[PATCH v2 0/2] gpio: shared-proxy: Fix line accounting

[PATCH v2 0/2] gpio: shared-proxy: Fix line accounting

[Shawn Guo]

Two fixes for the shared GPIO proxy's accounting of users and votes.

Patch 1 fixes a race between consumers that request a shared line
concurrently. Each of them sees usecnt > 1, assumes another user has
already configured the line, and fails with -EPERM. On Nord RRD this
intermittently leaves both DAC supply regulators without their enable
GPIO.

Patch 2 fixes a stale vote left behind when a proxy releases the line
and restoring the default value fails. The line stays at the
non-default value for the remaining users, and votecnt can underflow
when the line is requested again.

Changes in v2:
- Patch 1: Also use gpio_shared_proxy_may_init() instead of
  usecnt > 1 in set_config, so concurrent first users no longer skip
  the hardware config or log a spurious conflict (Sashiko)
- Patch 2: New. Fix the votecnt underflow on a failed write in free.
  It predates patch 1 (Sashiko)
- Link to v1: https://lore.kernel.org/all/20261015122740.971792-1-shengchao.guo@oss.qualcomm.com

Shawn Guo (2):
  gpio: shared-proxy: Fix race between concurrent first users
  gpio: shared-proxy: Drop the vote of a released proxy on write failure

 drivers/gpio/gpio-shared-proxy.c | 80 ++++++++++++++++++++++++++++----
 drivers/gpio/gpiolib-shared.h    |  1 +
 2 files changed, 71 insertions(+), 10 deletions(-)

-- 
2.43.0

[PATCH v2 1/2] gpio: shared-proxy: Fix race between concurrent first users

[Shawn Guo]

The proxy lets a consumer set the line direction and reset the
default value and vote tally only when usecnt == 1, i.e. when it is
the sole requester. But request() and direction_output() take the
descriptor mutex separately. When two consumers probe concurrently,
both request the line before either configures it, both see
usecnt == 2, both fall into the shared path, read back the still
unconfigured direction (input) and fail with -EPERM.

On Nord RRD the two DAC supplies are reg-fixed-voltage regulators
sharing tlmm GPIO 41. They probe asynchronously and intermittently
both fail, leaving the codecs without supplies:

  reg-fixed-voltage regulator-3p3-dac: setup of GPIO (default) failed: -1
  reg-fixed-voltage regulator-5p0-dac: setup of GPIO (default) failed: -1

Requesting the line is not evidence that a consumer has claimed it.
Instead, count proxies that have actually configured the line (set its
direction or voted for a value) in a new cfgcnt, with a per-proxy
configured flag, and allow (re)initialization only while no other
proxy has done so. A consumer obtained with GPIOD_ASIS that votes via
gpiod_set_value() without ever setting the direction counts as
configured, so a later user can never reset def_val/votecnt behind its
vote. The count is dropped when the proxy frees the line.

Fixes: e992d54c6f97 ("gpio: shared-proxy: implement the shared GPIO proxy driver")
Assisted-by: LLM
Signed-off-by: Shawn Guo <shengchao.guo@oss.qualcomm.com>
---
 drivers/gpio/gpio-shared-proxy.c | 74 ++++++++++++++++++++++++++++----
 drivers/gpio/gpiolib-shared.h    |  1 +
 2 files changed, 66 insertions(+), 9 deletions(-)

diff --git a/drivers/gpio/gpio-shared-proxy.c b/drivers/gpio/gpio-shared-proxy.c
index bc69b8729d19..0d1d724a6a37 100644
--- a/drivers/gpio/gpio-shared-proxy.c
+++ b/drivers/gpio/gpio-shared-proxy.c
@@ -21,9 +21,33 @@ struct gpio_shared_proxy_data {
 	struct gpio_chip gc;
 	struct gpio_shared_desc *shared_desc;
 	struct device *dev;
+	bool configured;
 	bool voted_change;
 };
 
+/*
+ * A proxy may (re)initialize the line - set its direction and reset the
+ * default value and the vote tally - only while no other proxy has set the
+ * direction or voted for a value. Requesting the line is not enough: consumers
+ * may request concurrently, so counting requests lets every one of them
+ * conclude that it is not first.
+ */
+static bool gpio_shared_proxy_may_init(struct gpio_shared_proxy_data *proxy)
+{
+	unsigned int cfgcnt = proxy->shared_desc->cfgcnt;
+
+	return !cfgcnt || (cfgcnt == 1 && proxy->configured);
+}
+
+static void gpio_shared_proxy_mark_configured(struct gpio_shared_proxy_data *proxy)
+{
+	if (proxy->configured)
+		return;
+
+	proxy->configured = true;
+	proxy->shared_desc->cfgcnt++;
+}
+
 static int
 gpio_shared_proxy_set_unlocked(struct gpio_shared_proxy_data *proxy, int value)
 {
@@ -117,6 +141,11 @@ static void gpio_shared_proxy_free(struct gpio_chip *gc, unsigned int offset)
 
 	proxy->shared_desc->usecnt--;
 
