Set GPIO at boot on Raspberry Pi

[Ian Lepore]

On Sun, 2015-12-20 at 11:20 -0600, Karl Denninger wrote:
> On 12/20/2015 11:02, Toby wrote:
> > Hi all
> > 
> > Is there a way to set GPIOs at boot? I'd like to set GPIO 28 to out
> > and
> > high before usb devices are probed, in order to be able to use my
> > usb
> > disk. If I have it plugged in during boot, I can't get it to work.
> > It
> > works though if I boot the pi, then plug in the disk and then set
> > the
> > pin to high.
> > 
> > Thanks,
> > Toby
> I've had good luck setting the state FIRST, then the pin config to
> desired (in this case, OUT.)
> 
> I have this general problem with driving a relay board, in that the
> jackwads that designed the board set it up to be active LOW, so if
> you
> set the config first you chatter the outputs, and FreeBSD's GPIO
> config
> rejects "active low" config as a setting.
> 
> The pins should come up configured as inputs so the peripheral
> shouldn't
> be upset about that.
> 

I'm not sure what you mean by "FreeBSD's GPIO config rejects active
low" -- what config are you refering to?

But I don't think pin state when changing config is quite what Toby was
asking; he needs a way to configure driven pins very early in boot.

Right now we have no way of preconfiguring gpio pins to be driven a
certain way early in boot.  By manipulating the device-tree data you
can configure a pin as input or output, and usually you can configure
pull up/down values for inputs, but there's no standard device-tree
syntax to configure the drive value when configuring a pin to be
driven.

Maybe we could add some dev.gpioc tunables so that pins could be
configured via loader.conf.

-- Ian