DFSDM Linux driver

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1. Article purpose[edit source]

The purpose of this article is to introduce the Linux^® driver for the DFSDM internal peripheral:

Which DFSDM features are supported by the driver
How to configure, use and debug it
What is the driver structure, where to the find source code.

2. Short Description[edit source]

The DFSDM Linux^® driver (kernel space) is based on the IIO and ALSA frameworks. It offers various modes:

IIO direct mode: single capture on a channel
IIO software buffer: capture one or more channels
IIO triggered buffer mode: capture on one or more channels using triggers
It uses hardware triggers available in IIO. See TIM Linux driver and LPTIM Linux driver.

It offers an extended API^[1] for audio usage which allows PDM microphones capture through a SPI interface.

3. Configuration[edit source]

3.1. Kernel configuration[edit source]

Activate the DFSDM Linux^® driver in the kernel configuration by using the menuconfig tool.

3.1.1. IIO driver[edit source]

Configuration flag: CONFIG_STM32_DFSDM_ADC

Device Drivers  --->
   <*> Industrial I/O support  --->
      Analog to digital converters  --->
         <*> STMicroelectronics STM32 dfsdm adc
         <*>   STMicroelectronics STM32 adc                        # DFSDM ADC IIO driver

When using the DFSDM ADC IIO driver (only), the user must also enable the driver for the external analog front-end (e.g. sigma delta modulator). The generic sigma delta modulators driver may be used for instance (CONFIG_SD_ADC_MODULATOR):

Device Drivers  --->
   <*> Industrial I/O support  --->
      Analog to digital converters  --->
         <*> Generic sigma delta modulator                         # sigma delta modulator driver

3.1.2. Audio driver[edit source]

Configuration flag: CONFIG_SND_SOC_STM32_DFSDM (Note: This configuration depends on CONFIG_STM32_DFSDM_ADC)

Device Drivers  --->
   <*> Sound card support  --->
      <*>   Advanced Linux Sound Architecture  --->
         <*>   ALSA for SoC audio support  --->
            STMicroelectronics STM32 SOC audio support  --->
               <*> SoC Audio support for STM32 DFSDM               # DFSDM Audio driver for digital microphone capture

3.2. Device tree[edit source]

Refer to the DFSDM device tree configuration article when configuring the DFSDM Linux kernel driver.

4. How to use[edit source]

4.1. IIO driver[edit source]

In "IIO direct mode", the conversion result can be read directly from sysfs, see: How to do a simple ADC conversion using the sysfs interface.

In "IIO triggered buffer mode", the configuration must be performed by using sysfs first. Then, character device (/dev/iio:deviceX) is used to read data, see Convert one or more channels using triggered buffer mode.

For information on the standard IIO consumer interface, please refer to How to use IIO kernel API which gives an example of the IIO consumer kernel API.

4.2. Audio driver[edit source]

The DFSDM Linux driver can be accessed from userland through an ALSA device. Refer to ALSA overview for information on how to list and use ALSA devices.

5. How to trace and debug[edit source]

5.1. How to monitor[edit source]

The DFSDM driver uses resources such as clocks and GPIOs.

Refer to Pinctrl_overview#How_to_monitor to check DFSDM GPIOs.
Refer to Clock_overview#How_to_monitor_with_debugfs to check DFSDM clocks.

5.1.1. How to monitor with debugfs[edit source]

The DFSDM registers are accessed using REGMAP by DFSDM Linux^® driver.

It comes with debugfs entries to dump registers:

$ cd /sys/kernel/debug/regmap/4400d000.dfsdm/
$ cat registers
000: 00000000
004: 00000000
008: 00000000

5.1.2. Other ways to monitor[edit source]

Man can check the DFSDM interrupts and/or the DFSDM DMA interrupts:

$ cat /proc/interrupts
           CPU0       CPU1
...
 99:          0          0     GIC-0 142 Level     4400d000.dfsdm:filter@0
100:          0          0     GIC-0 143 Level     4400d000.dfsdm:filter@1
101:          0          0     GIC-0 144 Level     4400d000.dfsdm:filter@2
...

5.2. How to trace[edit source]

5.2.1. IIO driver[edit source]

Refer to How to trace with dynamic debug for how to enable the debug logs in the driver and in the framework.

 dmesg -n8
 echo "file drivers/iio/adc/stm32-dfsdm* +p" > /sys/kernel/debug/dynamic_debug/control

To enable dynamic debug at boot time, append the following arguments on the kernel command line:

loglevel=8 dyndbg="file drivers/iio/adc/stm32-dfsdm* +p"

5.2.2. Audio driver[edit source]

Refer to ALSA_overview#How_to_trace for details on trace tools.

5.3. How to debug[edit source]

5.3.1. Audio driver[edit source]

Refer to ALSA_overview#How_to_debug for details on debugging tools.

6. Source code location[edit source]

It is composed of:

stm32-dfsdm-core.c , core part of DFSDM Linux driver to handle common resources: registers, clock
stm32-dfsdm-adc.c , ADC part of DFSDM Linux driver to handle ADC operations
stm32_adfsdm.c , ASoC DAI part of DFSDM Linux driver to handle audio operations

7. References[edit source]

↑ include/linux/iio/adc/stm32-dfsdm-adc.h , DFSDM IIO custom API

[stm32-dfsdm-adc-h-1] ude/linux/iio/adc/stm32-dfsdm-adc.h , DFSDM IIO custom API

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