Regaining Control of the UART

Regaining Control of the UART

The default boot loader appropriates the PI’s UART to be used as a serial console. If you want, or need, to hook up your GPS to the UART, you have to recompile the boot loader. On FreeBSD, the port for the Raspberry PI 2 boot loader can be found in /usr/ports/sysutils/u-boot-rpi2/. The port is based on the U-Boot software base.

Don't undertake this step until your system is booting correctly. After disabling the serial console on the UART, you won't see the boot-level messages. You only need to do this if you GPS requires the Transmit and Receive pins on the Raspberry Pi!

Disabling the UART for serial console use during boot does not seem to be easily configurable at boot-time, but one of you may know better how to proceed. So far, I’ve figured out how to disable the serial console by compiling needed options directly into the loader. I also keep two versions of the binary files handy just in case I need to see the boot process in its entirety!

Two changes are needed to the source code:

1. Add definitions to enable silent booting in the core files.
2. Add boot-time parameters to turn off use of the serial console during the very initial stages of the boot. This seems to be needed in order to keep the serial console from trying to read characters from the GPS. In this case, the boot just hangs and the console echoes gibberish.

Both changes are encapusulated as patch files. In the next two sections, I'll walk you through the process.

Enabling Silent Boot

Following the instructions in README.silent (in the docs directory of the U-Boot port), you should enable the following options. Adding these options merely enables code in the loader. You need subsequent environment settings to actually disable the messages.

#define CONFIG_SILENT_CONSOLE
#define CONFIG_SYS_DEVICE_NULLDEV
#define CONFIG_SILENT_CONSOLE_UPDATE_ON_RELOC
#define CONFIG_SILENT_CONSOLE_UPDATE_ON_SET

To integrate this change into the ports environment, I created a patch file patch-include_configs_rpi__2.h that the port system can apply automatically. This patch file must be added into the directory /usr/ports/sysutils/u-boot-rpi2/files. The file can be found in the patches directory of this repository.

*** include/configs/rpi_2.h.orig        Fri Nov 27 10:05:31 2015
--- include/configs/rpi_2.h     Fri Nov 27 10:04:15 2015
***************
*** 12,15 ****
--- 12,20 ----
  
  #include "rpi-common.h"
  
+ #define CONFIG_SILENT_CONSOLE
+ #define CONFIG_SYS_DEVICE_NULLDEV
+ #define CONFIG_SILENT_CONSOLE_UPDATE_ON_RELOC
+ #define CONFIG_SILENT_CONSOLE_UPDATE_ON_SET
+ 
  #endif

The Boot-Time Options

While the full U-Boot system offers multiple dynamic configuration options, none of the ones that I tried were sufficient. Those tried included placing settings in /boot/loader.conf, in config.txt, and in uEnv.txt (after compiling in a boot-time parameter to read uEnv.txt).

What did work was to add three lines to the rpi_common.h configuration file. Those three lines update the bootargs string that gets compiled into the binary file. Did I mention messy? The rpi_common.h file is itself patched as part of the port, so in the end I have generated a patch file for the patch file. Of course, you can edit the lines in yourself, either in the port's original patch file or in the resulting rpi_common.h. However, if you edit the rip_common.h, it will get overwritten next time you generate a full build of the port, and your changes will be lost.

The three lines added are the equivalent of

stdin=nulldev
stdout=nulldev
silent=1

Patching a Patch File

The patch file (for the patch file) can be found in patches/patch_patch_include_configs_rpi__common.h The contents of the patch file are:

% more patch_patch_include_configs_rpi__common.h 
*** patch-include_configs_rpi__common.h.orig    Thu Dec  3 04:22:15 2015
--- patch-include_configs_rpi__common.h Fri Nov 27 12:51:10 2015
***************
*** 1,6 ****
  --- include/configs/rpi-common.h.orig 2015-04-13 11:53:03.000000000 -0300
  +++ include/configs/rpi-common.h
! @@ -183,4 +183,63 @@
   
   #define CONFIG_BOOTDELAY 2
   
--- 1,6 ----
  --- include/configs/rpi-common.h.orig 2015-04-13 11:53:03.000000000 -0300
  +++ include/configs/rpi-common.h
! @@ -183,4 +183,66 @@
   
   #define CONFIG_BOOTDELAY 2
   
***************
*** 36,41 ****
--- 36,44 ----
  +     "pxefile_addr_r=0x00100000\0" \
  +     "kernel_addr_r=0x01000000\0" \
  +     "ramdisk_addr_r=0x02100000\0" \
+ +     "stdin=nulldev\0" \
+ +     "stdout=nulldev\0" \
+ +     "silent=1\0" \
  +     "Fatboot=" \
  +       "env exists loaderdev || env set loaderdev ${fatdev}; " \
  +       "env exists UserFatboot && run UserFatboot; " \

The lines prefixed by + + are the new lines.

Patching the patch file

1. Copy the patch file into the files/ directory of the port.
2. Run the patch command to patch the port’s original patch file.

 % patch patch-include_configs_rpi__common.h \ 
          patch_patch_include_configs_rpi__common.h
 ```
(That command is all on one line.)
3. Move the `patch_patch_include_configs_rpi__common.h` file to a safe place. Don’t leave it in the `files/` directory, where it might get removed when you update the port.
4. Now rebuild the port. The Makefile will automatically apply the patch files in `files/`, including the one for `configs/rpi_2.h`. 
5. Remember to install (or reinstall) the port. This will have the side effect of restoring the U-boot configuration file `config.txt` to its original state. If you have addional customizations, be sure to update your installed `config.txt`. By default, it can be found in `/usr/local/share/u-boot/u-boot-rpi2/`.

## Using the rebuilt files

Crochet will automatically pick up the files from the installed port. Once your PI boots, the files will be found on the PI in `/boot/msdos`. You can edit `config.txt` there, or even replace the boot loader files with ones that use the UART.