Device Drivers

NuttX supports a variety of device drivers, which can be broadly divided in three classes:

• Character Device Drivers
• Block Device Drivers
• Specialized Device Drivers
• Thermal Framework
• Oneshot Timer Drivers

Note

Device driver support depends on the in-memory, pseudo file system that is enabled by default.

Lower-half and upper-half

Drivers in NuttX generally work in two distinct layers:

• An upper half which registers itself to NuttX using a call such as register_driver() or register_blockdriver() and implements the corresponding high-level interface (read, write, close, etc.). implements the interface. This upper half calls into the lower half via callbacks.

• A “lower half” which is typically hardware-specific. This is usually implemented at the architecture or board level.

Details about drivers implementation can be found in OS Drivers Design and Device Drivers.

Subdirectories of nuttx/drivers

• 1wire/ One Wire Drivers

  1wire device drivers.

• analog/ Analog (ADC/DAC) Drivers

  This directory holds implementations of analog device drivers. This includes drivers for Analog to Digital Conversion (ADC) as well as drivers for Digital to Analog Conversion (DAC).

• audio/ Audio Device Drivers

  Audio device drivers.

• bch/ Block Driver to Character Driver

  Contains logic that may be used to convert a block driver into a character driver. This is the complementary conversion as that performed by loop.c.

• can/ CAN Drivers

  This is the CAN drivers and logic support.

• clk/Clock management (CLK)

  Clock management (CLK) device drivers.

• contactless/ Constactless Devices

  Contactless devices are related to wireless devices. They are not communication devices with other similar peers, but couplers/interfaces to contactless cards and tags.

• crypto/ Crypto Drivers

  Contains crypto drivers and support logic, including the /dev/urandom device.

• devicetree/ Device Tree support

  Device Tree support.

• dma/ DMA Drivers

  DMA drivers support.

• eeprom/ EEPROM

  EEPROM support as character drivers. Support as Memory Technology Device (MTD) is located in the mtd/ directory.

• efuse/ EFUSE Drivers

  EFUSE drivers support.

• i2c/ I2C Device Drivers

  I2C drivers and support logic.

• i2s/ I2S Drivers

  I2S drivers and support logic.

• i3c/ I3C Device Drivers

  I3C drivers and support logic.

• input/ Input Devices

  This directory holds implementations of human input device (HID) drivers. This includes such things as mouse, touchscreen, joystick, keyboard and keypad drivers.

  Note that USB HID devices are treated differently. These can be found under usbdev/ or usbhost/.

• ioexpander/ IO Expander Device Drivers

  IO Expander drivers.

• ipcc/ IPCC (Inter Processor Communication Controller) Driver

  IPCC (Inter Processor Communication Controller) driver.

• lcd/ LCD Character Drivers

  Drivers for parallel and serial LCD and OLED type devices.

• leds/ LEDS

  Various LED-related drivers including discrete as well as PWM- driven LEDs.

• loop/ Loop Device

  Supports the standard loop device that can be used to export a file (or character device) as a block device.

  See losetup() and loteardown() in include/nuttx/fs/fs.h.

• math/ Math Acceleration Drivers

  MATH Acceleration drivers.

• misc/ dev_null.c and dev_zero.c rwbuffer.c ramdisk.c

  Various drivers that don’t fit elsewhere.

• mmcsd/ SDIO Device Drivers MMCSD Device Drivers

  Support for MMC/SD block drivers. MMC/SD block drivers based on SPI and SDIO/MCI interfaces are supported.

• modem/ Modem Device

  Modem Support.

• motor/ Motor Drivers

  Motor control drivers.

• mtd/ Memory Technology Device Drivers

  Memory Technology Device (MTD) drivers. Some simple drivers for memory technologies like FLASH, EEPROM, NVRAM, etc.

  (Note: This is a simple memory interface and should not be confused with the “real” MTD developed at infradead.org. This logic is unrelated; I just used the name MTD because I am not aware of any other common way to refer to this class of devices).

• net/ Network interface drivers

  Network interface drivers.

• notes/ Note Driver Interface

  Note Driver Support.

• pinctrl/ Pinctrl Device Drivers

  Configure and manage pin.

• pipes/ FIFO and named pipe drivers

  FIFO and named pipe drivers. Standard interfaces are declared in include/unistd.h

• power/ Power-related Drivers

  Various drivers related to power management.

• rc/ Remote Control Devices

  Remote Control Device Support.

• regmap/ drivers/regmap

  Regmap Subsystems Support.

• reset/ Reset Driver

  Reset Driver Support.

• rf/ RF Devices

  RF Device Support.

• rptun/ Remote Proc Tunnel Drivers

  Remote Proc Tunnel Driver Support.

• segger/ Segger RTT drivers

  Segger RTT drivers.

• sensors/ Sensor Drivers

  Drivers for various sensors. A sensor driver differs little from other types of drivers other than they are use to provide measurements of things in environment like temperature, orientation, acceleration, altitude, direction, position, etc.

  DACs might fit this definition of a sensor driver as well since they measure and convert voltage levels. DACs, however, are retained in the analog/ sub-directory.

• serial/Serial Device Drivers

  Front-end character drivers for chip-specific UARTs. This provide some TTY-like functionality and are commonly used (but not required for) the NuttX system console.

• spi/ SPI Device Drivers

  SPI drivers and support logic.

• syslog/ SYSLOG

  System logging devices.

• timers/ Timers Drivers

  Includes support for various timer devices.

• usbdev/ USB Device-Side Drivers

  USB device drivers.

• usbhost/ USB Host-Side Drivers

  USB host drivers.

• usbmisc/ USB Miscellaneous Drivers

  USB Miscellaneous drivers.

• usbmonitor/ USB Monitor support

  USB Monitor support.

• usrsock/ Usrsock Driver

  Usrsock Driver Support.

• video/ Video Device Drivers

  Video-related drivers.

• virtio/ Virtio Device Drivers

  Virtio Device Support.

• wireless/ Wireless Drivers

  Drivers for various wireless devices.

Skeleton Files

Skeleton files are “empty” frameworks for NuttX drivers. They are provided to give you a good starting point if you want to create a new NuttX driver. The following skeleton files are available:

• drivers/lcd/skeleton.c Skeleton LCD driver

• drivers/mtd/skeleton.c Skeleton memory technology device drivers

• drivers/net/skeleton.c Skeleton network/Ethernet drivers

• drivers/usbhost/usbhost_skeleton.c Skeleton USB host class driver

Drivers Early Initialization

To initialize drivers early in the boot process, the drivers_early_initialize() function is introduced. This is particularly beneficial for certain drivers, such as SEGGER SystemView, or others that require initialization before the system is fully operational.

It is important to note that during this early initialization phase, system resources are not yet available for use. This includes memory allocation, file systems, and any other system resources.