Re: [PATCH 0/3] iio: adc: add mt6397 PMIC AUXADC support

[Andy Shevchenko]

On Thu, Sep 17, 2026 at 03:24:05PM -0500, Ryan Brue wrote:
> On 9/16/26 4:50 AM, Andy Shevchenko wrote:
> > On Tue, Sep 15, 2026 at 11:15:25PM -0500, Ryan Brue wrote:
> > > The MediaTek mt6397 PMIC has a 10-bit AUXADC that nothing in-tree can
> > > reach. On boards built around it that ADC is the only path to the battery:
> > > the SoC's AUXADC is wired to board thermistors, and the charger ICs these
> > > boards use have no ADC at all, so without it there is no pack voltage and
> > > no state of charge.
> > This doesn't explain why a brand new driver? Perhaps we have existing code that
> > may be updated to support this device?
> I considered adding mt6397 support to either mt6323-auxadc or mt6359-auxadc,
> and both had problems.
> 
> Both mt6323-auxadc and mt6359-auxadc select channels through a request
> register (1 bit per channel), while mt6397 uses a 4-bit numeric field CHSEL
> in CON1 (10:7), and then pulses a START bit (CON1 bit 0). That was the
> biggest reason I made the new driver.
> 
> For mt6323-auxadc, which is the closest I could find to the mt6397
> (CON0..CON27), it has 13 more registers than the mt6397 (CON0..CON14). It
> uses CON22 for its request register, and reads the result value from the
> same register as the ready bit. We don't do that - the mt6397 has a factory
> calibrated value for each channel at 0x16 higher than the raw value. mt6323
> also has a 1800 mV / 15 bit scale / resolution while we have 1200 mV / 10
> bits. We also have some per-channel preparation that we have to do before
> the burst, that the mt6323 doesn't have to do.
> 
> For mt6359-auxadc, it has a more generic framework for describing the
> AUXADC, but it assumes requests are channel-per-bit, and so we would have to
> basically ignore req_idx, req_mask, rdy_idx, and rdy_mask.

> We also have our own software sampling, which the vendor does too (Amazon
> Fire OS based on Linux 3.18). We'd have to have our own sampling callback to
> do it.

If it's not an upstream, we do not care.

> I drafted two other versions of these patches adding mt6397 support to both
> of those drivers, but the differences meant I had to add a lot of extra
> boilerplate to each driver and to me it didn't make sense.
> 
> In v2 I will add the justification to the cover letter and commits for why I
> chose a new driver.

Yes, please. Always do that when submitting a brand new driver that might sound
like a (partially) compatible with something already existing.

> If you'd like me to instead send the exploratory patches I made adapting
> mt6323-auxadc or mt6359-auxadc, let me know. I'm fine if it ends up seeming
> like we should adapt one of the existing drivers, but I think the mechanism
> for controlling this AUXADC is unique and merits its own driver.
> 
> Thanks again for the review, I am going through each one, and sorry for the
> delay. I'm rather new to kernel development.

No problem, and welcome!

-- 
With Best Regards,
Andy Shevchenko