[U-Boot-Users] OneNAND alternative to X-Load

[Pete MacKay <linux@architechnical.net>]

We're developing a PXA-270-based machine that will boot from OneNAND.  I
wanted to share my ideas (and code) with anyone faced with this challenge,
since this forum (esp. Kyungmin Park - thank you!!) was quite helpful.
 I'll start off explaining my u-boot code, then when I get some time I'll
submit my PXA-270 platform driver as a patch (currently works on 2.6.18).

We use a common approach: in lacking JTAG we have chip select jumpers that
swap CS0/CS1 between NOR and OneNAND.  The OneNAND has a bottom-mapped 2K
boot buffer of which 1K is valid, which is where the level 1 boot loader
fits.  It uses two memory-mapped 'bufferrams' to ping-pong data out of the
NAND pages, but you want to use BUFFERRAM1 only as writing BUFFERRAM0 can
corrupt BOOTRAM0 on some revs of the chip.  (We're executing out of
BOOTRAM0 so that's bad :).

There is open-source code called "x-loader" that is a stripped-down u-boot
for the level 1 boot loader.  What I did was integrate the level 1
function into u-boot itself with a custom configuration and linker script.
In the top u-boot Makefile I added this:

myboard_config	:	unconfig
	@./mkconfig $(@:_config=) arm pxa myboard

myboard_onenand_config	:	unconfig
	@./mkconfig $(@:_config=) arm pxa myboard
	@echo "LDSCRIPT := $(TOPDIR)/board/myboard/u-boot.onenand.lds" >>
include/config.mk

In include/configs/myboard_onenand.h I simply define CONFIG_BOOT_ONENAND
and then include myboard.h, with conditional switches inside for base
address, etc (i.e. define CONFIG_IDENT_STRING as " from OneNAND" or " from
NOR").

The linker script replaces cpu/pxa/start.S with oneboot.S and makes sure
the C code to load the other pages positions there too.  Then it aligns
the rest of u-boot at the 2K boundary (NAND page 1).  The oneboot.S code
melds start.S with lowlevel_init but calls C code to read the rest of the
block 0 pages into RAM.  I later optimized it to replace start.S entirely
for the NOR version.

BTW, I lacked originality and stole (liked) the name: oneboot is not to be
confused with the script that merges x-load and u-boot into one binary
image.

This is the relevant portion of the u-boot linker script:

	. = 0x00000000;

	. = ALIGN(4);
	.text :
	{
	  board/myboard/oneboot.o	(.text)
	  board/myboard/onenand_boot.o	(.text)
	  __oneboot_end = .;	/* must not exceed 0x400! */

	  . = 0x800;		/* reposition to 2K page boundary */
	  __page1_start = .;	/* OneNAND loading starts here */
	  *				(.text)
	}

Look at System.map to make sure your level 1 code fits within 1K.  Also in
oneboot.S I commented out labels and used relative branches to save symbol
space and replaced ldr with mov wherever possible.  And with the 270
optimizations in oneboot.S our system screams!

The full code is at http://code.architechnical.net/onenand, including the
2.6.18 pxa27x OneNAND driver.  Please don't laugh at my lame ancient web
site.  Oh yeah, I'm using arm-linux-gcc 3.4.4.
-- 
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