Re: [PATCH v9 5/6] iio: adc: ad4691: add oversampling support

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 30 Apr 2026 13:16:47 +0300
Radu Sabau via B4 Relay <devnull+radu.sabau.analog.com@kernel.org> wrote:

> From: Radu Sabau <radu.sabau@analog.com>
> 
> Add per-channel oversampling ratio (OSR) support for CNV burst mode.
> The accumulator depth register (ACC_DEPTH_IN) is programmed with the
> selected OSR at buffer enable time and before each single-shot read.
> 
> Supported OSR values: 1, 2, 4, 8, 16, 32.
> 
> Introduce AD4691_MANUAL_CHANNEL() for manual mode channels, which do
> not expose the oversampling ratio attribute since OSR is not applicable
> in that mode. A separate manual_channels array is added to
> struct ad4691_channel_info and selected at probe time; offload paths
> reuse the same arrays with num_channels capping access before the soft
> timestamp entry.
> 
> in_voltageN_sampling_frequency represents the effective output rate for
> channel N, defined as osc_freq / osr[N]. The chip has one internal
> oscillator shared by all channels; each channel independently
> accumulates osr[N] oscillator cycles before producing a result.
> 
> Writing sampling_frequency computes needed_osc = freq * osr[N] and
> snaps down to the largest oscillator table entry that satisfies both
> osc <= needed_osc and osc % osr[N] == 0, guaranteeing an exact integer
> read-back. The result is stored in target_osc_freq_Hz and written to
> OSC_FREQ_REG at buffer enable and single-shot time, so sampling_frequency
> and oversampling_ratio can be set in any order.
> 
> in_voltageN_sampling_frequency_available is computed dynamically from
> the channel's current OSR, listing only oscillator table entries that
> divide evenly by osr[N], expressed as effective rates. The list becomes
> sparser as OSR increases, capping at max_rate / osr[N].
> 
> Writing oversampling_ratio stores the new OSR for that channel;
> target_osc_freq_Hz is left unchanged. The effective rate read back via
> in_voltageN_sampling_frequency becomes target_osc_freq_Hz / new_osr
> automatically. The two attributes are orthogonal: sampling_frequency
> controls the oscillator, oversampling_ratio controls the averaging depth.
> 
> OSR defaults to 1 (no accumulation) for all channels.
> 
> Signed-off-by: Radu Sabau <radu.sabau@analog.com>
Just one thing - from Sashiko again.

J
>  
>  static int ad4691_read_avail(struct iio_dev *indio_dev,
> @@ -540,10 +655,30 @@ static int ad4691_read_avail(struct iio_dev *indio_dev,
>  	unsigned int start = ad4691_samp_freq_start(st->info);
>  
>  	switch (mask) {
> -	case IIO_CHAN_INFO_SAMP_FREQ:
> -		*vals = &ad4691_osc_freqs_Hz[start];
> +	case IIO_CHAN_INFO_SAMP_FREQ: {
> +		unsigned int osr = st->osr[chan->channel];
> +		int n = 0;
> +
Sashiko shouts about possibly getting a torn set in here if osr were to be changed
whilst you were computing the array.  That's probably worth using locks to protect against.
> +		/*
> +		 * Only oscillator frequencies evenly divisible by the channel's
> +		 * OSR yield an integer effective rate; expose those as effective
> +		 * rates (osc / osr) so the user works entirely in output-sample
> +		 * space.
> +		 */
> +		for (unsigned int i = start; i < ARRAY_SIZE(ad4691_osc_freqs_Hz); i++) {
> +			if (ad4691_osc_freqs_Hz[i] % osr != 0)
> +				continue;
> +			st->samp_freq_avail[n++] = ad4691_osc_freqs_Hz[i] / osr;
> +		}
> +		*vals = st->samp_freq_avail;
>  		*type = IIO_VAL_INT;
> -		*length = ARRAY_SIZE(ad4691_osc_freqs_Hz) - start;
> +		*length = n;
> +		return IIO_AVAIL_LIST;
> +	}
> +	case IIO_CHAN_INFO_OVERSAMPLING_RATIO:
> +		*vals = ad4691_oversampling_ratios;
> +		*type = IIO_VAL_INT;
> +		*length = ARRAY_SIZE(ad4691_oversampling_ratios);
>  		return IIO_AVAIL_LIST;
>  	default:
>  		return -EINVAL;