misc: add LogicalUnit::from_ls_bits and use it

src/connection/mod.rs

@@ -101,18 +101,31 @@ pub enum LogicalUnit {
     Three,
 }
 
+impl LogicalUnit {
+    /// Construct a `LogicalUnit` from the two lowest bits of `value`,
+    /// ignoring all other bits.
+    pub fn from_low_bits(value: u8) -> Self {
+        let value = value & 0b11;
+
+        match value {
+            0b00 => Self::Zero,
+            0b01 => Self::One,
+            0b10 => Self::Two,
+            0b11 => Self::Three,
+            _ => unreachable!("Value bitmasked with 0b11 has value greater than 3"),
+        }
+    }
+}
+
 impl TryFrom<u8> for LogicalUnit {
     type Error = ();
 
     fn try_from(value: u8) -> Result<Self, Self::Error> {
-        let val = match value {
-            0 => Self::Zero,
-            1 => Self::One,
-            2 => Self::Two,
-            3 => Self::Three,
-            _ => return Err(()),
-        };
-        Ok(val)
+        if value <= 0b11 {
+            Ok(Self::from_low_bits(value))
+        } else {
+            Err(())
+        }
     }
 }
 

src/storage/sdr/record/fru_device_locator.rs

@@ -68,8 +68,7 @@ impl FruDeviceLocator {
             })
         };
 
-        // NOTE(unwrap): `LogicalUnit::try_from` always succeeds for data masked with 0b11.
-        let lun = LogicalUnit::try_from((record_data[2] >> 3) & 0b11).unwrap();
+        let lun = LogicalUnit::from_low_bits(record_data[2] >> 3);
         let private_bus_id = record_data[2] & 0b111;
         let channel_number = record_data[3];
 

src/storage/sdr/record/mod.rs

@@ -66,9 +66,8 @@ impl SensorKey {
         // NOTE(unwrap): value is guaranteed to be in the correct range due to mask + shift.
         let owner_channel = Channel::new((owner_channel_fru_lun & 0xF0) >> 4).unwrap();
 
-        let fru_inv_device_owner_lun =
-            LogicalUnit::try_from((owner_channel_fru_lun >> 2) & 0x3).unwrap();
-        let owner_lun = LogicalUnit::try_from(owner_channel_fru_lun & 0x3).unwrap();
+        let fru_inv_device_owner_lun = LogicalUnit::from_low_bits(owner_channel_fru_lun >> 2);
+        let owner_lun = LogicalUnit::from_low_bits(owner_channel_fru_lun & 0b11);
 
         let sensor_number =
             SensorNumber(NonMaxU8::new(record_data[2]).ok_or(ParseError::InvalidSensorNumber)?);

src/storage/sel/mod.rs

@@ -93,7 +93,7 @@ impl From<(u8, u8)> for EventGenerator {
                 channel_number,
             }
         } else {
-            let lun = (value.1 & 0x3).try_into().unwrap();
+            let lun = LogicalUnit::from_low_bits(value.1);
 
             Self::RqSAAndLun {
                 i2c_addr: i2c_or_sid,