arm-devices/platforms: create page, document RPi.

Author: ericonr

src/SUMMARY.md

@@ -13,6 +13,7 @@
       - [Installation via chroot (x86/x86_64)](./installation/guides/chroot.md)
       - [Full Disk Encryption](./installation/guides/fde.md)
       - [ARM Devices](./installation/guides/arm-devices/index.md)
+         - [Supported platforms](./installation/guides/arm-devices/platforms.md)
    - [musl](./installation/musl.md)
 - [Configuration](./config/index.md)
    - [Manual Pages](./config/man.md)

src/installation/guides/arm-devices/platforms.md

@@ -0,0 +1,86 @@
+# Supported platforms
+
+## Raspberry Pi
+
+The `rpi-kernel` packages for all Raspberry Pi variants are built from the
+Raspberry Pi Foundation's kernel tree, which should enable all special
+functionality that isn't available with mainline kernels. The RPi kernel
+packages also have their own header packages, `rpi-kernel-headers`. These
+packages should be installed if you want to use any DKMS packages. Void ships
+`rpi-base` meta packages that depend on the relevant kernel and `rpi-firmware`
+packages, which together enable Wi-Fi and Bluetooth functionality.
+
+The [command line](../../../config/kernel.md#cmdline) parameters passed to the
+kernel are in the `rootfs/boot/cmdline.txt` file. Some of the relevant
+parameters are documented in the [official
+documentation](https://www.raspberrypi.org/documentation/configuration/cmdline-txt.md).
+
+### Raspberry Pi 3: 32-bit or 64-bit
+
+It is possible to run the RPi 2 images on an RPi 3, as the RPi 3's CPU supports
+both the Armv8 and Armv7 instruction sets. The difference between these images
+is that the RPi 2 provides a 32-bit system with packages from the Void `armv7l`
+repositories, while the RPi 3 image provides a 64-bit system with packages from
+the Void `aarch64` repositories.
+
+### Enabling hardware RNG device
+
+By default, the
+[HWRNG](https://en.wikipedia.org/wiki/Hardware_random_number_generator) device
+is not used by the system, which may result in the random devices taking long to
+seed on boot. This can be annoying if you want to start `sshd` and expect to be
+able to connect immediately.
+
+In order to fix this, install the `rng-tools` package and
+[enable](../../../config/services/index.md#enabling-services) the `rngd`
+service, which uses the `/dev/hwrng` device to seed `/dev/random`.
+
+### Graphical session
+
+The `mesa-dri` package contains drivers for all the Raspberry Pi variants, and
+can be used with the [modesetting Xorg
+driver](../../../config/graphical-session/xorg.md#modesetting) or
+[Wayland](../../../config/graphical-session/wayland.md).
+
+### Hardware
+
+More configuration information can be found in the Raspberry Pi Foundation's
+[official
+documentation](https://www.raspberrypi.org/documentation/configuration/). The
+`raspi-config` utility isn't available for Void Linux, so editing the
+`/boot/config.txt` file is usually required.
+
+#### Audio
+
+To enable the soundchip, add `dtparam=audio=on` to `/boot/config.txt`.
+
+#### Serial
+
+To enable serial console logins,
+[enable](../../../config/services/index.md#enabling-services) the
+`agetty-ttyAMA0` service. See
+[securetty(5)](https://man.voidlinux.org/securetty.5) for interfaces that allow
+root login and the kernel command line in `/boot/cmdline.txt` for start-up
+configuration of the serial port, done through the `console=ttyAMA0,115200`
+parameter.
+
+### I2C
+
+To enable [I2C](https://en.wikipedia.org/wiki/I%C2%B2C), add
+`device_tree_param=i2c_arm=on` to `/boot/config.txt`, and
+`bcm2708.vc_i2c_override=1` to `/boot/cmdline.txt`. Then, create a
+[modules-load(8)](https://man.voidlinux.org/modules-load.8) `.conf` file with
+the content below:
+
+```
+i2c-dev
+```
+
+Finally, install the `i2c-tools` package and use
+[i2cdetect(8)](https://man.voidlinux.org/i2cdetect.8) to verify your
+configuration. It should show:
+
+```
+$ i2cdetect -l
+i2c-1i2c          bcm2835 I2C adapter                 I2C adapter
+```