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

[Sascha Hauer <s.hauer-bIcnvbaLZ9MEGnE8C9+IrQ@public.gmane.org>]

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-bIcnvbaLZ9MEGnE8C9+IrQ@public.gmane.org> 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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