Re: [PATCH v2 3/3] iio: adc: ltc2497: add 2x conversion speed mode

On Thu, Aug 13, 2026 at 07:01:37PM +0300, Andrei Stancovici wrote:
> The LTC2499 supports a 2x output rate (SPD bit in the second
> configuration byte).  In 2x mode the offset auto-calibration is
> disabled, roughly doubling the conversion rate (~13.6Hz vs ~6.8Hz in
> simultaneous 50/60Hz rejection) while leaving linearity and full-scale
> errors unchanged (datasheet).  During a temperature measurement the part
> always converts at 1x regardless of SPD.
> 
> Expose the rate through the standard sampling_frequency /
> sampling_frequency_available ABI on the voltage channels only: SPD is
> ignored for temperature conversions, so the temperature channel
> deliberately carries no SAMP_FREQ attribute.  A new has_speed_mode
> capability flag gates the feature (LTC2499); the two-byte command path
> is now taken for has_temp || has_speed_mode, since both features need the
> second config byte.
> 
> The conversion-time wait becomes mode dependent: 150ms at 1x, 76ms at 2x
> (datasheet t_CONV max, simultaneous rejection, rounded up).  The wait is
> keyed on the conversion currently in flight, whose duration is fixed by
> the mode that was active when it started - not by the newly selected
> mode.  This matters on a 1x->2x switch: a 1x conversion may still be
> running when the first 2x read arrives, and reprogramming the device
> before it finishes would be NACKed with -EIO.  Timing is centralized in
> ltc2497core_conv_time_ms() so a future FA/FB rejection-mode selection
> can extend it into a [rejection][speed] lookup without touching callers.
> 
> LTC2496/LTC2497 (no speed mode) keep the single-byte path and the
> unchanged 150ms wait.
> 
> Validated on a live LTC2499: 20 reads take ~3.1s at 1x and ~1.6s at 2x
> (~0.5x, no -EIO), voltage and temperature readings stay sane in both
> modes, and the temperature/voltage interleave (sticky-PTAT) regression
> still passes at 1x and 2x.

...

> -static int ltc2497core_wait_conv(struct ltc2497core_driverdata *ddata)
> +static unsigned int ltc2497core_conv_time_ms(struct ltc2497core_driverdata *ddata,
> +					     u8 address)

Wondering if you use --histogram when preparing patches. If not, try it, it may give
more human-readable diffs.

...

> +static int ltc2497core_write_raw(struct iio_dev *indio_dev,
> +				 struct iio_chan_spec const *chan,
> +				 int val, int val2, long mask)
> +{
> +	struct ltc2497core_driverdata *ddata = iio_priv(indio_dev);
> +	bool sped_2x;
> +	unsigned int i;

Reversed xmas tree ordering (longer lines first).

> +	switch (mask) {
> +	case IIO_CHAN_INFO_SAMP_FREQ:
> +		/* Match the (val, val2) pair against the advertised rates. */
> +		for (i = 0; i < ARRAY_SIZE(ltc2497core_samp_freq_avail); i += 2) {
> +			if (val == ltc2497core_samp_freq_avail[i] &&
> +			    val2 == ltc2497core_samp_freq_avail[i + 1])
> +				break;
> +		}
> +		if (i == ARRAY_SIZE(ltc2497core_samp_freq_avail))
> +			return -EINVAL;
> +
> +		sped_2x = i / 2;
> +
> +		mutex_lock(&ddata->lock);
> +		ddata->sped_2x = sped_2x;
> +		/*
> +		 * The new speed only takes effect once the second command byte
> +		 * is reprogrammed, so force the next read to reprogram rather
> +		 * than reuse the value already latched for this address.
> +		 * LTC2497_CONFIG_DEFAULT is not a valid channel/temperature
> +		 * address, so it is a safe re-arm sentinel (as used at probe).
> +		 *
> +		 * A conversion started under the old speed may still be in
> +		 * flight; its own duration (conv_time_prev), not the new mode's,
> +		 * still gates the next reprogram, so the timing state is left
> +		 * untouched here.
> +		 */
> +		ddata->addr_prev = LTC2497_CONFIG_DEFAULT;
> +		mutex_unlock(&ddata->lock);
> +
> +		return 0;
> +
>  	default:
>  		return -EINVAL;
>  	}

>  };

...

> +	/*
> +	 * Parts with a speed mode expose in_voltage_sampling_frequency /
> +	 * _available on the voltage channels only.  SPD is ignored during a

Spell the node name in full, I can't really get if you meant
in_voltage_available or something else.

> +	 * temperature measurement, so the temperature channel deliberately
> +	 * carries no SAMP_FREQ attribute.  Patch a private copy of the shared
> +	 * channel array so parts without a speed mode stay untouched.
> +	 */

...

>  	/*
> -	 * Parts with the internal PTAT sensor (LTC2499) latch their converter
> -	 * configuration via a second command byte and only re-evaluate it when
> -	 * that byte has EN2 set; a single byte, or a second byte with EN2 = 0,
> -	 * means "keep previous". A one-byte channel select therefore cannot pull
> -	 * the device back out of temperature mode, so a voltage read after a
> -	 * temperature read would keep returning the PTAT result. Always drive the
> -	 * second byte with EN2 set on these parts: IM = 1 for a temperature read,
> -	 * EN2 alone (IM = 0) to (re)select an external input. FA = FB = 0 keeps
> -	 * the power-on simultaneous 50/60Hz rejection, whose worst-case
> -	 * conversion time the driver's wait already covers.
> +	 * Parts with a second config byte (LTC2499: internal PTAT sensor and/or
> +	 * the 2x speed mode) latch their converter configuration from that byte
> +	 * and only re-evaluate it when EN2 is set; a single byte, or a second
> +	 * byte with EN2 = 0, means "keep previous". A one-byte channel select

Please, be consistent with 1-space versus double-space at the start of a new
sentences. See just above comment and compare.

> +	 * therefore cannot pull the device back out of temperature mode, so a
> +	 * voltage read after a temperature read would keep returning the PTAT
> +	 * result. Always drive the second byte with EN2 set on these parts:
> +	 *   - temperature read: IM = 1 (SPD is ignored by the part in
> +	 *     temperature mode and is left 0 here);
> +	 *   - voltage read: IM = 0 (external input), plus SPD when 2x is
> +	 *     selected.
> +	 * FA = FB = 0 keeps the power-on simultaneous 50/60Hz rejection, whose
> +	 * worst-case conversion time the driver's wait already covers.
>  	 *
>  	 * The two bytes are assembled in the DMA-safe st->data buffer rather than
>  	 * on the stack, so the pointer handed to i2c_master_send() stays valid on
>  	 * adapters that DMA the transfer (e.g. with CONFIG_VMAP_STACK).
>  	 */


-- 
With Best Regards,
Andy Shevchenko