Porting NetBSD to HummingBoard Pulse, Part 1

This report was written by Saurav Prakash as part of Google Summer of Code 2019.

My venture into the first phase of The Google Summer of Code is nearing an end. The experience was enriching in every dimension, and the learning exposure I was subjected to was genuinely worthwhile. Here is a brief report on the work I have performed during this coding period.

About HummingBoard Pulse

HummingBoard Pulse is an arm64 board developed by SolidRun, with iMX8 System-on-Chip, Dual/Quad core Cortex A53 processor. This project involves extending the GENERIC64 kernel to support iMX8 SoC.

Device Tree Files

We compare the compatible property of the root node in the device tree with the list of ARM platforms compiled into the kernel. We add the .dts file for imx8 which is compatible with the imx8mq.dtsi file adopted from the Linux mainline 5.1.4. At this early stage, nodes for uart, iomux and clock only were created.

Board Platform Code

The platform code provides SoC specific code needed early at boot. The arm_platform structure for imx8 is initialised here. It contains function pointers like .ap_attach_init_args, .ap_device_register, .ap_reset, .ap_delay, .ap_uart_freq, .ap_mpstart and a, platform_early_putchar_function so that we can send a character to UART.

Clock Driver

During the booting stage we only need to enable the uart clocks (IMX8MQ_CLK_UART*_ROOT) and its parents. This includes writing drivers for fixed clocks (OSC_25M, OSC_27M, OSC_32K, and more), fixed-factor clocks, divider clocks, mux clocks, composite clocks (IMX8MQ_CLK_UART*), gate clocks (IMX8MQ_CLK_UART*_ROOT), sccg-pll clocks, and frac-pll clocks.

UART Driver

The imx drivers are separated between "core driver" and "bus glue". <arch/arm/imx/imxuart.c> is a "core driver", and we need to write a fdt based "bus glue" for the custom bus, this non-fdt based imx code currently uses.

What's Next

The sccg-pll, frac-pll, composite clocks aren't complete yet. So the focus in the next weeks would be to complete the unfinished work as required for the board to be booting till root. Finalising the UART driver code. Modifying the drivers written now, to suit the needs later. And then moving on further to write iomux driver, and then a USB driver for a storage device to boot from.

Lastly I would like to thank my mentors, @jmcneill, @martin @Cryo for being a constant support and non-flickering guidance throughout the process.