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

Jonathan Cameron
2026-05-05T14:33:10Z

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;