On 03/03/2016 03:23 PM, Eric Anholt wrote:

Stephen Warren [off-list ref] writes:

quoted

On 02/26/2016 11:19 AM, Eric Anholt wrote:

quoted

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.

quoted

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

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+			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.

quoted

+			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?

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+			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.

quoted

+			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?

Nothing here precludes making alternative pin groups for special
situations like you're bringing up here.  I'm just trying to bring
sanity to the giant lists of pins we have currently, that happen to
correspond to these.

Of your suggestions, making uart1_gpio36 split out cts/rts like the rest
makes a lot of sense to me.  Of the others, they seem like speculation
more than "we should fix this because it's not what people want."  Can
you provide specific feedback of what you'd like changed to get an Ack?

All of the points I raised should be fixed. I don't believe any of the 
groups that affect more than minimal sets of pins are useful. Indeed, 
using groups at all is rather tenuous; it'd be far better to list the 
precise sets of pins only as and when they're used.