+	if (proxy->configured) {
+		proxy->configured = false;
+		shared_desc->cfgcnt--;
+	}
+
 	dev_dbg(proxy->dev, "Shared GPIO freed, number of users: %u\n",
 		proxy->shared_desc->usecnt);
 }
@@ -131,7 +160,7 @@ static int gpio_shared_proxy_set_config(struct gpio_chip *gc,
 
 	guard(mutex)(&shared_desc->mutex);
 
-	if (shared_desc->usecnt > 1) {
+	if (!gpio_shared_proxy_may_init(proxy)) {
 		if (shared_desc->cfg != cfg) {
 			dev_dbg(proxy->dev,
 				"Shared GPIO's configuration already set, accepting changes but users may conflict!!\n");
@@ -155,15 +184,21 @@ static int gpio_shared_proxy_direction_input(struct gpio_chip *gc,
 	struct gpio_shared_proxy_data *proxy = gpiochip_get_data(gc);
 	struct gpio_shared_desc *shared_desc = proxy->shared_desc;
 	struct gpio_desc *desc = shared_desc->desc;
-	int dir;
+	int ret, dir;
 
 	guard(mutex)(&shared_desc->mutex);
 
-	if (shared_desc->usecnt == 1) {
+	if (gpio_shared_proxy_may_init(proxy)) {
 		dev_dbg(proxy->dev,
-			"Only one user of this shared GPIO, allowing to set direction to input\n");
+			"No other user configured this shared GPIO, allowing to set direction to input\n");
 
-		return gpiod_direction_input(desc);
+		ret = gpiod_direction_input(desc);
+		if (ret)
+			return ret;
+
+		gpio_shared_proxy_mark_configured(proxy);
+
+		return 0;
 	}
 
 	dir = gpiod_get_direction(desc);
@@ -176,6 +211,8 @@ static int gpio_shared_proxy_direction_input(struct gpio_chip *gc,
 		return -EPERM;
 	}
 
+	gpio_shared_proxy_mark_configured(proxy);
+
 	return 0;
 }
 
@@ -189,18 +226,20 @@ static int gpio_shared_proxy_direction_output(struct gpio_chip *gc,
 
 	guard(mutex)(&shared_desc->mutex);
 
-	if (shared_desc->usecnt == 1) {
+	if (gpio_shared_proxy_may_init(proxy)) {
 		dev_dbg(proxy->dev,
-			"Only one user of this shared GPIO, allowing to set direction to output with value '%s'\n",
+			"No other user configured this shared GPIO, allowing to set direction to output with value '%s'\n",
 			str_high_low(value));
 
 		ret = gpiod_direction_output(desc, value);
 		if (ret)
 			return ret;
 
+		/* Only this proxy can have voted, so the tally is ours to reset */
 		shared_desc->def_val = value;
 		shared_desc->votecnt = 0;
 		proxy->voted_change = false;
+		gpio_shared_proxy_mark_configured(proxy);
 
 		return 0;
 	}
@@ -215,7 +254,13 @@ static int gpio_shared_proxy_direction_output(struct gpio_chip *gc,
 		return -EPERM;
 	}
 
-	return gpio_shared_proxy_set_unlocked(proxy, value);
+	ret = gpio_shared_proxy_set_unlocked(proxy, value);
+	if (ret)
+		return ret;
+
+	gpio_shared_proxy_mark_configured(proxy);
+
+	return 0;
 }
 
 static int gpio_shared_proxy_get_cansleep(struct gpio_chip *gc,
@@ -230,10 +275,21 @@ static int gpio_shared_proxy_set_cansleep(struct gpio_chip *gc,
 					  unsigned int offset, int value)
 {
 	struct gpio_shared_proxy_data *proxy = gpiochip_get_data(gc);
+	int ret;
 
 	guard(mutex)(&proxy->shared_desc->mutex);
 
-	return gpio_shared_proxy_set_unlocked(proxy, value);
+	/*
+	 * A GPIOD_ASIS consumer may vote without ever setting the direction;
+	 * count it as configured so a later user can't reset its vote.
+	 */
+	ret = gpio_shared_proxy_set_unlocked(proxy, value);
+	if (ret)
+		return ret;
+
+	gpio_shared_proxy_mark_configured(proxy);
+
+	return 0;
 }
 
 static int gpio_shared_proxy_get_direction(struct gpio_chip *gc,
diff --git a/drivers/gpio/gpiolib-shared.h b/drivers/gpio/gpiolib-shared.h
index 618756f6c6aa..ae10506b7029 100644
--- a/drivers/gpio/gpiolib-shared.h
+++ b/drivers/gpio/gpiolib-shared.h
@@ -41,6 +41,7 @@ struct gpio_shared_desc {
 	struct gpio_desc *desc;
 	unsigned long cfg;
 	unsigned int usecnt;
+	unsigned int cfgcnt;
 	unsigned int votecnt;
 	int def_val;
 	struct mutex mutex; /* serializes all proxy operations on this descriptor */
-- 
2.43.0

