Re: PMBus driver for FSP/3Y Power device with non-standard VOUT values (LINEAR11 vs LINEAR16)

[Guenter Roeck <linux@roeck-us.net>]

On Wed, Mar 13, 2019 at 12:31:53PM +0000, Grönke, Christian wrote:
> Hello all,
> 
> I am currently working on a hwmon/pmbus driver for a PMBus capable 3Y Power/FSP power supply (YH5301-1EAR).
> 
> The datasheet has limited information. But with the help of some online sources, other datasheets from the vendor and the pmbus sub-system I could figure out most of it.
> 
> However, I did run in some troubles with the VOUT values. To my understanding (and from what I could validate) the device does encode _all_ values as "LINEAR11". Meaning all values have a mantissa of 11 bit and the exponent in the other 5 bits of the register word. This causes some troubles with the read out of the VOUT registers (e.g. READ_VOUT). The pmbus subsystem expects these values to be in "LINEAR16" encoding. Hence the full word register is the mantissa and the exponent is supposed to be in VOUT_MODE.
> Sadly, the VOUT_MODE register isn't supported. It reads 0xFF.
> 
... violating its own specification...

> In my first attempt to work around this, I provided a custom read_word_data function that would fixup the value. However, that did lead to problems with negative values and also had a precision loss (12,09V -> 12V). I tried to compensate by faking the values as 'direct' and adjusting the m/b/R values to match. This is also not perfect, as it is messy and also seems to report the wrong values in some cases.

Messy is relative. Polluting generic code is just as messy.
What are those cases where a wrong value is reported ?

> 
> I think the best solution would be to prevent pmbus (more specifically 'pmbus_reg2data_linear') to treat the obtained value as LINEAR16. A quick hack shows me that this would work with all the reported values of the device. However, this means proving da vendor/device specific workaround by changing a generic component.
> 
> I am not sure how you feel about this. Ultimately, I'd like to upstream the driver (and potential fix), so I would like to find a solution that is fine with you all.
> 
> My current proposal would be to introduce a flag in pmbus_platform_data.flags that allows to disable the LINEAR16 switch in case the sensor has the class PSC_VOLTAGE_OUT. At least this seems the change with the smallest impact. I am not sure how to name the flag but to propose something I'd say 'PMBUS_VOUT_IS_LINEAR11'
> 
> What do you think?

Not sure if that is less messy.

Have you tried to fake the VOUT_MODE command and to provide an exponent
that makes sense ? While LINEAR16 does not support negative values,
I don't immediately see that as a practical problem, unless the power
supply indeed reports negative output voltages.

Thanks,
Guenter

P.s.: I asked for more information from the vendor, but I don't really expect to
get anything. Worth trying, though.