Re: [PATCH 1/3] Input: add `SW_BOOT_ALT`
From: Robin Murphy
Date: Tue Sep 27 2022 - 08:58:11 EST
On 2022-09-27 09:51, Quentin Schulz wrote:
Hi Jeff,
On 9/27/22 03:56, Jeff LaBundy wrote:
Hi Quentin,
On Mon, Sep 26, 2022 at 11:37:45AM +0200, Quentin Schulz wrote:
Hi all,
On 9/23/22 19:19, Jeff LaBundy wrote:
[...]
I have this switch on the board and I want to expose its state to
the user,
if this new event code is not possible/a good idea what would you
suggest we
could use?
Note that we already support the same switch but in a different
way:
https://urldefense.proofpoint.com/v2/url?u=https-3A__git.kernel.org_pub_scm_linux_kernel_git_torvalds_linux.git_tree_arch_arm64_boot_dts_rockchip_rk3399-2Dpuma-2Dhaikou.dts-23n167&d=DwIBAg&c=_sEr5x9kUWhuk4_nFwjJtA&r=LYjLexDn7rXIzVmkNPvw5ymA1XTSqHGq8yBP6m6qZZ4njZguQhZhkI_-172IIy1t&m=vObtDja6hUlL_kVvKRILCvaQfklw1gl7TAW7cm5_9H_Yi00zkzGV6-D3yzazR2gU&s=0Xo-7CDmMoriaTNn_hJdMwftLlcY-tgopPcgzXfQtJE&e=
We are just configuring the GPIOs into the GPIO mode with a
pull-up, and
then it's up to the user to use gpiod or gpio-sysfs to check the
state of
the GPIO used for this switch. I don't like this, very not
user-friendly and
was looking for something better :)
Actually, that's exactly what I was going to suggest. What in
particular
is not user-friendly about it?
The implementation we currently have in place for Puma Haikou is just
muxing
the pins in their GPIO function with a pull-up. Nothing else. This
means the
user still needs to know exactly which GPIO number is for which
signal/event. Not that great. Also does not allow for a consistent user
"experience" across modules on the same devkit (different GPIOs
depending on
the SoM).
I had hoped gpio-hog in input mode would expose it to userspace but
it does
not seem to?
No, I don't believe that is the case. That being said, you should be
able to grep /sys/kernel/debug/gpio for the hog node's label and get
the pin state (lo or hi).
Anything to recommend so that we can export a GPIO line for a specific
function to usersapce without needing the user to know which GPIO number
they should look at?
If debugfs won't work for your purposes, another option is to request
the GPIO from a general-purpose platform driver and return one of two
strings via sysfs.
This board will be fully supported upstream and I don't feel like this
general-purpose platform driver would be accepted, but maybe I'm wrong.
I didn't know about the /sys/kernel/debug/gpio file just yet for some
reason, so that's one way to do it, thanks for the info.
Good thing is that the kernel requests this pin in GPIO input mode so at
least that's one less possible misconfiguration from userspace. Not so
good thing is that you need to grep in this file and it's not that great
for scripts :) We'll discuss internally what feels the best to us and go
for it, thanks for the tips!
I'm wondering if allowing sysfs/libgpiod to access the state of GPIOs
requested by the kernel would make sense on the subsystem level though.
I'm sure I'm missing obvious security/HW-limitations pitfalls, but I
think allowing userspace to get the value of a GPIO input pin requested
by the kernel would be nice? Nothing for the input subsystem folks
though :)
FWIW there was a series a while back proposing to add a boot status
interface, but I'm not sure if there was any further progress:
https://lore.kernel.org/lkml/20220204072234.304543-1-joel@xxxxxxxxx/
Either way though, these kinds of GPIO (or ADC) configuration inputs for
things like boot functionality or board ID are just about common enough
that even if not tied in to any other specific interface, I reckon there
might be a valid argument to be made for a simple generic driver for
reporting the state of a named configuration input in a standardised
manner. Perhaps hwmon might be a better fit?
Robin.