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Building generic kernels

Even if kernel sources are not provided by the vendor, it's still possible to package kernel and make the phone boot.

Contents

Find generic kernel for the device

To find a needed kernel, you have to:

  1. Find out the information about the SoC/board used in the device. One SoC may have multiple boards. For example, Spreadtrum SC7731C has 2 boards: sc7731cea and sc7731ceb.
  2. Try to search for kernels on the internet. Use different queries to find as much kernels as possible. Kernel is considered matching if it has files for your board, such as device tree source (located at arch/device_arch(e.g. arm, arm64)/boot/dts).

Patch vendor dts in kernel

Get dts from stock boot.img, replace the board dts with it and make a patch.

Adapt kernel config

This step is needed to make a proper kernel config which works for the device. You have to find detailed information about the hardware. Possible ways to do it are:

  1. Get dts from stock boot.img
  2. Follow instructions from tuning sysfs page to get contents of sysfs. It is recommended to do it on a rooted phone in Android, since dump made from recovery will probably have less details about hardware.
  3. Find photos of device's internals on the web. If there are none, disassemble the device yourself or ask someone to do it.

After one of these steps write down all information you've found and use it to adjust kernel config. Run pmbootstrap kconfig edit device-codename and use built-in search (press / key) to find options related to found hardware.

There is no driver for X in the kernel. What to do?

  1. Check if driver is available in other generic kernels you have found for the device. If driver is available, check step 6 and further.
  2. Try to find this driver in other kernels that don't match your device. It will be helpful to understand how it works, though you'll most likely have to rewrite(port) it.
  3. Find datasheet to have more information for porting the driver.
  4. Find a dummy driver for the driver's group(e.g. lcd_dummy.c for LCD panel drivers) in your kernel. Read it as well as other drivers for devices similar to yours.
  5. Use drivers and datasheet to make a new driver.
  6. Patch the kernel with driver, Makefile and Kconfig files related to your drivers group to add support. Give a proper name to driver option in Kconfig.
  7. Add a line to linux-device-codename's config to make the driver option appear in menuconfig. Example: # CONFIG_FB_LCD_ST7796S_MIPI is not set
  8. Run pmbootstrap kconfig edit device-codename and enable the option. Tap / on the keyboard to search for it.