doc/qemu_mips: add doc howto debug u-boot with gdb

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>

doc/README.qemu_mips

 
 Notes for the Qemu MIPS port
 
-Example usage:
+I) Example usage:
 
 # ln -s u-boot.bin mips_bios.bin
 start it:
@@ -16,3 +16,65 @@ create image:
 # dd of=flash bs=1k conv=notrunc if=u-boot.bin
 start it:
 # qemu-system-mips -M mips -pflash flash -monitor null -nographic
+
+II) How to debug U-Boot
+
+In order to debug U-Boot you need to start qemu with gdb server support (-s)
+and waiting the connection to start the CPU (-S)
+
+# qemu-system-mips -S -s -M mips -pflash flash -monitor null -nographic
+
+in an other console you start gdb
+
+1) Debugging of U-Boot Before Relocation
+
+Before relocation, the addresses in the ELF file can be used without any problems
+buy connecting to the gdb server localhost:1234
+
+# mipsel-unknown-linux-gnu-gdb u-boot
+GNU gdb 6.6
+Copyright (C) 2006 Free Software Foundation, Inc.
+GDB is free software, covered by the GNU General Public License, and you are
+welcome to change it and/or distribute copies of it under certain conditions.
+Type "show copying" to see the conditions.
+There is absolutely no warranty for GDB.  Type "show warranty" for details.
+This GDB was configured as "--host=i486-linux-gnu --target=mipsel-unknown-linux-gnu"...
+(gdb)  target remote localhost:1234
+Remote debugging using localhost:1234
+_start () at start.S:64
+64              RVECENT(reset,0)        /* U-boot entry point */
+Current language:  auto; currently asm
+(gdb)  b board.c:289
+Breakpoint 1 at 0xbfc00cc8: file board.c, line 289.
+(gdb) c
+Continuing.
+
+Breakpoint 1, board_init_f (bootflag=<value optimized out>) at board.c:290
+290             relocate_code (addr_sp, id, addr);
+Current language:  auto; currently c
+(gdb) p/x addr
+$1 = 0x87fa0000
+
+2) Debugging of U-Boot After Relocation
+
+For debugging U-Boot after relocation we need to know the address to which
+U-Boot relocates itself to 0x87fa0000 by default.
+And replace the symbol table to this offset.
+
+(gdb) symbol-file
+Discard symbol table from `/private/u-boot-arm/u-boot'? (y or n) y
+Error in re-setting breakpoint 1:
+No symbol table is loaded.  Use the "file" command.
+No symbol file now.
+(gdb) add-symbol-file u-boot 0x87fa0000
+add symbol table from file "u-boot" at
+        .text_addr = 0x87fa0000
+(y or n) y
+Reading symbols from /private/u-boot-arm/u-boot...done.
+Breakpoint 1 at 0x87fa0cc8: file board.c, line 289.
+(gdb) c
+Continuing.
+
+Program received signal SIGINT, Interrupt.
+0xffffffff87fa0de4 in udelay (usec=<value optimized out>) at time.c:78
+78              while ((tmo - read_c0_count()) < 0x7fffffff)