linux-kernel - Re: [PATCH] iio: industrialio-core: look for aliases to request device index

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Message-ID: <ZfejyEvPIncygKJ9@atmark-techno.com>
Date: Mon, 18 Mar 2024 11:15:36 +0900
From: Dominique Martinet <dominique.martinet@...ark-techno.com>
To: David Lechner <dlechner@...libre.com>
Cc: Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
	Jonathan Cameron <jic23@...nel.org>,
	Syunya Ohshio <syunya.ohshio@...ark-techno.com>,
	Guido Günther <agx@...xcpu.org>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Rob Herring <robh+dt@...nel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	Conor Dooley <conor+dt@...nel.org>, linux-iio@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iio: industrialio-core: look for aliases to request
 device index

David Lechner wrote on Fri, Mar 15, 2024 at 10:53:36AM -0500:
> How about using udev rules to create symlinks for each device based on
> the label attribute? No changes to the kernel are needed.

Right, it's definitely possible to make symlinks for each "device" -- my
patch comment links to such an udev script "solution":
https://git.toradex.com/cgit/meta-toradex-bsp-common.git/tree/recipes-core/udev/files/verdin-imx8mm/toradex-adc.sh?h=kirkstone-6.x.y
(the script is launched by udev here:
https://git.toradex.com/cgit/meta-toradex-bsp-common.git/tree/recipes-core/udev/files/verdin-imx8mm/99-toradex.rules
)

My conceptual problem with this is that this makes symlinks in /dev to
files in /sys and it feels like we're crossing boundaries.
As far as I can tell there is no way for userspace to create arbitrary
symlinks in /sys, so I think we could have an interface more
user-friendly by allowing paths to be static for users with multiple
devices.
(I guess that's a weak argument given e.g. disks etc will also have an
unreliable name in /sys in the general case, but simple programs don't
interact with them in /sys and can use stable links in /dev so my
expectations here aren't quite the same)

Ultimately, the problem might run deeper in that we're having userspace
interact with the device through /sys and not the /dev char dev... As
far as I could see /dev/iio:deviceX only allows reading buffered values
and doesn't have any ioctl or other way of reading immediate values as
is possible in /sys though, so that'd require quite a bit of work to
duplicate the interface there...

Perhaps I'm just thinking too much and symlinks from /dev to /sys are a
thing in the IIO world? I've not seen it done anywhere except in that
toradex tree when I was looking earlier.

Andy Shevchenko wrote on Sat, Mar 16, 2024 at 10:14:35PM +0200:
> [...]

Thank you for the review!

>> +#include <linux/of.h>
>
> What about ACPI?
> Please try avoid hard to use OF-specific code for the new features.

Given my suggestion here relied on users giving manual hints in the DTB
I'm not sure how that could be interfaced with ACPI, but if you have a
suggestion to make paths static that'd work with either interfaces I'd
be more than happy to give it a try.

I'd also like to add that in my particular case it's a problem created
by the OF interface in the first place: devices are currently created in
the order they're parsed from OF, and it just so happens that this order
doesn't work well for us; I'm not aware of how IIO interacts with ACPI
but perhaps the way the list of devices processed from ACPI is "stable
enough" in practice?

Thank you,
-- 
Dominique