linux-kernel - Re: [PATCH v2 4/4] ARM: dts: mt8135: Add pinctrl node for mt8135.

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Message-ID: <20141006071841.GN4992@pengutronix.de>
Date:	Mon, 6 Oct 2014 09:18:41 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	"Hongzhou.Yang" <srv_hongzhou.yang@...iatek.com>,
	Rob Herring <robh+dt@...nel.org>,
	Matthias Brugger <matthias.bgg@...il.com>,
	srv_heupstream@...iatek.com, Sascha Hauer <kernel@...gutronix.de>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Russell King <linux@....linux.org.uk>,
	Grant Likely <grant.likely@...aro.org>,
	Hongzhou Yang <hongzhou.yang@...iatek.com>,
	"Joe.C" <yingjoe.chen@...iatek.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Vladimir Murzin <vladimir.murzin@....com>,
	Ashwin Chaugule <ashwin.chaugule@...aro.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, dandan.he@...iatek.com
Subject: Re: [PATCH v2 4/4] ARM: dts: mt8135: Add pinctrl node for mt8135.

On Thu, Oct 02, 2014 at 04:02:58PM +0200, Linus Walleij wrote:
> On Wed, Sep 24, 2014 at 2:40 PM, Sascha Hauer <s.hauer@...gutronix.de> wrote:
> > On Wed, Sep 24, 2014 at 01:23:09PM +0200, Linus Walleij wrote:
> 
> >> I haven't got to reviewing the driver, but this looks just wrong.
> >>
> >> Have the magic numbers in the driver.
> >>
> >> Use strings to describe functions, not integers.
> >
> > Interrupts, clocks, gpios, dma channels, nearly everything in the device tree is
> > arbitrarily numbered. Instead of "irq-i2c0" we have <0 36 IRQ_TYPE_LEVEL_HIGH>
> > in the device tree. These numbers can be resolved efficiently in the
> > driver by shifting them to get a bitmask or by adding them as offset to
> > a register base.
> > Why do you want to make pinctrl different?
> 
> Because pin control is about combining groups of pins with
> certain functions.
> 
> > Thanks to the recently
> > introduced defines in the device trees these numbers are not magic at
> > all anymore.
> 
> Yeah that is good but not what I'm after here.
> 
> >> We need to move toward standardized device tree bindings
> >> for this stuff, and that means using strings, not magic
> >> numbers.
> >
> > Agreed for standardized device tree bindings, but not for using strings.
> 
> What is the alternative? Device Tree is very much about strings,
> as is shown by the pin config bindings.

The alternative is to use numbers. The majority of SoCs have a bit field
per pad which is used for muxing the pad to different functions. The
natural way to describe this is a pair of numbers: <pad-number>
<function-number>.  The pad number can normally be directly translated
into a register offset and the function number to a value written to
that register. This is true for most SoCs I know of and makes it very
easy to generate code for and to prove for correctness of both the code
and the device tree.

Sascha

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