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iio: adc: stm32-dfsdm: add support for buffer modes
DFSDM conversions can be launched continuously, or using various
triggers:
- by software
- hardware triggers (e.g. like in stm32-adc: TIM, LPTIM, EXTI)
- synchronously with DFSDM filter 0. e.g. for filters 1, 2
Launching conversions can be done using two methods:
a - injected:
- scan mode can be used to convert several channels each time a
trigger occurs.
- When not is scan mode, channels are converted in sequence, one upon
each trigger.
b - regular:
- supports software triggers or synchronous with filter 0
- single or continuous conversions
This patch finalizes DFSDM operating modes using IIO buffer modes:
- INDIO_BUFFER_SOFTWARE: regular continuous conversions (no trigger)
but limited to 1 channel. Users must set sampling frequency in this case.
For filters > 1, conversions can be started synchronously with filter 0.
- INDIO_BUFFER_TRIGGERED: triggered conversions uses injected mode for
launching conversions. DFSDM can use hardware triggers (e.g. STM32 timer
or lptimer), so add INDIO_HARDWARE_TRIGGERED to supported modes.
- INDIO_DIRECT_MODE: Only support DMA-based buffer modes. In case no DMA is
available, only support single conversions.
From userland perspective, to summarize various use cases:
1 - single conversion on any filter:
$ cd iio:deviceX
$ cat in_voltageY_raw
This uses regular a conversion (not continuous)
2 - Using sampling frequency without trigger (single channel, buffer)
$ cd iio:deviceX
$ echo 100 > sampling_frequency
$ echo "" > trigger/current_trigger
$ echo 1 > scan_elements/in_voltageY_en
$ echo 1 > buffer/enable
This uses regular conversion in continuous mode (Frequency is achieved
by tuning filter parameters)
3 - sync mode with filter 0: other filters can be converted when using
"st,filter0-sync" dt property. The conversions will get started at the
same time as filter 0. So for any filters > 1:
$ cd iio:deviceX
$ echo 100 > sampling_frequency
$ echo "" > trigger/current_trigger
$ echo 1 > scan_elements/in_voltageY_en
$ echo 1 > buffer/enable
Then start filter 0 as in 2 above.
4 - Using a hardware trigger (with one channel):
- check trigger, configure it:
$ cat /sys/bus/iio/devices/trigger1/name
tim6_trgo
$ echo 100 > /sys/bus/iio/devices/trigger1/sampling_frequency
- go to any filter:
$ echo 1 > scan_elements/in_voltageY_en
$ echo tim6_trgo > trigger/current_trigger
$ echo 1 > buffer/enable
This uses injected conversion as it uses a hardware trigger (without scan)
5 - Using a hardware trigger (with 2+ channel):
Same as in 4/ above, but enable two or more channels in scan_elements.
This uses injected conversion as it uses a hardware trigger (with scan mode)
Signed-off-by: 
Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>