python: correct i2c detection logic

The absence of I2C functionality does not imply SMBus ability. Instead we check for required SMBus functionality and find the bus with the most SPD EEPROMs.

On my system the old code hits bus 3 and stops. The new code finds bus 6 with 0x50 and bus 1 with both 0x50 and 0x51, and correctly selects bus 1.

Author: Artoria2e5

spd-eeprom.py

@@ -29,7 +29,17 @@
     sys.exit("This operating system is not supported.")
 
 # To determine what functionality is present
-I2C_FUNC_I2C = 0x00000001
+I2C_FUNC_SMBUS_QUICK = 0x00010000
+I2C_FUNC_SMBUS_READ_BYTE = 0x00020000
+I2C_FUNC_SMBUS_WRITE_BYTE = 0x00040000
+I2C_FUNC_SMBUS_READ_BYTE_DATA = 0x00080000
+I2C_FUNC_SMBUS_WRITE_BYTE_DATA = 0x00100000
+I2C_FUNC_SMBUS_READ_WORD_DATA = 0x00200000
+I2C_FUNC_SMBUS_WRITE_WORD_DATA = 0x00400000
+
+REQUIRED_FUNC = I2C_FUNC_SMBUS_READ_BYTE | I2C_FUNC_SMBUS_WRITE_BYTE | \
+                I2C_FUNC_SMBUS_READ_BYTE_DATA | I2C_FUNC_SMBUS_WRITE_BYTE_DATA | \
+                I2C_FUNC_SMBUS_QUICK
 
 # Data for SMBus Messages
 class i2c_smbus_data(ctypes.Union):
@@ -105,6 +115,10 @@ def print_usage():
     print("    -w           write data to SPD EEPROM (input from file)")
     print("    -d DIMM      DIMM slot index (0 - 7)")
     print("    -f FILE      input or output file path")
+    print()
+    print("Environment Variables:")
+    print("    SPD_EEPROM_SMBUS   specify the SMBus index to use (e.g. 1 for /dev/i2c-1)")
+    print()
 
 def spd_set_page(fd, page):
     try:
@@ -213,48 +227,75 @@ def smbus_probe(dimm_slot = None):
     except Exception:
         pass
 
-    smbus_idx = ""
+    fd = None
+    smbus_idx = None
 
     files = os.listdir("/dev")
     files = list(filter(lambda x: x.startswith("i2c-"), files))
 
-    for entry in files:
-        fd = os.open("/dev/" + entry, os.O_RDWR)
-        if not i2c_smbus_get_funcs(fd) & I2C_FUNC_I2C:
-            smbus_idx = entry[4:]
-            break
-
-    if smbus_idx.isdigit():
-        smbus_idx = int(smbus_idx)
+    if os.getenv("SPD_EEPROM_SMBUS") is not None:
+        fd, smbus_idx, n = try_smbus("i2c-" + os.getenv("SPD_EEPROM_SMBUS"), dimm_slot)
+        if fd is None:
+            sys.exit("Could not access SMBus %s." % os.getenv("SPD_EEPROM_SMBUS"))
+        else:
+            return fd, smbus_idx
     else:
-        sys.exit("No SMBus adapter found.")
-
-    if dimm_slot == None:
-        print("Probing for SPD EEPROM on SMBus %d" % smbus_idx)
+        rets = [try_smbus(x, dimm_slot) for x in files]
+        rets = list(filter(lambda x: x[0] is not None, rets))
+        rets = sorted(rets, key=lambda x: x[2], reverse=True)
+        if len(rets) == 0:
+            sys.exit("No supported SMBus found.")
+        else:
+            fd, smbus_idx = rets[0][0], rets[0][1]
+            print("Selecting /dev/i2c-%d by EEPROM count" % smbus_idx)
+            return fd, smbus_idx
+
+
+def try_smbus(file: str, dimm_slot = None):
+    fd = os.open("/dev/" + file, os.O_RDWR)
+    smbus_name: str = file[4:]
+    if i2c_smbus_get_funcs(fd) & REQUIRED_FUNC != REQUIRED_FUNC:
+        print("I2C adapter %s does not support required SMBus functions." % file)
         print()
+        os.close(fd)
+        return None, None, None
 
+    if smbus_name.isdigit():
+        smbus_idx: int = int(smbus_name)
+    else:
+        print("Invalid SMBus index: %s" % smbus_name)
+        return None, None, None
+
+    print("Probing for SPD EEPROM on SMBus %d" % smbus_idx)
+    if dimm_slot is None:
         eeprom = 0
         ee1004 = spd_set_page(fd, 0)
 
         for slot in range(0, 8):
             try:
                 i2c_smbus_read_byte(fd, 0x50 + slot)
-
                 print("DIMM slot %d: %s SPD EEPROM" % (slot, "512 Byte EE1004" if ee1004 else "256 Byte AT24"))
-
                 eeprom += 1
             except IOError:
                 pass
 
         if eeprom == 0:
-            print("No SPD EEPROM detected.")
+            print("No SPD EEPROM detected on SMBus %d." % smbus_idx)
+            print()
+            os.close(fd)
+            return None, None, None
     else:
         try:
             i2c_smbus_read_byte(fd, 0x50 + dimm_slot)
+            eeprom = 1
         except IOError:
-            sys.exit("DIMM slot %d is empty." % dimm_slot)
+            print("DIMM slot %d is empty." % dimm_slot)
+            os.close(fd)
+            print()
+            return None, None, None
 
-        return fd, smbus_idx
+    print()
+    return fd, smbus_idx, eeprom
 
 def main():
     if os.getuid():
@@ -266,7 +307,7 @@ def main():
         sys.exit(print_usage())
 
     op_code = 0
-    dimm_slot = ""
+    dimm_slot = None
     file_path = ""
 
     for opt, arg in opts:
@@ -282,7 +323,9 @@ def main():
             file_path = arg
 
     if op_code == 1 and len(opts) == 1:
-        smbus_probe()
+        _, smbus_idx = smbus_probe()
+        print("To save time for next time, you can specify the SMBus index" \
+        " by: `export SPD_EEPROM_SMBUS=%d`." % smbus_idx)
     elif op_code != 0 \
         and len(opts) == 3 \
         and len(file_path) != 0 \