linux-kernel - pmbus: dealing with unsigned mantissa in linear reading

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Date:   Tue, 15 Jun 2021 23:34:53 +0000
From:   Chris Packham <Chris.Packham@...iedtelesis.co.nz>
To:     Guenter Roeck <linux@...ck-us.net>,
        Jean Delvare <jdelvare@...e.com>
CC:     "linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: pmbus: dealing with unsigned mantissa in linear reading

Hi Guenter,

I've had a report from someone testing the BPA-RS600.

When they have the input voltage set to >= 256V (apparently the PSU can 
handle input up to 264V). The Vin reading goes negative.

CMD 0x88, VAL=0xf3f8, Vin=254
CMD 0x88, VAL=0xf3fc, Vin=255
CMD 0x88, VAL=0xf400, Vin=-256

Looking at pmbus_reg2data_linear() the mantissa is cast to s16 which is 
correct according to section 7.1 of the PMBUS spec which says that the 
mantissa is an 11-bit two's complement value.

It seems that the BPA-RS600 is using a non-standard format for the 
mantissa (probably because the range of the standard linear encoding 
can't support values >255V). Does the pmbus infrastructure provide a way 
for me to define a custom format/conversion for a given sensor?

Thanks,
Chris