linux-kernel - Re: [PATCH 1/2] leds: pca955x: Make the gpiochip always expose all pins

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Message-Id: <ea912be8-ad2d-4332-97b4-5626b19eac5f@www.fastmail.com>
Date:   Mon, 01 Nov 2021 12:37:11 +1030
From:   "Andrew Jeffery" <andrew@...id.au>
To:     linux-leds@...r.kernel.org, linux-gpio@...r.kernel.org
Cc:     Cédric Le Goater <clg@...d.org>,
        "Rob Herring" <robh+dt@...nel.org>,
        "Joel Stanley" <joel@....id.au>, "Pavel Machek" <pavel@....cz>,
        "Linus Walleij" <linus.walleij@...aro.org>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-aspeed@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        "Andy Shevchenko" <andy.shevchenko@...il.com>
Subject: Re: [PATCH 1/2] leds: pca955x: Make the gpiochip always expose all pins

On Tue, 21 Sep 2021, at 14:09, Andrew Jeffery wrote:
> The devicetree binding allows specifying which pins are GPIO vs LED.
> Limiting the instantiated gpiochip to just these pins as the driver
> currently does requires an arbitrary mapping between pins and GPIOs, but
> such a mapping is not implemented by the driver. As a result,
> specifying GPIOs in such a way that they don't map 1-to-1 to pin indexes
> does not function as expected.
>
> Establishing such a mapping is more complex than not and even if we did,
> doing so leads to a slightly hairy userspace experience as the behaviour
> of the PCA955x gpiochip would depend on how the pins are assigned in the
> devicetree. Instead, always expose all pins via the gpiochip to provide
> a stable interface and track which pins are in use.
>
> Specifying a pin as `type = <PCA955X_TYPE_GPIO>;` in the devicetree
> becomes a no-op.
>
> I've assessed the impact of this change by looking through all of the
> affected devicetrees as of the tag leds-5.15-rc1:
>
> ```
> $ git grep -l 'pca955[0123]' $(find . -name dts -type d)
> arch/arm/boot/dts/aspeed-bmc-ibm-everest.dts
> arch/arm/boot/dts/aspeed-bmc-ibm-rainier.dts
> arch/arm/boot/dts/aspeed-bmc-opp-mihawk.dts
> arch/arm/boot/dts/aspeed-bmc-opp-mowgli.dts
> arch/arm/boot/dts/aspeed-bmc-opp-swift.dts
> arch/arm/boot/dts/aspeed-bmc-opp-tacoma.dts
> arch/arm/boot/dts/aspeed-bmc-opp-witherspoon.dts
> ```
>
> These are all IBM-associated platforms. I've analysed both the
> devicetrees and schematics where necessary to determine whether any
> systems hit the hazard of the current broken behaviour. For the most
> part, the systems specify the pins as either all LEDs or all GPIOs, or
> at least do so in a way such that the broken behaviour isn't exposed.
>
> The main counter-point to this observation is the Everest system whose
> devicetree describes a large number of PCA955x devices and in some cases
> has pin assignments that hit the hazard. However, there does not seem to
> be any use of the affected GPIOs in the userspace associated with
> Everest.
>
> Regardless, any use of the hazardous GPIOs in Everest is already broken,
> so let's fix the interface and then fix any already broken userspace
> with it.
>
> Signed-off-by: Andrew Jeffery <andrew@...id.au>

Hello LED maintainers,

Just checking in on the state of this as it hasn't appeared in for-next.

Andrew