Re: [PATCH] i2c: i801: Fix handling SMBHSTCNT_PEC_EN

[Heiner Kallweit <hkallweit1@gmail.com>]

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@suse.de>
> 
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?