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

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Date:   Tue, 15 Jun 2021 18:46:17 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Chris Packham <Chris.Packham@...iedtelesis.co.nz>
Cc:     Jean Delvare <jdelvare@...e.com>,
        "linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: pmbus: dealing with unsigned mantissa in linear reading

On Tue, Jun 15, 2021 at 11:34:53PM +0000, Chris Packham wrote:
> 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?

Essentially you'll have to implement a custom read word function and
convert the value reported by the power supply to a valid linear11
value. In this case, you may have to shift the reported value by 1 bit
and increase the exponent by one.

Not sure what you mean with "standard linear encoding can't support values
>255V". Why not ? That is what the exponent is for, after all. It rather
seems to me that the PS vendor decided to violate the standard to get
another bit of accuracy. 

Guenter