i2c: core: Add i2c_update_timeout() helper for dynamic transfer timeouts

The transfer timeout for an I2C controller should reflect the actual
message length and bus frequency rather than a static 1-second value.
A static timeout causes unnecessary delays on error paths for short
messages, and may be insufficient for very long transfers.

Add i2c_update_timeout() to i2c-core which computes a transfer-specific
timeout and stores it directly in the standard adap->timeout field.  The
formula accounts for 9 bits per byte (8 data + 1 ACK) at the configured
bus frequency.  The caller supplies a safety multiplier and a minimum
floor so that each driver retains full control over its timing policy
without those values becoming public API.

Storing the result in adap->timeout makes it visible to all consumers of
that field, including the arbitration-loss retry loop in __i2c_transfer().

The function is gated by CONFIG_I2C_DYNAMIC_TIMEOUT.  When the config is
disabled, i2c_update_timeout() compiles to a no-op inline stub so drivers
that call it build cleanly and the existing static 1-second default is
preserved unchanged.

A timeout explicitly configured by userspace via the I2C_TIMEOUT ioctl is
stored in a new adap->user_timeout field and always takes precedence over
the kernel-computed value.  When userspace has not configured a timeout,
the computed value is used.  The ioctl keeps writing adap->timeout as well,
so adapters that never call i2c_update_timeout() continue to honour it
exactly as before.

As i2c_update_timeout() is an exported helper, guard against a zero bus
frequency from a misbehaving caller with WARN_ON_ONCE() and return early,
leaving the existing timeout untouched as a safe fallback rather than
dividing by zero.

Signed-off-by: Aniket Randive <aniket.randive@oss.qualcomm.com>
Reviewed-by: Mukesh Kumar Savaliya <mukesh.savaliya@oss.qualcomm.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://patch.msgid.link/20260911-master-v9-1-77ac458344e2@oss.qualcomm.com

drivers/i2c/Kconfig

@@ -136,6 +136,19 @@ config I2C_SLAVE_TESTUNIT
 
 endif
 
+config I2C_DYNAMIC_TIMEOUT
+	bool "Dynamic per-transfer timeout based on message length"
+	depends on I2C
+	help
+	  When enabled, the I2C core computes a per-transfer timeout from the
+	  message length and bus frequency instead of using a static 1-second
+	  default.  A timeout explicitly configured via the I2C_TIMEOUT userspace
+	  interface always takes precedence over the computed value.
+
+	  When disabled, the existing static 1-second timeout is preserved.
+
+	  If unsure, say N.
+
 config I2C_DEBUG_CORE
 	bool "I2C Core debugging messages"
 	help

drivers/i2c/i2c-core-base.c

@@ -29,6 +29,7 @@
 #include <linux/irq.h>
 #include <linux/jump_label.h>
 #include <linux/kernel.h>
+#include <linux/math64.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
 #include <linux/of_device.h>
@@ -2010,6 +2011,48 @@ void i2c_parse_fw_timings(struct device *dev, struct i2c_timings *t, bool use_de
 }
 EXPORT_SYMBOL_GPL(i2c_parse_fw_timings);
 
+#ifdef CONFIG_I2C_DYNAMIC_TIMEOUT
+/**
+ * i2c_update_timeout - compute and set a dynamic transfer timeout on an adapter
+ * @adap: the i2c_adapter whose timeout field will be updated
+ * @bus_freq_hz: I2C bus clock frequency in Hz
+ * @len: transfer length in bytes
+ * @safety_coeff: multiplier applied over the theoretical wire time
+ * @min_usec: minimum timeout floor in microseconds
+ *
+ * Computes a transfer-specific timeout from the message length and bus
+ * frequency, applies a safety multiplier and a minimum floor, then stores
+ * the result in adap->timeout (in jiffies).  The caller supplies the policy
+ * constants so they remain internal to the driver.
+ *
+ * A timeout explicitly configured by userspace via the I2C_TIMEOUT ioctl
+ * always takes precedence; the computed value is used only when userspace
+ * has not configured one.
+ */
+void i2c_update_timeout(struct i2c_adapter *adap, u32 bus_freq_hz,
+			size_t len, unsigned int safety_coeff,
+			unsigned int min_usec)
+{
+	u64 bit_usec, total_usec;
+	unsigned long jiffies_val;
+
+	if (adap->user_timeout > 0) {
+		adap->timeout = adap->user_timeout;
+		return;
+	}
+
+	if (WARN_ON_ONCE(!bus_freq_hz))
+		return;
+
+	bit_usec = mul_u64_u32_div(len * 9, USEC_PER_SEC, bus_freq_hz);
+	total_usec = bit_usec * safety_coeff + min_usec;
+
+	jiffies_val = usecs_to_jiffies((unsigned int)min_t(u64, total_usec, UINT_MAX));
+	adap->timeout = (int)min_t(unsigned long, jiffies_val, INT_MAX);
+}
+EXPORT_SYMBOL_GPL(i2c_update_timeout);
+#endif /* CONFIG_I2C_DYNAMIC_TIMEOUT */
+
 /* ------------------------------------------------------------------------- */
 
 int i2c_for_each_dev(void *data, int (*fn)(struct device *dev, void *data))

drivers/i2c/i2c-dev.c

@@ -495,6 +495,9 @@ static long i2cdev_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 			return -EINVAL;
 
 		client->adapter->timeout = msecs_to_jiffies(arg * 10);
+#ifdef CONFIG_I2C_DYNAMIC_TIMEOUT
+		client->adapter->user_timeout = client->adapter->timeout;
+#endif
 		break;
 	default:
 		/* NOTE:  returning a fault code here could cause trouble

include/linux/i2c.h

@@ -757,6 +757,9 @@ struct i2c_adapter {
 	struct rt_mutex mux_lock;
 
 	int timeout;			/* in jiffies */
+#ifdef CONFIG_I2C_DYNAMIC_TIMEOUT
+	int user_timeout;		/* I2C_TIMEOUT ioctl value in jiffies */
+#endif
 	int retries;
 	struct device dev;		/* the adapter device */
 	unsigned long locked_flags;	/* owned by the I2C core */
@@ -928,6 +931,16 @@ unsigned int i2c_adapter_depth(struct i2c_adapter *adapter);
 
 void i2c_parse_fw_timings(struct device *dev, struct i2c_timings *t, bool use_defaults);
 
+#ifdef CONFIG_I2C_DYNAMIC_TIMEOUT
+void i2c_update_timeout(struct i2c_adapter *adap, u32 bus_freq_hz,
+			size_t len, unsigned int safety_coeff,
+			unsigned int min_usec);
+#else
+static inline void i2c_update_timeout(struct i2c_adapter *adap, u32 bus_freq_hz,
+				      size_t len, unsigned int safety_coeff,
+				      unsigned int min_usec) {}
+#endif
+
 /* Return the functionality mask */
 static inline u32 i2c_get_functionality(struct i2c_adapter *adap)
 {