Mainline Linux kernel on the Gemini PDA 🐧

This is my attempt to compile a mainline kernel for the Gemini PDA.

⚠️ For regular users, this kernel does nothing useful (display, keyboard, etc. don't work). Don't use it unless you are interested in kernel development and willing to risk frying your Gemini. ⚠️

What works

Currently, only serial console works. The system boots into BusyBox, which is accessible over the serial console (with the USB FTDI cable).

Docs and notes about the Gemini PDA

See the docs directory.

Compiling

⚠️ Building with buildroot requires around 10 gigabytes of disk space.

1. Clone this repository.
2. Run make init, to build mkbootimg and to initialize buildroot to defconfig.
3. Run make toolchain to build the cross-compilation toolchain. This will take a while.
4. Run make to build the image. This will also take a while. The image will appear in output/boot_linux.img.
5. dd the image to the recovery partition, connect your UART cable, boot and enjoy the mainline kernel!

Build cache

The defconfig for buildroot uses build cache. The cache is in ~/.buildroot-ccache. Buildroot does not clear it, you must manually empty the directory to clear the build cache.

Custom after-build script.

You can add a custom script to run after the build in scripts/custom-after-build.sh. It will be executed after a successful build (full-image builds only, i.e. make, not make rebuild-linux). The script runs in the root of the repository. You can e.g. copy the image to a server for easy flashing.

Changing things

Linux kernel

When making any changes to the kernel configuration, sources, or the device tree, the kernel has to be rebuilt separately:

make linux-rebuild all

all is the default make target, so the above command will first rebuild the kernel, followed by rest of the build operations.

Device tree

The device tree is located at board/gemini-pda/gemini-pda.dts.

Menuconfig

make linux-menuconfig

NOTE: the resulting .config will be saved in ext/buildroot/output/build/linux-*/.config, and you will need to copy it to board/gemini-pda/kernel.config to yourself to preserve your changes.

Linux kernel source

You can edit the Linux source in ext/buildroot/output/build/linux-*. Note that the changes will be lost if the kernel is cleaned with make linux-dirclean, so make sure to export the modifications as patches.

Buildroot menuconfig

make menuconfig 

NOTE: the Buildroot .config file will be saved in ext/buildroot/.config, and you will need to copy it to configs/defconfig yourself to preserve your changes.

Running the kernel without flashing

You can kexec into the new kernel dynamically. To facilitate this, there are two scripts: kexecboot.sh and run-kexecboot.sh. kexecboot.sh runs on the device, and run-kexecboot.sh runs on the host. run-kexecboot.sh ttyUSB0 opens ttyUSB0, writes to the terminal /usr/local/bin/kexecboot.sh, and then transfers the latest build results to the PDA. The PDA then patches the device tree and kexecs into the new kernel. Note that the transfer takes about a minute, so it's not significantly faster than booting to Gemian and dding the image to recovery.

You can make scripts/custom-after-build.sh run run-kexecboot.sh after every build.

Directory structure

Directory	Description
board/gemini-pda	Contains the DTS and kernel, uClibc-ng, and busybox configs.
board/gemini-pda/root-overlay	Contains the files copied over the root filesystem.
configs	Contains Buildroot's defconfig
docs	Notes about the Gemini PDA hardware and configuration.
ext	External dependencies, as
logs	Contains log files from terminal runs.
output	Will contain the built boot image.
package	Custom Buildroot packages. Contains the mt6797_debug module.
scripts	run-minicom.sh and build-related scripts.

Terminal utility

Running ./scripts/run-minicom.sh ttyUSB0 will open a minicom terminal with the correct settings for Gemini. It will also store logs into the logs/ directory.

mt6797_debug module

The included mt6797_debug module provides MT6797-specific files in /sys/kernel/debug/mt6797. The module is loaded by default.

Debugfs files

File	Description
emi_mpu	Information about memory regions protected with EMI MPU
pll_bus	Measured frequency of the main AXI bus clock. ⚠️ Reading this file has a race condition with the clock driver.
psci_status	PSCI-reported status of CPUs
regs/EMI_CONM	Contents of the EMI_CONM register (memory clocks)
regs/M4U_PT_BASE_ADDR	Base address of M4U page table
regs/M4U_PT_BASE_ADDR_SEC	Second base address of M4U page table
regs/M4U_CTRL	Control registers of M4U
regs/M4U_IVRP_PADDR	Address of M4U's violation info area
regs/*	Various other registers

Rebuilding the module

The module can be rebuilt with make mt6797_debug-rebuild all.

Kernel patch sets

The following patch sets are included:

Fix clk

This patchset marks critical clocks as such. Without these patches, the memory would get corrupted.

The patch set has been sent to MediaTek's Linux mailing list.

Fix scpsys

This patchset fixes scpsys to allow gaps in the power domains, otherwise the probe for it would fail.

The patch set has been sent to MediaTek's Linux mailing list.

Add infracfg resets

PMIC wrapper requires a reset signal. A patch set adds the required resets to infracfg.

Running on other devices

The kernel has been tested on Umidigi Umi Z. It's preloader does not attempt to detect the UART cable, so the Force UART option of mt6797_debug has to be selected in make menuconfig.