Re: "leds: gpio: Use generic support for composing LED names" breaks gpio debugfs

From: Jacek Anaszewski <jacek.anaszewski@gmail.com>
To: Russell King - ARM Linux admin <linux@armlinux.org.uk>,
	Linus Walleij <linus.walleij@linaro.org>,
	Pavel Machek <pavel@ucw.cz>
Cc: linux-leds@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: "leds: gpio: Use generic support for composing LED names" breaks gpio debugfs
Date: Wed, 4 Dec 2019 21:52:49 +0100	[thread overview]
Message-ID: <fb88206c-a094-df37-04af-71a1174fb0f6@gmail.com> (raw)
In-Reply-To: <20191204110219.GV25745@shell.armlinux.org.uk>

Hi Russel,

On 12/4/19 12:02 PM, Russell King - ARM Linux admin wrote:
> Hi,
> 
> Patch d7235f5feaa0 ("leds: gpio: Use generic support for composing LED
> names") changed the way labels are used.
> 
> devm_fwnode_get_gpiod_from_child() is passed led.name, which used to be
> set to the label or node name, resulting in /sys/kernel/debug/gpio
> containing useful information.  For example:
> 
>  gpio-487 (                    |udpu:green:power    ) out lo
>  gpio-488 (                    |udpu:red:power      ) out hi
>  gpio-489 (                    |udpu:green:network  ) out hi
>  gpio-490 (                    |udpu:red:network    ) out hi
>  gpio-491 (                    |udpu:green:alarm    ) out hi
>  gpio-492 (                    |udpu:red:alarm      ) out hi
> 
> After this commit, it now contains a rather less useful:
> 
>  gpio-487 (                    |?                   ) out hi ACTIVE LOW
>  gpio-488 (                    |?                   ) out hi ACTIVE LOW
>  gpio-489 (                    |?                   ) out hi ACTIVE LOW
>  gpio-490 (                    |?                   ) out hi ACTIVE LOW
>  gpio-491 (                    |?                   ) out hi ACTIVE LOW
>  gpio-492 (                    |?                   ) out hi ACTIVE LOW
> 
> This is because led.name is now NULL prior to the call to
> devm_fwnode_get_gpiod_from_child().

Ugh, my bad. But it actually has been wrong since the patch
a96aa64cb5723 ("leds/led-class: Handle LEDs with the same name"),
which enabled LED core to amend the LED name just beforehand
the registration of a LED class device.

It seem that the only proper solution would be introducing a new
pre_register_cdev(const char *name) op to the LED core, that would allow
drivers to come up with their implementation thereof. In this particular
case leds-gpio driver would need to put there gpiod initialization.

The pre_register_device op would need to be called (when available)
from the led_classdev_register_ext() after calling
led_classdev_next_name() and before device_create_with_groups().
The op would have to be passed the final_name variable then.

> While this is not an API, it is useful for debugging, and given the
> dynamic allocation of GPIOs to physical GPIOs, having the labels
> present is an advantage.
> 
> Can this be fixed / old behaviour restored?

If there are no objections I can come up with a fix, but no
sooner than at the weekend.

-- 
Best regards,
Jacek Anaszewski

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