Add an initial Android tips guide draft

Change-Id: I93ad93a2848f81a2f90f62a75e00cb59dbb420e2

content/guides/android.md

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+---
+title: "Android tips"
+summary: "Android handbook for mainlined devices"
+draft: true
+---
+
+While bringing up Linux for these devices is our main focus, we would also like to make it work with Android. We think that Android is a good way to "stress" all the devices' features and it might also be a good way to gain more users and have more reports.
+
+This page will contain some useful information regarding Android and its support for mainline kernels.
+
+For reference, we maintain a common Android device tree that is used for all of our supported devices. You can find it [here](https://github.com/Mainline4Lumia/android_device_linux_mainline-common).
+
+## Kernel
+
+Typically tagged Android releases (aka major releases like Android 13) only works with LTS kernels.
+
+If you want to directly use the latest mainline kernel, it is highly recommended to use the `main` branch of the Android Open Source Project (AOSP). Use the `android-mainline` branch from Android common kernel tree to make sure you have all the required patches. You will also need to apply the Android config fragment to your kernel config.
+
+The fastest way to get started is to build the latest AOSP release and build a LTS GKI (Generic Kernel Image). You can then use the `gki_defconfig` kernel config and write a GKI fragment making sure only the drivers your devices needs will be built.
+
+NXP wrote a nice introduction to GKI for their products, you can find it [here](https://community.nxp.com/t5/i-MX-Processors-Knowledge-Base/i-MX-Android-12-GKI-General-Kernel-Image-development-tips/ta-p/1600993).
+
+Bonus: If you build a custom ROM like LineageOS, you will be able to build the kernel inline with the ROM, which will make it easier to build the kernel together with the required modules.
+
+## Graphics
+
+### Without GPU
+
+Android is able to boot even when the GPU hasn't been bringed up yet, of course, you must have something to draw onto, typically achieved with a framebuffer provided by the bootloader.
+
+All you have to do is:
+- Make sure `/dev/fb0` exists and works (you can try booting the Android recovery to quickly check if Android likes it)
+  - If it doesn't exist, try to set the `video` kernel command line parameter (e.g. with efifb, set it to `video=efifb`)
+- Build `[email protected]` (the framebuffer adapter doesn't implement the methods required by newer versions, `@2.1` would work too)
+- Build `gralloc.default` together with `@2.0` allocator and `@2.1` mapper HIDL implementations
+- Build `ANGLE` + SwiftShader's `pastel` Vulkan HAL
+
+### With GPU
+
+Android supports using mesa as a hardware composer and a graphics allocator. This is the recommended way to use a GPU with Android. Refer to device/linaro/dragonboard configuration for this case.