[PATCH v2 2/2] gpio: shared-proxy: Drop the vote of a released proxy on write failure

[Shawn Guo]

When a proxy that voted for a value change frees the line, it calls
gpio_shared_proxy_set_unlocked() to withdraw its vote. If it is the
last voter and restoring the default value fails, that function
returns early, leaving votecnt incremented and voted_change set.

The consumer is gone, but its vote is not. The line is held at the
non-default value for all remaining users, since votecnt can never
drop to zero again. And the proxy data outlives the request, so
voted_change is still set when the line is requested again. If
another user has reinitialized the line in between, resetting
votecnt to 0, the stale proxy's next vote for the default value
decrements votecnt from 0 and underflows it.

Drop the vote unconditionally on release, after reporting the
failure, since the consumer is going away regardless.

Fixes: e992d54c6f97 ("gpio: shared-proxy: implement the shared GPIO proxy driver")
Assisted-by: LLM
Signed-off-by: Shawn Guo <shengchao.guo@oss.qualcomm.com>
---
 drivers/gpio/gpio-shared-proxy.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/gpio/gpio-shared-proxy.c b/drivers/gpio/gpio-shared-proxy.c
index 0d1d724a6a37..24241c981df1 100644
--- a/drivers/gpio/gpio-shared-proxy.c
+++ b/drivers/gpio/gpio-shared-proxy.c
@@ -134,9 +134,13 @@ static void gpio_shared_proxy_free(struct gpio_chip *gc, unsigned int offset)
 
 	if (proxy->voted_change) {
 		ret = gpio_shared_proxy_set_unlocked(proxy, shared_desc->def_val);
-		if (ret)
+		if (ret) {
 			dev_err(proxy->dev,
 				"Failed to unset the shared GPIO value on release: %d\n", ret);
+			/* The consumer is going away regardless, drop its vote */
+			shared_desc->votecnt--;
+			proxy->voted_change = false;
+		}
 	}
 
 	proxy->shared_desc->usecnt--;
-- 
2.43.0

Re: [PATCH v2 1/2] gpio: shared-proxy: Fix race between concurrent first users

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] gpio: shared: set_value(0) is a no-op on ASIS lines driven HIGH
--

commit b2e913f9e7adbb02eb24728f50149f4b95e20857
Author: Shawn Guo <shengchao.guo@oss.qualcomm.com>

gpio: shared-proxy: Fix race between concurrent first users

This commit fixes a race condition where multiple consumers probing concurrently
can both request a shared GPIO before either configures it, causing both to fail.
It tracks the number of proxies that have actually configured the line, allowing
initialization only when no other proxy has done so.

> diff --git a/drivers/gpio/gpio-shared-proxy.c b/drivers/gpio/gpio-shared-proxy.c
> index bc69b8729d196..0d1d724a6a376 100644
> --- a/drivers/gpio/gpio-shared-proxy.c
> +++ b/drivers/gpio/gpio-shared-proxy.c

[ ... ]

> @@ -230,10 +275,21 @@ static int gpio_shared_proxy_set_cansleep(struct gpio_chip *gc,
>  					  unsigned int offset, int value)
>  {
>  	struct gpio_shared_proxy_data *proxy = gpiochip_get_data(gc);
> +	int ret;
>  
>  	guard(mutex)(&proxy->shared_desc->mutex);
>  
> -	return gpio_shared_proxy_set_unlocked(proxy, value);
> +	/*
> +	 * A GPIOD_ASIS consumer may vote without ever setting the direction;
> +	 * count it as configured so a later user can't reset its vote.
> +	 */
> +	ret = gpio_shared_proxy_set_unlocked(proxy, value);

[Severity: Medium]
If a consumer requests a shared GPIO with GPIOD_ASIS and the physical line
is already driven HIGH, does shared_desc->def_val still default to 0?

When the consumer later calls gpiod_set_value(0) to drive the line LOW,
gpio_shared_proxy_set_unlocked() checks if value != shared_desc->def_val.
Since both value and def_val are 0, it skips calling
gpiod_set_value_cansleep() and returns success without changing the
hardware state.

Does this result in set_value(0) acting as a no-op on ASIS lines that are
initially driven HIGH?

> +	if (ret)
> +		return ret;
> +
> +	gpio_shared_proxy_mark_configured(proxy);
> +
> +	return 0;
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261016144919.1362846-1-shengchao.guo@oss.qualcomm.com?part=1