about to CONFIG_LOGBUFFER ..

I do not understand the CONFIG_LOGBUFFER.
What's the CONFIG_LOGBUFFER?
How to use the CONFIG_LOGBUFFER in U-Boot and Linux Kernel?

Why that excluded log buffer in linux system memory?

It's a feature so that both U-Boto and Linux can use the same memory as log buffer.

In U-Boot, for example the POST code can output messages to the log buffer. Or you can redirect the standard output of (some or all) commands to be stored in the log buffer.

In Linux, the syslog mechanism makes use of the log buffer.

This way you can for example have the standard Linux systlog toolset process the boot messages or the POST diagnosis. Or you can use U-Boot to print the Linux kernels log buffer after a crash.

This is to make sure that the log buffer is kept intact over reboots. Even after a Linux crash you can use U-Boot (or Linux after the next boot) to print the log buffer contents.

Best regards,

Wolfgang Denk

Software Engineering:  Embedded and Realtime Systems,  Embedded Linux
Phone: (+49)-8142-4596-87  Fax: (+49)-8142-4596-88  Email: wd at denx.de
"Pull the wool over your own eyes!"                - J.R. "Bob" Dobbs