linux-kernel - Re: [PATCH 2/5] ARM: bcm2835: Replace alt0/i2s

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Date:	Thu, 3 Mar 2016 14:26:12 -0700
From:	Stephen Warren <swarren@...dotorg.org>
To:	Eric Anholt <eric@...olt.net>
Cc:	linux-rpi-kernel@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Lee Jones <lee@...nel.org>,
	Florian Fainelli <f.fainelli@...il.com>,
	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>, devicetree@...r.kernel.org,
	Linus Walleij <linus.walleij@...aro.org>,
	linux-gpio@...r.kernel.org, Stefan Wahren <stefan.wahren@...e.com>
Subject: Re: [PATCH 2/5] ARM: bcm2835: Replace alt0/i2s_alt[02] with standard
 groups.

On 02/26/2016 11:19 AM, Eric Anholt wrote:
> Since all of these pins were documented, we can use their names to
> explain what's going on.

> diff --git a/arch/arm/boot/dts/bcm2835-rpi-a-plus.dts b/arch/arm/boot/dts/bcm2835-rpi-a-plus.dts

>   &gpio {
> +	pinctrl-0 = <&i2c0_gpio0
> +		     &i2c1_gpio2
> +		     &gpclk0_gpio4
> +		     &gpclk1_gpio5
> +		     &spi0_gpio7
> +		     &pcm_gpio18
> +		     &pwm0_gpio40
> +		     &pwm1_gpio45
> +		     &gpioout
> +		     &alt3>;
>   };

Why not convert alt3 to the new scheme too?

I think this configures too many pins, which in turn makes assumptions 
about what those pins are used for that may not be valid.

Recent RPi firmware configures almost all expansion connector GPIOs as 
GPIO-in. This ensures that no matter what is connected to the expansion 
connector, there can be no signal conflicts due to both the bcm283x and 
some external device both attempting to drive the same pin. I believe 
the default Linux pinmux should adopt the same approach, by simply not 
configuring any expansion connector pins except those known to have a 
100% hard-coded usage. For example, the HAT I2C pins must only be used 
for that purpose on the RPi, so even if the HW supported using them as 
arbitrary GPIO or PWM or ..., we know they're actually I2C.

So, I think this list should only include configuration for pins 
connected to on-board devices, or expansion pins that have a 100% known 
purpose.

(I can't quite remember how many pins are being configured in the 
upstream kernel's DT files at present; it's possible the complying with 
this rule may involve removing some pinctrl settings that are currently 
present to avoid conflicts. User-specific additions should come from DT 
overlays or manual DT edits.)