On 22.07.2021 10:34, Jean Delvare wrote: > On Wed, 21 Jul 2021 14:46:20 +0200, Jean Delvare wrote: >> As for testing, I also don't have a PEC-cable device at hand. However, >> we may still be able to test this change: >> * If you have a device at 0x69 on the i801 SMBus of any of your system, >> that would be a clock device, which almost always support PEC. >> * If you have EEPROMs on your i801 SMBus, you may be lucky and find a >> sequence of bytes where the PEC computation leads to exactly the >> value of the following byte. I remember doing that years ago, sadly I >> can no longer find the script I wrote at that time. Be careful when >> accessing SPD EEPROMs, you want to read from them, not write to them >> ;-) Incidentally i2c-tools was just improved to allow arbitrary SMBus >> block read commands so i2cget can be used for easy testing from >> user-space. > > Well, what I wrote above wasn't accurate (bad memory I suppose). While > SMBus Block Read commands are OK to test the clock devices at 0x69, > they are not the best choice to poke a read-only EEPROM, as the first > byte will be interpreted as the block length, and if it is not between > 1 and 32, it is invalid and the transaction will fail, regardless of > PEC. Which in turn dramatically decreases the chances that at least one > offset in your EEPROM will work and be usable for testing purposes. > > Furthermore, i2cget has a safety to prevent you from messing up with > your SPD EEPROMs, that will deny using PEC at all in the I2C address > range 0x50-0x57. Which is exactly what I was suggesting to do. So I had > to recompile i2cget without the safety in order to preform my tests. To > be honest I think the safety is overkill (as far as I can see PEC would > only trash data in "c" mode so we could limit the safety to that mode) > but my testing being clearly a protocol abuse, I'm fine with having to > modify the source code to do it. > > Anyway, for the record, my hackish testing protocol is as follows: > > # rmmod at24 > # modprobe i2c-dev > # for i in `seq 0 254` ; do echo $i ; ./tools/i2cget -y 4 0x50 $i bp ; sleep 1 ; done > > Then I basically look at commands failing (on PEC error), until I am > lucky enough that the next byte in the EEPROM matches the expected PEC > value. I had 2 such offsets on my first SPD EEPROM (82 and 163). > Meaning that I was able to test your patch and I can confirm that it > works OK (testing limited to the 8 Series/C220 Series [8086:8c22] > device and SMBus Read Byte transactions, but I have no reason to > believe other devices or other transaction types would behave > differently). > > Tested-by: Jean Delvare <jdelvare@xxxxxxx> > Thanks for the comprehensive explanation, Jean. Now that you added your Tested-by: Would you prefer that I send a v2 that incorporates your two smaller comments? Or is it ok as-is?
