Documentation: Add Beaglebone Black documentation

Change-Id: If1a9808d1f20ee61048182d416f25e9a81c631af
Signed-off-by: Sam Lewis <sam.vr.lewis@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44389
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

Documentation/mainboard/index.md

@@ -176,6 +176,10 @@ The boards in this section are not real mainboards, but emulators.
 
 - [Lemur Pro](system76/lemp9.md)
 
+## Texas Instruments
+
+- [Beaglebone Black](ti/beaglebone-black.md)
+
 ## UP
 
 - [Squared](up/squared/index.md)

Documentation/mainboard/ti/beaglebone-black.md

@@ -0,0 +1,131 @@
+# Beaglebone Black
+This page gives some details about the [BeagleBone Black] coreboot port and
+describes how to build and run it.
+
+The port currently only supports booting coreboot from a micro SD card and has
+some other limitations listed below.
+
+## Supported Boards
+The Beaglebone port supports the following boards:
+
+- Beaglebone Black
+- Beaglebone Black Wireless
+- Beaglebone Pocket (untested, may need tweaking)
+- Beaglebone Blue (untested, may need tweaking)
+- Beaglebone Original (untested, may need tweaking)
+
+## Use Cases
+This port was primarily developed as a learning exercise and there is
+potentially little reason to use it compared to the defacto bootloader choice of
+U-Boot. However, it does have some interesting practical use cases compared to
+U-Boot:
+
+1. Choosing coreboot as a lightweight alternative to U-Boot. In this case,
+   coreboot is used to do the absolute minimum necessary to boot Linux, forgoing
+   some U-Boot features and functionality. Complex boot logic can then instead
+   be moved into Linux where it can be more flexibly and safely executed. This
+   is essentially the LinuxBoot philosophy. [U-Boot Falcon mode] has similar
+   goals to this as well.
+2. Facilitating experimenting with coreboot on real hardware. The Beaglebone
+   Black is widely available at a low pricepoint (~$65) making it a great way to
+   experiment with coreboot on real ARMv7 hardware. It also works well as a
+   development platform as it has exposed pads for JTAG and, due to the way it
+   boots, is effectively impossible to brick.
+3. The Beaglebone Black is often used as a external flasher and EHCI debug
+   gadget in the coreboot community, so many members have access to it and can
+   use it as a reference platform.
+
+## Quickstart
+1. Run `make menuconfig` and select _TI_/_Beaglebone_ in the _Mainboard_ menu.
+2. Add a payload as normal.
+3. Run `make`.
+4. Copy the resulting `build/MLO` file to the micro SD card at offset 128k - ie
+   `dd if=build/MLO of=/dev/sdcard seek=1 bs=128k`.
+
+**NOTE**: By default, the Beaglebone is configured to try to boot first from
+eMMC before booting from SD card. To ensure that the Beaglebone boots from SD,
+either erase the internal eMMC or hold the _S2_ button while powering on (note
+that this has to be while powering on - ie when plugging in the USB or DC barrel
+jack - the boot order doesn't change on reset) to prioritize SD in the boot
+order.
+
+## Serial Console
+By default, coreboot uses UART0 as the serial console. UART0 is available
+through the J1 header on both the Beaglebone Black and Beaglebone Black
+Wireless. The serial runs at 3.3V and 115200 8n1.
+
+The pin mapping is shown below for J1.
+
+    ```eval_rst
+    +----------------------------+------------+
+    | Pin number                 | Function   |
+    +============================+============+
+    | 1 (Closest to barrel jack) | GND        |
+    +----------------------------+------------+
+    | 4                          | RX         |
+    +----------------------------+------------+
+    | 5                          | TX         |
+    +----------------------------+------------+
+    ```
+
+## Boot Process
+The AM335x contains ROM code to allow booting in a number of different
+configurations. More information about the boot ROM code can be found in the
+AM335x technical reference manual (_SPRUH73Q_) in the _Initialization_ section.
+
+This coreboot port is currently configured to boot in "SD Raw Mode" where the
+boot binary, with header ("Table of Contents" in TI's nomenclature), is placed
+at the offset of 0x20000 (128KB) on the SD card. The boot ROM loads the coreboot
+bootblock stage into SRAM and executes it.
+
+The bootblock and subsequent romstage and ramstage coreboot stages expect that
+the coreboot image, containing the CBFS, is located at 0x20000 on the SD card.
+All stages directly read from the SD card in order to load the next stage in
+sequence.
+
+## Clock Initialization and PMIC
+To simplify the port, the TPS65217C Power Management IC (PMIC) on the Beaglebone
+Black is not configured by coreboot. By default, the PMIC reset values for
+VDD_MPU (1.1V) and VDD_CORE (1.8V) are within the Operating Performance Point
+(OPP) for the MPU PLL configuration set by the boot ROM of 500 MHz.
+
+When using Linux as a payload, the kernel will appropriately scale the core
+voltages for the desired MPU clock frequency as defined in the device tree.
+
+One significant difference because of this to the U-Boot port is that the DCDC1
+rail that powers the DDR3 RAM will be 1.5V by default. The Micron DDR3 supports
+both 1.35V and 1.5V and U-Boot makes use of this by setting it to 1.35V to
+conserve power. Fortunately, Linux is again able to configure this rail but it
+involves adding an entry to the device tree:
+
+    &dcdc1_reg {
+    regulator-name = "vdd_ddr3";
+    regulator-min-microvolt = <1350000>;
+    regulator-max-microvolt = <1350000>;
+    regulator-boot-on;
+    regulator-always-on;
+    };
+
+If this port was to be extended to work with boards or SoCs with different
+requirements for the MPU clock frequency or different Operating Performance
+Points, then the port may need to be extended to set the core voltages and MPU
+PLL within coreboot, prior to loading a payload. Extending coreboot so that it
+can configure the PMIC would also be necessary if there was a requirement for
+coreboot to run at a different MPU frequency than the 500 MHz set by the boot
+ROM.
+
+# Todo
+- Allow coreboot to run from the Beaglebone Black's internal eMMC. This would
+  require updating the `mmc.c` driver to support running from both SD and eMMC.
+- Support the boot ROMs *FAT mode* so that the coreboot binary can be placed on
+  a FAT partition.
+- Increase the MMC read speed, it currently takes ~15s to read ~20MB which is a
+  bit slow. To do this, it should be possible to update the MMC driver to:
+    - Increase the supported blocksize (currently is always set to 1)
+    - Support 4-bit data width (currently only supports 1-bit data width)
+- Convert the while loops in the MMC driver to timeout so that coreboot does not
+  hang on a bad SD card or when the SD card is removed during boot.
+
+
+[Beaglebone Black]: https://beagleboard.org/black [U-Boot Falcon mode]:
+https://elixir.bootlin.com/u-boot/v2020.07/source/doc/README.falcon