linux-kernel - Re: [PATCH v2 04/10] gpio: shared-proxy: implement the shared GPIO proxy driver

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Message-ID: <aPkcpTWfTb0HOF51@smile.fi.intel.com>
Date: Wed, 22 Oct 2025 21:04:21 +0300
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Bartosz Golaszewski <brgl@...ev.pl>
Cc: Kees Cook <kees@...nel.org>, Mika Westerberg <westeri@...nel.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Manivannan Sadhasivam <mani@...nel.org>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Saravana Kannan <saravanak@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andy Shevchenko <andy@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Srinivas Kandagatla <srini@...nel.org>,
	Liam Girdwood <lgirdwood@...il.com>,
	Mark Brown <broonie@...nel.org>, Jaroslav Kysela <perex@...ex.cz>,
	Takashi Iwai <tiwai@...e.com>, linux-hardening@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-sound@...r.kernel.org,
	linux-arm-msm@...r.kernel.org,
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v2 04/10] gpio: shared-proxy: implement the shared GPIO
 proxy driver

On Wed, Oct 22, 2025 at 03:10:43PM +0200, Bartosz Golaszewski wrote:
> 
> Add a virtual GPIO proxy driver which arbitrates access to a single
> shared GPIO by multiple users. It works together with the core shared
> GPIO support from GPIOLIB and functions by acquiring a reference to a
> shared GPIO descriptor exposed by gpiolib-shared and making sure that
> the state of the GPIO stays consistent.
> 
> In general: if there's only one user at the moment: allow it to do
> anything as if this was a normal GPIO (in essence: just propagate calls
> to the underlying real hardware driver). If there are more users: don't
> allow to change the direction set by the initial user, allow to change
> configuration options but warn about possible conflicts and finally:
> treat the output-high value as a reference counted, logical "GPIO
> enabled" setting, meaning: the GPIO value is set to high when the first
> user requests it to be high and back to low once the last user stops
> "voting" for high.

I have two Q:s about the design:
1) why can't the value be counted on the struct gpio_desc level?
2) can gpio-aggregator facilities be reused (to some extent)?

...

> +#include <linux/auxiliary_bus.h>
> +#include <linux/cleanup.h>
> +#include <linux/device.h>
> +#include <linux/gpio/consumer.h>
> +#include <linux/gpio/driver.h>
> +#include <linux/module.h>

+ types.h

> +#include "gpiolib-shared.h"

...

> +out:
> +	if (shared_desc->highcnt)
> +		dev_dbg(proxy->dev,
> +			"Voted for value '%s', effective value is 'high', number of votes for 'high': %u\n",
> +			value ? "high" : "low", shared_desc->highcnt);
> +	else
> +		dev_dbg(proxy->dev, "Voted for value 'low', effective value is 'low'\n");

You can unify and maybe save a few bytes here and there by doing something like
this:

	const char *tmp; // name is a placeholder

	tmp = str_high_low(shared_desc->highcnt);
	dev_dbg(proxy->dev,
		"Voted for value '%s', effective value is '%s', number of votes for '%s': %u\n",
		str_high_low(value), tmp, tmp, shared_desc->highcnt);

...

> +		dev_dbg(proxy->dev,
> +			"Only one user of this shared GPIO, allowing to set direction to output with value '%s'\n",
> +			value ? "high" : "low");

str_high_low() ?

> +		ret = gpiod_direction_output(desc, value);
> +		if (ret)
> +			return ret;
> +
> +		if (value) {
> +			proxy->voted_high = true;
> +			shared_desc->highcnt = 1;
> +		} else {
> +			proxy->voted_high = false;
> +			shared_desc->highcnt = 0;
> +		}
> +
> +		return 0;
> +	}

-- 
With Best Regards,
Andy Shevchenko