This is the hardware independent GPIO (General Purpose Input Output) Interface for Mynewt.
Contains the basic operations to set and read General Purpose Digital I/O Pins within a Mynewt system.
Individual GPIOs are referenced in the APIs as pins. However, in this interface the pins are virtual GPIO pins. The MCU header file maps these virtual pins to the physical GPIO ports and pins.
Typically, the BSP code may define named I/O pins in terms of these virtual pins to describe the devices attached to the physical pins.
Here's a brief example so you can get the gist of the translation.
Suppose my product uses the stm32F4xx processor. There already exists support for this processor within Mynewt. The processor has N ports (A,B,C..) of 16 GPIO pins per port. The MCU hal_gpio driver maps these to a set of virtual pins 0-N where port A maps to 0-15, Port B maps to 16-31, Port C maps to 32-47 and so on. The exact number of physical port (and virtual port pins) depends on the specific variant of the stm32F4xx.
So if I want to turn on port B pin 3, that would be virtual pin 1*16 + 3 = 19. This translation is defined in the MCU implementation of hal_gpio.c for the stmf32F4xx. Each MCU will typically have a different translation method depending on its GPIO architecture.
Now, when writing a BSP, it's common to give names to the relevant port pins that you are using. Thus, the BSP may define a mapping between a function and a virtual port pin in the bsp.h header file for the BSP. For example,
#define SYSTEM_LED (37) #define FLASH_SPI_CHIP_SELECT (3)
would map the system indicator LED to virtual pin 37 which on the stm32F4xx would be Port C pin 5 and the chip select line for the external SPI flash to virtual pin 3 which on the stm32F4xxis port A pin 3.
Said another way, in this specific system we get
SYSTEM_LED --> hal_gpio virtual pin 37 --> port C pin 5 on the stm34F4xx
Blinky uses the hal_gpio to blink the system LED. The blinky source code is available in the blinky repo. Examine how blinky_task_handler initializes and toggles the GPIO to control the LED.