Correcting I2C MAC address read

src/hardware/configuration.rs

@@ -495,13 +495,10 @@ pub fn setup(
             .set_speed(hal::gpio::Speed::VeryHigh);
     }
 
-    let mac_addr = match eeprom::read_eui48(&mut eeprom_i2c) {
-        Err(_) => {
-            info!("Could not read EEPROM, using default MAC address");
-            smoltcp::wire::EthernetAddress([0x10, 0xE2, 0xD5, 0x00, 0x03, 0x00])
-        }
-        Ok(raw_mac) => smoltcp::wire::EthernetAddress(raw_mac),
-    };
+    let mac_addr = smoltcp::wire::EthernetAddress(eeprom::read_eui48(
+        &mut eeprom_i2c,
+        &mut delay,
+    ));
 
     let network_devices = {
         // Configure the ethernet controller

src/hardware/eeprom.rs

@@ -1,12 +1,42 @@
-use embedded_hal::blocking::i2c::WriteRead;
+use embedded_hal::blocking::{delay::DelayMs, i2c::WriteRead};
 
+// The EEPROM is a variant without address bits, so the 3 LSB of this word are "dont-cares".
 const I2C_ADDR: u8 = 0x50;
 
-pub fn read_eui48<T>(i2c: &mut T) -> Result<[u8; 6], T::Error>
+// The MAC address is stored in the last 6 bytes of the 256 byte address space.
+const MAC_POINTER: u8 = 0xFA;
+
+pub fn read_eui48<T>(i2c: &mut T, delay: &mut impl DelayMs<u8>) -> [u8; 6]
 where
     T: WriteRead,
 {
-    let mut buffer = [0u8; 6];
-    i2c.write_read(I2C_ADDR, &[0xFA_u8], &mut buffer)?;
-    Ok(buffer)
+    let mut previous_read: Option<[u8; 6]> = None;
+    // On Stabilizer v1.1 and earlier hardware, there is a fault where the I2C bus is not connected
+    // to the CPU until the P12V0A rail enables, which can take many seconds, or may never come up
+    // at all. During these transient turn-on conditions, we may fail the I2C read operation. To
+    // accomodate this, we repeat the I2C read for a set number of attempts with a fixed delay
+    // between them. Then, we wait for the bus to stabilize by waiting until the MAC address
+    // read-out is identical for two consecutive reads.
+    for _ in 0..40 {
+        let mut buffer = [0u8; 6];
+        if i2c
+            .write_read(I2C_ADDR, &[MAC_POINTER], &mut buffer)
+            .is_ok()
+        {
+            if let Some(old_read) = previous_read {
+                if old_read == buffer {
+                    return buffer;
+                }
+            }
+
+            previous_read.replace(buffer);
+        } else {
+            // Remove any pending previous read if we failed the last attempt.
+            previous_read.take();
+        }
+
+        delay.delay_ms(100);
+    }
+
+    panic!("Failed to read MAC address");
 }