Drivers

List of drivers

Category	Device	Status	Ticket
filesystem	TARFS	read-only	54
PLIC	generic PLIC	max 31 IRQs
RTC	Goldfish	no alarms, read-only	8
UART	NS16550A	no output IRQ	18
UART	SiFive	no IRQs

Coding conventions

Device structure

By convention each driver is represented as at least 8-byte structure, where initial 4 bytes act as driver's ID (but actually it's a base address), and the second 4-bytes is a pointer to a config function.

Below there is an example of 16-byte structure representing UART device:

struct hal_uart {
    u32 base_addr;
    u32 (*config)(u32 base_addr, u32 mask, u32 flags);
    u32 (*putc)(u32 base_addr, char c);
    byte (*getc)(u32 base_addr);
}

Device config

Getting ad setting device's configuration is achieved by a function of the following signatur:

u32 config(u32 base_addr, u32 mask, u32 config);

The config flags can be specific per device, but by convention bit 0 should indicate whether the device is enabled (1) or disabled (0). The above allows common interface for handling any device config. I.e. system function #6 (SYSFN_GET_DEV_CFG) returns a configuration of any device passed to it as a parameter, i.e.

li a0, DEV_UART_0              # const representing device ID in device manager
call device_get                # function returns device's structure address
lw a0, (a0)                    # load first word from the structure (base_addr)
li a5, SYSFN_GET_DEV_CFG       # system function #6
ecall                          # execute system function and fetch the config