Re: [PATCH 2/2] hwmon: (pmbus/max20830) Add support for MAX20810, MAX20815, LTM4739, LTM4740

[Carlos Jones]

From: Carlos Jones Jr <carlosjr.jones@xxxxxxxxxx>

Hi Guenter,

Thanks for the review.

> According to the datasheets, MAX20810 reports "MAX20810", and MAX20815
> reports "MAX20815". I find no reference to the "B" variants on the
> Analog Web page.
>
> LTM4739, according to its datasheet, reports "LTM4739", and LTM4740
> reports "LTM4740".
>
> Please explain the discrepancy.

The discrepancy is between the datasheets and what the parts actually store
in IC_DEVICE_ID (0xAD).

The LTM4739 and LTM4740 are uModule products built around the MAX20810 and
MAX20815 dies, respectively. On the hardware I tested, reading IC_DEVICE_ID
returns the internal MAX die identifier, not the LTM part name:

    LTM4739 -> "MAX20810B"
    LTM4740 -> "MAX20815B"

So the modules report the die's ID (with a "B" die/stepping suffix), which is
not the string the LTM datasheets currently document. This is a datasheet vs
silicon mismatch, not a driver choice: the driver matches what the silicon
actually returns so that probe succeeds on real hardware.

I only added the IDs I could confirm on hardware (MAX20810B / MAX20815B, as
reported by the LTM4739 / LTM4740 modules). I deliberately did not add the
"MAX20810" / "MAX20815" / "LTM4739" / "LTM4740" strings, since no part I have
reports them. The driver keeps the LTM4739/LTM4740 naming and device-tree
compatibles for the modules while matching on the MAX die IDs they report.

I raised this with the product line and a datasheet correction has been
initiated to document the MAX20810B / MAX20815B IC_DEVICE_ID values.

If you'd prefer the discrepancy spelled out in-tree, I'm happy to add a note to
the commit message and a code comment in a v2.

Thanks,
Carlos