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Add oversampling ratio (OSR) support for CNV burst mode. The accumulator depth register (ACC_DEPTH_IN(0)) 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. The OSR is shared across all channels (in_voltage_sampling_frequency and in_voltage_oversampling_ratio are info_mask_shared_by_all) because the chip has one internal oscillator and a single accumulator depth register (ACC_DEPTH_IN(0)) for all channels. in_voltage_sampling_frequency represents the effective output rate, defined as osc_freq / osr. Writing it computes needed_osc = freq * osr and snaps down to the largest oscillator table entry that satisfies both osc <= needed_osc and osc % osr == 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_voltage_sampling_frequency_available is precomputed at probe for each OSR value, listing only oscillator table entries that divide evenly by that OSR, expressed as effective rates (osc_freq / osr). The list becomes sparser as OSR increases, capping at max_rate / osr. read_avail picks the precomputed list for the current OSR, making the returned pointer stable and race-free. Writing oversampling_ratio stores the new shared OSR and snaps target_osc_freq_Hz to the largest oscillator table entry that is both <= old_effective_rate * new_osr and evenly divisible by new_osr. This preserves an integer read-back of in_voltage_sampling_frequency after the OSR change while keeping the oscillator as close as possible to the previous effective rate. OSR defaults to 1 (no accumulation). Signed-off-by: Radu Sabau <radu.sabau@analog.com> Signed-off-by: Jonathan Cameron <jic23@kernel.org>
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