How to set GPIOs to pre-defined value

[Andrew Jeffery]

On Thu, 9 May 2019, at 17:12, Brad Chou wrote:
> 
> > On May 8, 2019, at 9:53 AM, Andrew Jeffery <andrew at aj.id.au> wrote:
> > 
> > 
> > 
> > On Tue, 7 May 2019, at 18:23, Brad Chou wrote:
> >> Hi Samuel,
> >> Thanks for your reply, I am using AST2500.
> >> I tried add gpio-hog settings into my device tree, and yes, the GPIO 
> >> works as it defined.
> >> But all GPIOs defined by gpio-hog can not be modified in user space by 
> >> gpioset / gpioget utility.
> >> I need to set all GPIOs to pre-defined state and can change it at run 
> >> time.
> >> Set GPIOs in Device tree is trying to lock it by a fixed direction and 
> >> value.
> >> 
> > 
> > This problem is probably best taken up with upstream. Currently GPIO
> > nodes in the devicetree are ignored if they do not have the `gpio-hog`
> > property:
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpio/gpiolib-of.c?h=v5.1#n454
> > 
> > Changing that might be a hard argument - it may impact existing
> > devicetrees that expect the current behaviour.
> > 
> > However, I'm interested in what you see wrong with doing this from
> > userspace? What requirements do you have that would need this to
> > be done in the kernel?
> > 
> > Cheers,
> > 
> > Andrew
> 
> I don’t really need it to be set in the kernel.
> Just curious about how openBMC initial all GPIOs, especially for 
> AST2500 that has over 200+ GPIOs.

Well, the GPIOs are muxed with other functionality (LPC, I2C, SPI etc),
and generally your platform design is going to only use a subset of the
pins that are left for GPIO functionality, so you don't tend to need to
initialise the state of 200+ pins. Some take fixed values, in which case
you can use the kernel's gpio-hog mechanism to configure them.
Accessing GPIOs at run-time generally means you want to change their
state, in which case we usually have dedicated applications that handle
the state transitions based on other events (e.g. "Power-on the host")
and as such there's no need for a general "GPIO initialisation" script.
These applications should apply the right state as part of their
initialisation.

Does that help?

If you have requirements outside of what's outlined above then I can't
see a problem with using a shell script to drive the libgpiod tools to
configure the necessary GPIOs, it just depends on your requirements
and how you feel like doing the system integration.

Cheers,

Andrew