Add per-byte timeout budget for rp2 I2C (#5189)

* Add per-byte timeout budget for rp2 I2C

* run goimports

Author: sparques

src/machine/machine_rp2_i2c.go

@@ -280,8 +280,6 @@ func (i2c *I2C) deinit() (resetVal uint32) {
 
 // tx performs blocking write followed by read to I2C bus.
 func (i2c *I2C) tx(addr uint8, tx, rx []byte) (err error) {
-	const timeout_us = 4_000
-	deadline := ticks() + timeout_us
 	if addr >= 0x80 || isReservedI2CAddr(addr) {
 		return errInvalidTgtAddr
 	}
@@ -292,6 +290,14 @@ func (i2c *I2C) tx(addr uint8, tx, rx []byte) (err error) {
 		return nil
 	}
 
+	// Base 4ms for small register pokes.
+	// Add per-byte budget. 100us/byte is conservative at 400kHz and still ok at 100kHz for modest sizes.
+	timeout_us := uint64(4_000) + uint64(txlen+rxlen)*100
+	// Cap so it doesn't go insane:
+	timeout_us = min(timeout_us, 500_000)
+
+	deadline := ticks() + timeout_us
+
 	err = i2c.disable()
 	if err != nil {
 		return err