README.commands: Document what UNDEF_SYM does

Changes in v2: - Reword a bit more Signed-off-by: Tom Rini <trini@ti.com>

README.commands: Document what UNDEF_SYM does
Changes in v2: - Reword a bit more Signed-off-by: Tom Rini <trini@ti.com>
Tom Rini
1 parent 244e6f9705
Showing 1 changed file with 7 additions and 0 deletions Side-by-side Diff
doc/README.commands
@@ -22,6 +22,13 @@
 compiled into any object code and construct a static array so the
 command can be found in an array starting at __u_boot_cmd_start.
  
+To ensure that the linker does not discard these symbols when linking
+full U-Boot we generate a list of all the commands we have built (based
+on the sections mentioned above) and use that to force the linker to
+first enter the symbol as undefined in the output object so that there
+is then a need for the symbol to be kept (this is the UNDEF_SYM logic in
+the Makefile).
+
 If a new board is defined do not forget to define the command section
 by writing in u-boot.lds ($(TOPDIR)/board/boardname/u-boot.lds) these
 3 lines:
...	...	@@ -22,6 +22,13 @@
22	22	compiled into any object code and construct a static array so the
23	23	command can be found in an array starting at __u_boot_cmd_start.
24	24
	25	+To ensure that the linker does not discard these symbols when linking
	26	+full U-Boot we generate a list of all the commands we have built (based
	27	+on the sections mentioned above) and use that to force the linker to
	28	+first enter the symbol as undefined in the output object so that there
	29	+is then a need for the symbol to be kept (this is the UNDEF_SYM logic in
	30	+the Makefile).
	31	+
25	32	If a new board is defined do not forget to define the command section
26	33	by writing in u-boot.lds ($(TOPDIR)/board/boardname/u-boot.lds) these
27	34	3 lines: