Re: [PATCH] power: supply: qcom_battmgr: do not report power_now on SM8750

[Qianhui Xu]

Hi Konrad,

You are right, and thanks for stopping on that sentence.  "Maximum-load
updater" is a phrase I invented; it is not a QBG or battmgr term from any
document, so I should not have used it in a commit message.  Let me
restate what I actually observed and what I am and am not claiming.

What the driver does (no bug here)

qcom_battmgr_bat_get_property() does not sample anything.  For a given
power_supply property N it reads the 32-bit slot that the QBG
notification buffer reserves for that property (in the SM8750 variant the
battery property array starts at context + 0x6a8 and is indexed by
property id, so BATT_POWER_NOW, id 22, is context + 0x6a8 + 4 * 22).
The driver faithfully returns whatever firmware left there.

What I observed on the unit

  - charging, actual input well under 1 W: power_now reported ~89 W
  - discharging, actual draw ~4 W:           power_now reported >100 W
  - current_now * voltage_now in the same samples stays in the expected
    magnitude range, i.e. the other battery properties are consistent

So on this platform the BATT_POWER_NOW slot does not hold a measurement of
present power; it holds something closer to a configured/estimated ceiling.
That is a statement about the value, and it is reproducible on my unit.

What I do not have

QBG firmware sources, or a public document describing the semantics of the
BATT_POWER_NOW slot.  The "it is a maximum-load figure" reading is my
inference from the two facts above plus the vendor battery property
interface, not something I can cite.  I also cannot yet say whether this
is a property of the SM8750 firmware build, of this OEM's configuration,
or of the slot in general: Fenglin's earlier reply (default supported,
OEM can customize) would predict the latter, and confirming it needs
power_now/BATT_POWER_NOW readouts from the stock Android build on the
same unit, which I have not done.

Proposed wording

I will resend the patch with the firmware-semantics sentence removed.  The
commit message will keep only what is verifiable: on SM8750 the reported
POWER_NOW is not a present-power reading (with the two counterexamples),
the driver returns the slot as-is, and therefore this platform should not
advertise POWER_NOW.  The code change is unaffected: add the SM8750
variant and reuse the SM8550 property list without POWER_NOW.

If you happen to know the intended semantics of that slot, or know of a
QBG interface document that defines it, I would rather cite that than
describe it in my own words.  Otherwise the factual version is what I will
send.

Thanks,
Qianhui