Erroneous sbs-battery sysfs info a MT8173 Chromebook

[Alper Nebi Yasak]

Hi,

On 2024-10-03 19:08 +03:00, Nícolas F. R. A. Prado wrote:
> On Thu, Oct 03, 2024 at 06:15:49PM +0300, Alper Nebi Yasak wrote:
>> Hello,
>>
>> I have a MT8173 Chromebook ("Lenovo 300e") where I'm getting a lot of 
>> battery-related errors:
>>
>> [   34.678473] sbs-battery 6-000b: sbs_read_string_data_fallback: Returned block_length is longer than 0x20
>> [   34.702079] power_supply_show_property: 5 callbacks suppressed
>> [   34.702096] power_supply sbs-6-000b: driver failed to report `technology' property: -22
>> 
>> [...]
>>
>> I have _another_ MT8173 Chromebook ("ASUS C202X") where things work 
>> fine. But I couldn't find a good mainline kernel version for this one.
>> Anyone have any idea what is going on?
> 
> FWIW, I have also experienced issues reading properties from the SBS batteries
> before on chromebooks:
> 
> https://lore.kernel.org/all/924db470-8163-4454-8f59-f7372a132186@notapiano/
> 
> In those cases it was due to the EC firmware not implementing the SBS commands,
> but that had already been fixed in the latest EC firmware release, so simply
> updating the firmware fixed it.

(Replied there as well for a different board: cozmo)

> I don't know how ectool works, but it might be fetching the battery properties
> through a different mechanism (ie not SBS commands), so this might also be a bug
> in the EC FW despite ectool working.

AFAICT, ectool talks to /dev/cros_ec with EC commands, which makes EC 
send SBS commands to the battery on its own; whereas in the kernel 
driver case it passes kernel's SBS commands through the i2c tunnel.

But never mind all that, I should've looked into ChromeOS kernel 
sources. There's a limited i2c passthrough [1] that's implemented there. 
And this specific battery is failing to recognize a read command due to 
a stop bit or something, so they use this limited passthrough [2]. I've 
managed to apply those on top of linux-next and it makes things work, 
but I have no idea how I'd go about upstreaming it (comments on [1] 
point out it's quite hacky and not used in later boards).

[1] CHROMIUM: i2c: wire a limited smart battery passthrough
https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/2230594

[2] CHROMIUM: arm64: dts: mt8173-hana: Add google,limited-passthrough property
https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/422609