>> the ASIC-specific info, just like device tree files can override
>> properties from include files.
>>
>> Or if this is a bad idea.
>>
>> Nobody is doing that right now AFAICT, but it is surely possible....
>
> If I understand correctly, we would like drivers to be able to read
> some common "system" registers to figure out which SoC variant we are
> running on. Such feature should normally be provided by code in
> arch/arm/mach-*/ and called by drivers, but we are trying to eliminate
> all dependencies of driver code on architecture code, correct?
>
> So, wouldn't we need a small, architecture-independent, infrastructure,
> through which architecture-specific code could "register" at boot
> time which SoC we are running on, and drivers could query this
> information from the common infrastructure?
>
> Of course, the major problem is to figure out what is the good
> representation for this SoC identifier. Do we need a big list of SoCs
> like we had machine IDs? A simple string? Or maybe there is just no
> good way, and the whole idea is moot.

I think this is a bad idea, it means you can't try and re-use an
older kernel on an newer SoC design which may be broadly compatible
without changing the kernel source to add these flag checks.

I also don't like information hidden away that the user cannot see.
Having the device specifically named allows us to see from sysfs
exactly what we are dealing with.

-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius
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