W1: w1_therm.c is flagging 0C etc as invalid

commit d05074b1beb66d40283228bd226f7244407375bb

Author: David Fries <[email protected]>
Date:   Sat Jan 19 18:06:49 2008 -0600

The extra rom[0] check is flagging valid temperatures as invalid when there is already a CRC data transmission check.

w1_therm_read_bin()

	if (rom[8] == crc && rom[0])
		verdict = 1;

Requiring rom[0] to be non-zero will flag as invalid temperature conversions when the low byte is zero, specifically the temperatures 0C, 16C, 32C, 48C, -16C, -32C, and -48C.

The CRC check is produced on the device for the previous 8 bytes and is required to ensure the data integrity in transmission. I don't see why the extra check for rom[0] being non-zero is in there. Evgeniy Polyakov didn't know either. Just for a check I unplugged the sensor, executed a temperature conversion, and read the results. The read was all ff's, which also failed the CRC, so it doesn't need to protect against a disconnected sensor.

I have more extensive patches in the work, but these two trivial ones will do for today. I would like to hear from people who use the ds2490 USB to one wire dongle. 1 if you would be willing to test the patches as I currently only have the one sensor on a short parisite powered wire, 2 if there is any cheap sources for the ds2490.

index feed89e..112f4ec 100644
@@ -204,7 +204,7 @@ static ssize_t w1_therm_read_bin(struct kobject *kobj,
 
 				crc = w1_calc_crc8(rom, 8);
 
-				if (rom[8] == crc && rom[0])
+				if (rom[8] == crc)
 					verdict = 1;
 			}
 		}

David Fries <[email protected]>
http://fries.net/~david/ (PGP encryption key available)