Change read_bit() to return 0 or -1UL, rather than 0 or 1.

src/drivers/bitbash/bitbash.c

  * @v bit_id		Bit number
  * @ret data		Value read
  *
- * @c data will always be either 0 or 1.
+ * @c data will always be either 0 or -1UL.  The idea is that the
+ * caller can simply binary-AND the returned value with whatever mask
+ * it needs to apply.
  */
 int read_bit ( struct bit_basher *basher, unsigned int bit_id ) {
-	return ( basher->read ( basher, bit_id ) ? 1 : 0 );
+	return ( basher->read ( basher, bit_id ) ? -1UL : 0 );
 }

src/drivers/bitbash/i2c_bit.c

 	/* Receive byte */
 	for ( i = 8 ; i ; i-- ) {
 		value <<= 1;
-		value |= i2c_recv_bit ( basher );
+		value |= ( i2c_recv_bit ( basher ) & 0x1 );
 	}
 
 	/* Send NACK */

src/include/gpxe/i2c.h

 
 /** Bit indices used for I2C bit-bashing interface */
 enum {
+	/** Serial clock */
 	I2C_BIT_SCL = 0,
+	/** Serial data */
 	I2C_BIT_SDA,
 };