[PATCH v6 1/2] i2c: core: Add i2c_update_timeout() helper for dynamic transfer timeouts

[Aniket Randive]

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().

Signed-off-by: Aniket Randive <aniket.randive@xxxxxxxxxxxxxxxx>
---
 drivers/i2c/i2c-core-base.c | 24 ++++++++++++++++++++++++
 include/linux/i2c.h         |  3 +++
 2 files changed, 27 insertions(+)

diff --git a/drivers/i2c/i2c-core-base.c b/drivers/i2c/i2c-core-base.c
index 3ec04787a737..3280652e20d8 100644
--- a/drivers/i2c/i2c-core-base.c
+++ b/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>
@@ -2000,6 +2001,29 @@ void i2c_parse_fw_timings(struct device *dev, struct i2c_timings *t, bool use_de
 }
 EXPORT_SYMBOL_GPL(i2c_parse_fw_timings);
 
+/**
+ * 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.
+ */
+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 = mul_u64_u32_div(len * 9, USEC_PER_SEC, bus_freq_hz);
+
+	adap->timeout = usecs_to_jiffies(bit_usec * safety_coeff + min_usec);
+}
+EXPORT_SYMBOL_GPL(i2c_update_timeout);
+
 /* ------------------------------------------------------------------------- */
 
 int i2c_for_each_dev(void *data, int (*fn)(struct device *dev, void *data))
diff --git a/include/linux/i2c.h b/include/linux/i2c.h
index 14ab4d3055af..b52974eb7e58 100644
--- a/include/linux/i2c.h
+++ b/include/linux/i2c.h
@@ -912,6 +912,9 @@ void i2c_put_adapter(struct i2c_adapter *adap);
 unsigned int i2c_adapter_depth(struct i2c_adapter *adapter);
 
 void i2c_parse_fw_timings(struct device *dev, struct i2c_timings *t, bool use_defaults);
+void i2c_update_timeout(struct i2c_adapter *adap, u32 bus_freq_hz,
+			size_t len, unsigned int safety_coeff,
+			unsigned int min_usec);
 
 /* Return the functionality mask */
 static inline u32 i2c_get_functionality(struct i2c_adapter *adap)

-- 
2.34.1