[PATCH v3] gpio: shared: make the voting mechanism adaptable

[Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>]

The current voting mechanism in GPIO shared proxy assumes that "low" is
always the default value and users can only vote for driving the GPIO
"high" in which case it will remain high as long as there's at least one
user voting.

This makes it impossible to use the automatic sharing management for
certain use-cases such as the write-protect GPIOs of EEPROMs which are
requested "high" and driven "low" to enable writing. In this case, if
the WP GPIO is shared by multiple EEPROMs, and at least one of them
wants to enable writing, the pin must be set to "low".

Modify the voting heuristic to assume the value set by the first user on
request to be the "default" and subseqent calls to gpiod_set_value()
will constitute votes for a change of the value to the opposite. In the
wp-gpios case it will mean that the nvmem core requests the GPIO as
"out-high" for all EEPROMs sharing the pin, and when one of them wants
to write, the pin will be driven low, enabling it.

Fixes: e992d54c6f97 ("gpio: shared-proxy: implement the shared GPIO proxy driver")
Reported-by: Marek Vasut <marex@nabladev.com>
Closes: https://lore.kernel.org/all/20260511163518.51104-1-marex@nabladev.com/
Reviewed-by: Linus Walleij <linusw@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
---
Changes in v3:
- Rebase on top of locking changes in gpio-shared-proxy.c
- Link to v2: https://patch.msgid.link/20260528-gpio-shared-dynamic-voting-v2-1-820837ce8cd2@oss.qualcomm.com

Changes in v2:
- Rebased on top of current linux-next which contains a fix for the
  shared GPIO proxy release path
- Link to v1: https://patch.msgid.link/20260513-gpio-shared-dynamic-voting-v1-1-8e1c49961b7d@oss.qualcomm.com
---
 drivers/gpio/gpio-shared-proxy.c | 66 +++++++++++++++++++---------------------
 drivers/gpio/gpiolib-shared.h    |  3 +-
 2 files changed, 34 insertions(+), 35 deletions(-)

diff --git a/drivers/gpio/gpio-shared-proxy.c b/drivers/gpio/gpio-shared-proxy.c
index 10ca2ef77ef3410b7be46a6657ebea8cd80c4491..52a366f0ec4de71e9d6b8f88ef77ef007b660c88 100644
--- a/drivers/gpio/gpio-shared-proxy.c
+++ b/drivers/gpio/gpio-shared-proxy.c
@@ -22,7 +22,7 @@ struct gpio_shared_proxy_data {
 	struct gpio_chip gc;
 	struct gpio_shared_desc *shared_desc;
 	struct device *dev;
-	bool voted_high;
+	bool voted_change;
 };
 
 static int
@@ -34,52 +34,54 @@ gpio_shared_proxy_set_unlocked(struct gpio_shared_proxy_data *proxy, int value)
 
 	lockdep_assert_held(&shared_desc->mutex);
 
-	if (value) {
-	       /* User wants to set value to high. */
-		if (proxy->voted_high)
-			/* Already voted for high, nothing to do. */
+	if (value != shared_desc->def_val) {
+	       /* User wants to vote for a value change. */
+		if (proxy->voted_change)
+			/* Already voted for a change, nothing to do. */
 			goto out;
 
-		/* Haven't voted for high yet. */
-		if (!shared_desc->highcnt) {
+		/* Haven't voted for a value change yet. */
+		if (!shared_desc->votecnt) {
 			/*
-			 * Current value is low, need to actually set value
-			 * to high.
+			 * Current value is default, need to actually set value
+			 * to the opposite.
 			 */
-			ret = gpiod_set_value_cansleep(desc, 1);
+			ret = gpiod_set_value_cansleep(desc, value);
 			if (ret)
 				goto out;
 		}
 
-		shared_desc->highcnt++;
-		proxy->voted_high = true;
+		shared_desc->votecnt++;
+		proxy->voted_change = true;
 
 		goto out;
 	}
 
-	/* Desired value is low. */
-	if (!proxy->voted_high)
-		/* We didn't vote for high, nothing to do. */
+	/* Desired value is the default. */
+	if (!proxy->voted_change)
+		/* We didn't vote for change previously, nothing to do. */
 		goto out;
 
-	/* We previously voted for high. */
-	if (shared_desc->highcnt == 1) {
-		/* This is the last remaining vote for high, set value  to low. */
-		ret = gpiod_set_value_cansleep(desc, 0);
+	/* We previously voted for change. */
+	if (shared_desc->votecnt == 1) {
+		/* This is the last remaining vote for change, set value to default. */
+		ret = gpiod_set_value_cansleep(desc, shared_desc->def_val);
 		if (ret)
 			goto out;
 	}
 
-	shared_desc->highcnt--;
-	proxy->voted_high = false;
+	shared_desc->votecnt--;
+	proxy->voted_change = false;
 
 out:
-	if (shared_desc->highcnt)
+	if (shared_desc->votecnt)
 		dev_dbg(proxy->dev,
-			"Voted for value '%s', effective value is 'high', number of votes for 'high': %u\n",
-			str_high_low(value), shared_desc->highcnt);
+			"Voted for value '%s', effective value is '%s', number of votes: %u\n",
+			str_high_low(value), str_high_low(!shared_desc->def_val),
+			shared_desc->votecnt);
 	else
-		dev_dbg(proxy->dev, "Voted for value 'low', effective value is 'low'\n");
+		dev_dbg(proxy->dev, "Voted for value '%s', effective value is '%s'\n",
+			str_high_low(value), str_high_low(shared_desc->def_val));
 
 	return ret;
 }
@@ -107,8 +109,8 @@ static void gpio_shared_proxy_free(struct gpio_chip *gc, unsigned int offset)
 
 	guard(mutex)(&shared_desc->mutex);
 
-	if (proxy->voted_high) {
-		ret = gpio_shared_proxy_set_unlocked(proxy, 0);
+	if (proxy->voted_change) {
+		ret = gpio_shared_proxy_set_unlocked(proxy, shared_desc->def_val);
 		if (ret)
 			dev_err(proxy->dev,
 				"Failed to unset the shared GPIO value on release: %d\n", ret);
@@ -197,13 +199,9 @@ static int gpio_shared_proxy_direction_output(struct gpio_chip *gc,
 		if (ret)
 			return ret;
 
-		if (value) {
-			proxy->voted_high = true;
-			shared_desc->highcnt = 1;
-		} else {
-			proxy->voted_high = false;
-			shared_desc->highcnt = 0;
-		}
+		shared_desc->def_val = value;
+		shared_desc->votecnt = 0;
+		proxy->voted_change = false;
 
 		return 0;
 	}
diff --git a/drivers/gpio/gpiolib-shared.h b/drivers/gpio/gpiolib-shared.h
index bbdc0ab7b647a44fca714bed0a7ff75101da7460..618756f6c6aafd087df267a4a32732ccc03af38e 100644
--- a/drivers/gpio/gpiolib-shared.h
+++ b/drivers/gpio/gpiolib-shared.h
@@ -41,7 +41,8 @@ struct gpio_shared_desc {
 	struct gpio_desc *desc;
 	unsigned long cfg;
 	unsigned int usecnt;
-	unsigned int highcnt;
+	unsigned int votecnt;
+	int def_val;
 	struct mutex mutex; /* serializes all proxy operations on this descriptor */
 };
 

---
base-commit: efecde8a254d1f207b75c5ebcfba2c51f4c771d9
change-id: 20260512-gpio-shared-dynamic-voting-9aab4689f5e9

Best regards,
-- 
Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>