linux-kernel - Re: [PATCH 1/5] ARM: bcm2835: Define standard pinctrl groups in the gpio node.

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Date:	Thu, 3 Mar 2016 14:20:34 -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 1/5] ARM: bcm2835: Define standard pinctrl groups in the
 gpio node.

On 02/26/2016 11:19 AM, Eric Anholt wrote:
> The BCM2835-ARM-Peripherals.pdf documentation specifies what the
> function selects do for the pins, and there are a bunch of obvious
> groupings to be made.  With these created, we'll be able to replace
> bcm2835-rpi.dtsi's main "set all of these pins to alt0" with
> references to specific groups we want enabled.

> diff --git a/arch/arm/boot/dts/bcm283x.dtsi b/arch/arm/boot/dts/bcm283x.dtsi

> +			spi0_gpio7: spi0_gpio7 {
> +				brcm,pins = <7 8 9 10 11>;
> +				brcm,function = <BCM2835_FSEL_ALT0>;
> +			};

This is too many pins.

- It includes both MOSI and MISO, although a particular use-case may 
only use 1 of those.

- It includes both chip-select signals, whereas a particular use-case 
may use 0, 1, or 2 of those. This is especially true since IIRC the 
mainline bcm283x SPI driver wants to only use GPIOs for chip-selects, 
not SPI-controller-generated chip-select signals, to avoid some issues 
with the HW generation of these signals.

I believe a similar comment applies to other SPI nodes too.

> +			pcm_gpio18: pcm_gpio18 {
> +				brcm,pins = <18 19 20 21>;
> +				brcm,function = <BCM2835_FSEL_ALT0>;
> +			};

Here too, I wonder if some people might want only one of DIN/DOUT and 
not both?

> +			uart1_gpio36: uart1_gpio36 {
> +				brcm,pins = <36 37 38 39>;
> +				brcm,function = <BCM2835_FSEL_ALT2>;
> +			};

Similarly, I think for UARTS, TX/RX and RTS/CTS should always be in 
different nodes so people can choose 2- or 4-wire mode. Most of the UART 
nodes are already split like this, but this one isn't.

> +			emmc_gpio22: emmc_gpio22 {
> +				brcm,pins = <22 23 24 25 26 27>;
> +				brcm,function = <BCM2835_FSEL_ALT3>;
> +			};

1-wire (1 data wire, plus CLK/CMD) eMMC is possible in theory, although 
I don't know whether it makes sense to support this?