Silencing the boot process

All the output sent to the console is useful when developing your application, but when deploying the final product, this may be quite undesirable. Since the software running on the board is actually a set of distinct code bases, we have to tweak a few different places for a truly silent boot.

The u-boot code already supports the notion of booting up silently. The document README.silent covers most of the details:

The config option CONFIG_SILENT_CONSOLE can be used to quiet messages
on the console.  If the option has been enabled, the output can be
silenced by setting the environment variable "silent".
 
- CONFIG_SILENT_CONSOLE_UPDATE_ON_SET
	When the "silent" variable is changed with env set, the change
	will take effect immediately.
 
- CONFIG_SILENT_CONSOLE_UPDATE_ON_RELOC
	Some environments are not available until relocation (e.g. NAND)
	so this will make the value in the flash env take effect at
	relocation.
 
The following actions are taken if "silent" is set at boot time:
 
 - Until the console devices have been initialized, output has to be
   suppressed by testing for the flag "GD_FLG_SILENT" in "gd->flags".
 
 - When the console devices have been initialized, "stdout" and
   "stderr" are set to "nulldev", so subsequent messages are
   suppressed automatically. Make sure to enable "nulldev" by
   #defining CONFIG_SYS_DEVICE_NULLDEV in your board config file.
 
 - When booting a linux kernel, the "bootargs" are fixed up so that
   the argument "console=" will be in the command line, no matter how
   it was set in "bootargs" before. If you don't want the linux command
   line to be affected, define CONFIG_SILENT_U_BOOT_ONLY in your board
   config file as well, and this part of the feature will be disabled.

Some things to note:

• define CONFIG_SILENT_CONSOLE to enable support for silent mode
• set the u-boot envvar silent to disable u-boot output
• you should probably tweak the bootdelay to 1 second (or turn it off)
• you can still interrupt u-boot by pressing the specified key (you won't see output, but your input will be processed)
• you can use U-Boot tools under Linux to modify the bootdelay or silent env vars

Also note that U-Boot only adds console= to clear the default console. It does not touch earlyprintk= as this is a debug only option -- you need to remove this from your default command line yourself.

The Linux kernel too supports quieting the console output. This can be done in three ways. The first is to disable the console completely by using the console= option on the kernel command line. Since this may not be what you want during testing/development/deployment, you can also control the type of message which gets sent to the console by using the loglevel= option. The quiet option suppresses most messages; during a normal boot there won't be any.

The kernel-parameters.txt covers this briefly:

/* line 543 to 571 */
	console=	[KNL] Output console device and options.
 
		tty<n>	Use the virtual console device <n>.
 
		ttyS<n>[,options]
		ttyUSB0[,options]
			Use the specified serial port.  The options are of
			the form "bbbbpnf", where "bbbb" is the baud rate,
			"p" is parity ("n", "o", or "e"), "n" is number of
			bits, and "f" is flow control ("r" for RTS or
			omit it).  Default is "9600n8".
 
			See Documentation/serial-console.txt for more
			information.  See
			Documentation/networking/netconsole.txt for an
			alternative.
 
		uart[8250],io,<addr>[,options]
		uart[8250],mmio,<addr>[,options]
			Start an early, polled-mode console on the 8250/16550
			UART at the specified I/O port or MMIO address,
			switching to the matching ttyS device later.  The
			options are the same as for ttyS, above.
		hvc<n>	Use the hypervisor console device <n>. This is for
			both Xen and PowerPC hypervisors.
 
                If the device connected to the port is not a TTY but a braille
                device, prepend "brl," before the device type, for instance

So to disable the console, all you need is literally console= or console=null.

The kernel-parameters.txt covers this briefly:

/* line 1446 to 1460 */
	loglevel=	All Kernel Messages with a loglevel smaller than the
			console loglevel will be printed to the console. It can
			also be changed with klogd or other programs. The
			loglevels are defined as follows:
 
			0 (KERN_EMERG)		system is unusable
			1 (KERN_ALERT)		action must be taken immediately
			2 (KERN_CRIT)		critical conditions
			3 (KERN_ERR)		error conditions
			4 (KERN_WARNING)	warning conditions
			5 (KERN_NOTICE)		normal but significant condition
			6 (KERN_INFO)		informational
			7 (KERN_DEBUG)		debug-level messages

So you can simply add loglevel=0 to the u-boot bootargs envvar. Now the kernel will only show emergency messages to the console rather than informational/debug.

If, at runtime, you wish to change the console log level, you can do this a few ways:

• from the shell, run dmesg -n # to set the log level to #
• from your code, call the syslog kernel function

The syslog kernel function is pretty easy to utilize:

#include <unistd.h>
#include <sys/syscall.h>
 
static inline int syslog(int type, char *bufp, int len)
{
	return syscall(SYS_syslog, type, bufp, len);
}
 
int main()
{
	/* Set the console loglevel to 3 */
	syslog(8, 0, 3);
 
	return 0;
}

See the syslog(2) man page for more in-depth details of the syslog() system call.

The kernel-parameters.txt covers this briefly:

/* line 2476 to 2478 */
	quiet		[KNL] Disable most log messages

The busybox shell will display some helpful text at startup. This message can be disabled via the uClinux Vendor menuconfig:

uClinux-dist$ make config_menuconfig

Then navigate the menus:

BusyBox  --->
  [*] sh: Hide message on interactive shell startup

If you wish to disable the console altogether, then just add console= to the kernel command line option.

If you wish to disable the console shell on the serial port, turn off the CONFIG_USER_INIT_CONSOLE_SH build option in the uClinux Vendor menuconfig:

uClinux-dist$ make config_menuconfig

Then navigate the menus:

Core Applications  --->
  [*] init
  [ ]   enable console shell