Monday, April 15, 2024

[fedora-arm] Re: Mapping GPIO Pins from Wandboard to Fedora/libpiod

> >> I have a Wandboard (yeah, I know, OLD Hardware) currently running a
> >
> > As in the imx6 based Wandboard? Which model/rev? What release of
> > Fedora, what kernel etc?
> Yes, an imx6 Wandboard. I'll have to go upstairs and open the case to see
> the EXACT model/rev printed on the PCB. Running "cat /proc/cpuinfo" only
> shows a single core. And it's running Fedora 36, currently running
> 5.18.9-200.fc36.armv7hl and libgpiod-1.6.3-7.fc36.armv7hl
> >> project, and I'm trying to add some GPIO inputs to it. In previous GPIO
> >> handling (on a different platform) I used the /sys/class/gpio interfaces
> >> to set up the GPIO, but I've discovered this is now deprecated and
> >> removed
> >> from Fedora. So I installed libgpiod and am trying to use it.
> >
> > Yes, we stopped supporting that because it was deprecated and upstream
> > asked us to.
> >
> >> I'm using
> >>
> >> as a reference for the board layout. Specifically, on page 27, it shows
> >> me that the JP4 header connects to GPIO3_12, GPIO3_27, GPIO6_31,
> >> CPIO1_24,
> >> GPIO7_8, GPIO3_26, GPIO_18, and GPIO_19.
> >>
> >> I figure that GPIO_18 is going to be gpiochip0 line 18, and GPIO3_12
> >> will
> >> be gpiochip3 line 12.. Except that gpiodetect only tells me I have
> >> gpiochip0 through gpiochip6, so I'm not sure where GPIO7_8 would get
> >> mapped. That's the first issue I have.
> >
> > You'd likely need to look at the device tree and pinmappings to see
> > how they're mapped out.
> I was trying to follow the device tree, and found a dts in the
> wandboard-org tree that seemed to match the settings from that PDF I
> pointed to. Of course, that may or may not be correct to the actual
> hardware I have running upstairs.

Best ones are probably the upstream ones:

> >> The second issue is that the kernel does not have any named mappings, so
> >> running gpioinfo just displays generic data, like:
> >>
> >> gpiochip6 - 32 lines:
> >> line 0: unnamed unused input active-high
> >> line 1: unnamed unused input active-high
> >>
> >> Except, of course, looking at gpiochip3 line 12 shows me:
> >>
> >> gpiochip3 - 32 lines:
> >> ...
> >> line 12: unnamed "scl" output active-high [used
> >> open-drain]
> >>
> >> Which seems to imply that that GPIO3_12 is actually used for something
> >> else -- or my mapping strategy is off.
> >
> > Probably a kernel device, often GPIOs are used for other things.
> Being only ancillarily associated with Arm/Embedded HW -- what does it
> mean for a GPIO to be "used for other things"? And more importantly, why
> would it be wired to a header if it's being used for something else?

So in the case I mention below the GPIO pin is used for i2c and it's
on that header so you could add say a i2c based temp sensor or other
i2c device.

Also board designers may use a GPIO to hook up a mSD card detect pin,
or a WiFi interface reset pin, or something else on the board layout.

You can see the default pin allocation here from line 152-195:

And the GPIOs mapped to i2c here on lines 103-104 and again 113-114,
and then as a camera enable/reset at 139-140:

> > A quick look at the dtsi for the wandboards some of the GPIOs re used
> > for SCL/SDA pins on two of the i2c buses. The i2c1 seems to not have
> > anything attached so I guess in on a pin header for end user use, and
> > i2c12 has a audio codec and for the camera connector.
> How exactly is this done? Is the pin wired to two places on the PCB?

It depends, for example on a RPi header you can use a DT overlay to
change the default use of a PIN, by default is might be a standard
GPIO but you apply an overlay that remaps it so it routes a i2s audio
interface so you can use a DAC to output sound. So it's generally more
about being able to use the reduced amounts of external pins for
different usecases, someone might want it in a robot, someone else
might want it to output audio.

> >> I'd appreciate any guidance anyone might have on this subject.
> >
> > If it's a iMX6 based Wandboard the guidance will be limted as the
> > support for ARMv7 went EOL when F-36 did so I have no recollection of
> > what kernel, libgpiod version etc was shipped there.
> Understood. I just happen to have bought into the Wandboard platform a
> long time ago, so this is what I've been using for the project. It uses a
> USB Relay, leveraging CRelay, so theoretically I could swap the system out
> for a different board in order to get 4 GPIOs for external switch
> detction, but honestly I'd rather not spend the money... :)
> > Peter
> -derek
> --
> Derek Atkins 617-623-3745
> Computer and Internet Security Consultant
arm mailing list --
To unsubscribe send an email to
Fedora Code of Conduct:
List Guidelines:
List Archives:
Do not reply to spam, report it:

No comments:

Post a Comment