Re: OMAP3: Linux Without a Boot Loader?

[Greg Ungerer <gerg@snapgear.com>]

Hi Nishanth,

Nishanth Menon wrote:
> Hi,
> Before I start, a Disclaimer:
>     This is more of a "can it be done" project, Don't do this to a 
> production device and flame me.. at least not yet, IMHO ;)
> 
> In ELC2010(SFO), Greg Ungerer showed how to do a boot of kernel without 
> the need for a bootloader.
>     http://elinux.org/ELC_2010_Presentations
>     http://elinux.org/images/7/7b/LinuxWithoutABootLoader.pdf
> 
> How does this help?
> a) Use kernel as a bootloader in itself:
>     http://elinux.org/images/2/2f/ELC-2010-Damm-Kexec.pdf
> b) One more way to shift to using Device trees to describe connectivity 
> and peripherals (Grant Likely's work)
>     http://elinux.org/images/b/b6/ARM_Device_Tree_Status_Report.pdf
> c) Today some features are assumed(wrongly) to be already set in the 
> bootloader (such as pin muxing etc..). as a community whose code is used 
> in many products, we all know the variant bootloaders folks use (some 
> properitary and some community). kernel assumptions are sometimes proven 
> wrong when someone tries the kernel on thier own inhouse bootloader.
> d) Reduce dependency on numerous u-boot, x-loader versions.
> e) many bootloaders dont have advanced support such as filesystem, usb 
> host etc.. kinda a pain when you have to do things twice - do it once in 
> kernel, then again for a bootloader..
> 
> Few more things I can think about, but I was interested in seeing if I 
> could get it running on OMAP3..
> 
> What was done:
> As a proof of concept for OMAP3 (SDP3630 with OMAP3630), I formatted a 
> standard kernel build (bzImage) as I describe below and flashed it to 
> nand partition 0 using mmc boot. Then switched to nand boot and I have 
> kernel booting off the flash without x-loader.
> 
> Configuration header allows any image to boot off a I/O device such as 
> MMC/NAND device straight into SDRAM and proceed execution. ARM kernel 
> has a few additional requirements that CH is unable to cater to, 
> including disabling Caches/interrupts, setting up ATAGs, r0,r1,r2 
> settings with mach-id etc..
> 
> So I wrote a small code, which I called sty which setups the environment 
> for the kernel piggy to start off on.. sty is something you'd put in the 
> arch/arm/boot/head-xyz.S except I have kept it away from the kernel to 
> allow for independent development given multiple boards out there..
> 
> to describe the board, omap-uboot-utils has a configuration file in 
> configs/ which describes the machine ID, bootargs, sdrc config, dpll 
> settings etc.. things like sdrc_config, mach_id etc could potentially 
> change for platforms, but is mostly a copy paste from existing file..
> 
> The final image looks as follows:
>     +---------------------+
>     | Configuration       |
>     | Header              |
>     +---------------------+
>     | "Sty"               |
>     +---------------------+
>     | kernel piggy        |
>     | bzImage             |
>     +---------------------+
> 
> Image creation steps:
> make bzImage, copy the arch/arm/boot/zImage to required location,
> ./tagger -c configs/sample-3630.cfg -f zImage
> ./gpsign -c configs/sample-3630.cfg -f zImage.tag
> 
> I need to probably add some decent documentation around it, but please 
> feel free to poke around - it is pretty simple code..

Nicely done :-)

Have you put up the "Sty" code anywhere for us to look at?

Regards
Greg



> Risks:
> * Nand flash is pretty notorious for generating bad blocks run time.. 
> since the image written is pretty big, flash blocks in that area could 
> effectively kill you boot till u reflash again..
> 
> 
> Future work:
> [3] shows how to boot from an MMC card with u-boot (and no MLO). I 
> expect this to work with kernel as well..
> * maybe more.. left to everyone's imagination ofcourse ;)
> 
> References:
> -----------
> Arm booting requirements: 
> http://www.arm.linux.org.uk/developer/booting.php (or you can use the 
> google's cached version while Russel gets his network back)
> 
> Logs:
> [1] Flashing Steps: http://pastebin.mozilla.org/718671
> [2] Power on in NAND boot log: http://pastebin.mozilla.org/718674
> 
> Kernel used:    
> git://git.kernel.org/pub/scm/linux/kernel/git/khilman/linux-omap-pm.git 
> (pm branch)
>     Commit: 5914a1c9bd8083c65dcdaccae2c458e254573afb
> 
> Omap-uboot-utils used:
>     git://github.com/nmenon/omap-u-boot-utils.git (wip-taggersty branch)
>     Commit: 76a4328253fd53dbebea517f2339a7389d8242e1
>     (the utlity tagger is mostly a collection of code snippets from 
> u-boot.. so the licensing remains the same.. in case you are wondering.. 
> ;) )..
> 
> MMC boot without x-loader:
> [3] 
> http://omapedia.org/wiki/E-MMC_boot#You_can_boot_omap3630_without_x-loader


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Greg Ungerer  --  Principal Engineer        EMAIL:     gerg@snapgear.com
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