Re: [PATCH v3 2/3] iio: adc: ltc2497: add LTC2499 internal temperature channel

[Andy Shevchenko <andriy.shevchenko@intel.com>]

On Wed, Sep 02, 2026 at 09:44:11AM +0300, Andrei Stancovici wrote:
> The LTC2499 has an internal PTAT (proportional to absolute temperature)
> sensor that is activated by a second I2C configuration byte (EN2 | IM).
> Expose it as an IIO_TEMP channel providing raw, scale and offset so the
> standard IIO formula
> 
> 	T[m°C] = (raw + offset) * scale
> 
> reconstructs the temperature.
> 
> The PTAT sensor yields the absolute temperature as
> 
> 	T(K) = DATAOUT24 * Vref / 1570		(Vref in volts)
> 
> The raw value exported here is sign-extended and normalised to
> 2^(resolution + 1) == 2^25, i.e. raw = 2 * DATAOUT24, so on the IIO
> milli-degree-Celsius convention
> 
> 	scale[m°C/LSB] = Vref_uV / 3140000
> 	offset         = -273150 * 3140000 / Vref_uV
> 
> The scale and offset are derived from the reference voltage returned by
> regulator_get_voltage(); its error is propagated as before, so a board
> that fails to describe vref-supply gets a clear read error instead of a
> silently wrong temperature. No board-specific reference value is assumed
> in the driver.
> 
> The single temperature channel is appended as the last entry of the
> shared channel array and excluded via num_channels for parts without an
> internal sensor, so the existing LTC2497 channel layout and device name
> are unchanged.
> 
> The LTC2499 latches its converter configuration from the second command
> byte and only re-evaluates it when that byte has EN2 set. EN2 | IM
> selects the internal temperature sensor. Because a single-byte command,
> or a second byte with EN2 = 0, means "keep previous", a one-byte channel
> select cannot pull the device back out of temperature mode: after a
> temperature read every subsequent voltage read would keep returning the
> PTAT result instead of the selected input. Temperature support is
> therefore only correct if the voltage path also emits a second command
> byte that re-selects an external input.
> 
> Send two-byte commands for all conversions on parts that have the sensor
> (has_temp):
> 
> 	temperature:	EN2 | IM
> 	voltage:	EN2			(IM = 0 -> external input)
> 
> The LTC2497 and LTC2496, which lack the second-byte mechanism, keep using
> the original single-byte channel select and are unchanged.

Looks good to me,
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>

...

>  struct ltc2497_chip_info {
>  	u32 resolution;
> +	bool has_temp;
>  	const char *name;

While `pahole` probably has found no issues with this layout, I would still
move boolean to be the last member here.

>  };

-- 
With Best Regards,
Andy Shevchenko