Re: [beagleboard] Writing a Bootloader???

[Vladimir Pantelic]

vamsi53 wrote:
> Hello Guys,
> I would like to learn about writing boot loaders.Like how to go about
> writing a boot loader for the Beagle board. I tried web but couldn't
> find info about the same. I would greatly appreciate your suggestions
> and recommended readings.

read the uboot source code, then try to forget it :)

[Vamsi Krishna Baratam]

Thanks for your reply.

But I was expecting a better answer than that as I am seriously interested in learning the Bootloader code.

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[Ben Dooks]

On Wed, Dec 15, 2010 at 12:49:10PM -0800, vamsi53 wrote:
> Hello Guys,
> I would like to learn about writing boot loaders.Like how to go about
> writing a boot loader for the Beagle board. I tried web but couldn't
> find info about the same. I would greatly appreciate your suggestions
> and recommended readings.

I'm not sure there's a lot of good documentation on writing bootloaders
as it is somewhat of a specialist subject, and many people do it differently
depending on what they are after and how their system/soc works.

It sort of breaks down as so:

1) The system gets the initial code from somewhere, and this depends on
not only the SoC but also the configuration that the board may have
done. Some people even have weird setups which have a secondary CPU
to bootstrap the code into the primary.... these people can however
be safely ignored as nutters.

2) The code loaded in step 1 (by the SoC) usually has nothing it can rely
on and needs to ensure the minimum hardware seyup for the CPU core and
any periphreals it needs to load the next stage of the bootloader.

It will also ensure that the external memory is setup so that the
next step has somewhere to put the code to run.

Oftenn this code is only a killobyte or two in size, just to get
enough state to start loading the next part of the bootloader.

3) The next part is to load the rest of the bootloader, usually from the
same source as part 2. Once the code has loaded, the first loader will
start executing this new coe.

4) Now that part 3 has downloaded some more code into the main RAM, it
can now go about doing the rest of the initial setup ready to do
something useful for the user. At this stage there are multiple
choices of what to do.

A) Do minimal hardware setup, and directly execute a linux kernel
B) Load another bootloader from a different source and execute that
C) Run the main bootloader and continue.

--
Ben Dooks, b...@fluff.org, http://www.fluff.org/ben/

Large Hadron Colada: A large Pina Colada that makes the universe disappear.

[Mike Nelson]

Vamsi:

Google is your friend. Try searching for "Booting ARM Linux" or
"bootloader ARM Linux".

Here is a good starting place:
http://www.arm.linux.org.uk/developer/booting.php

As Vladimir says, u-Boot source code is also a good place to start,
but you need some background information to interpret what is going on
in u-Boot and other loaders. The "tip of the spear", so to speak is
in u-Boot code at "../arch/arm/lib/bootm.c". This is where the
"ATAGs" are set up to pass parameters to the kernel and the kernel is
called.

Regards:

Mike Nelson, http://michaeltnelson.com
mobile: 1-650-291-7343
office: 1-650-257-7565

[Vamsi Krishna Baratam]

Mark, Mike & Ben,

Thanks all for your prompt responses.
I would like to know why exactly do you about choosing two bootloaders for the board(like we have two in the case of Beagleboard) and when dont you need it?  And for now I want to concentrate on Uboot. Whats the approach for writing a bootloader(uboot) for a board that is not being supported currently by uboot, for instance a raw board? Look forward to hearing from you people.

Thanks

[Mark Lazarewicz]

http://omappedia.org/wiki/Bootloader_Project

---

From: Vamsi Krishna Baratam <vam...@gmail.com>
To: beagl...@googlegroups.com
Sent: Wed, December 15, 2010 4:44:55 PM
Subject: Re: [beagleboard] Writing a Bootloader???

[Dave Hylands]

Hi Vamsi,

On Wed, Dec 15, 2010 at 2:44 PM, Vamsi Krishna Baratam
<vam...@gmail.com> wrote:
> Mark, Mike & Ben,
>
> Thanks all for your prompt responses.
> I would like to know why exactly do you about choosing two bootloaders for
> the board(like we have two in the case of Beagleboard) and when dont you
> need it?  And for now I want to concentrate on Uboot. Whats the approach for
> writing a bootloader(uboot) for a board that is not being supported
> currently by uboot, for instance a raw board? Look forward to hearing from
> you people.

When the chip starts up, the SDRAM is typically not initialized. This
means that you need to load a small bootloader which will fit into
some internal RAM (sometimes you can lock down the cache for this) and
this small bootloader will then initialize SDRAM and load the next
stage of bootloader.

If your internal RAM is big enough, you might only need one stage of bootloader.

I've worked on chips where you only get 8K or 16K of internal SRAM,
which is nowhere big enough to load something like uboot.

It also depends on whether you have NOR flash (which you can execute
from) or NAND flash (which you can't execute from).

--
Dave Hylands
Shuswap, BC, Canada
http://www.DaveHylands.com/

[aos...@gmail.com]

I tried this a few years ago. The documentation is shotty at best. Bear in mind that you're more or less running on metal. Try looking into toy operating systems, they'll touch on bootloaders at least a little.

-Alex

-----Original Message-----
From: vamsi53 <vam...@gmail.com>
Sender: beagl...@googlegroups.com
Date: Wed, 15 Dec 2010 12:49:10
To: Beagle Board<beagl...@googlegroups.com>
Reply-To: beagl...@googlegroups.com
Subject: [beagleboard] Writing a Bootloader???

Hello Guys,
I would like to learn about writing boot loaders.Like how to go about
writing a boot loader for the Beagle board. I tried web but couldn't
find info about the same. I would greatly appreciate your suggestions
and recommended readings.

Thanks,
Vamsi

[Mark Lazarewicz]

Dave Hylands said

<<ve worked on chips where you only get 8K or 16K of internal SRAM,

<<whichs nowhere big enough to load something like uboot.

 

I worked on 25K COTS boards that had 64K Internal RAM and cost is not an issue

 

In that case the bootRom contained  multipe u-boot images and the the bootloader(u-boot) could  be reflashed and protection against bricking the board

 

Uboot used the internal RAM for stack and initialized DDR all peripherals and loaded the image into DDR. This level of functionality would be a tight  fit in internal RAM of OMAP

 

My point these high end  boards needed no xloader but this same at a cost

 

u-boot initialized DDR handled USB,ethernet tftp etc etc

 

To people coming from high cost systems x-loader looks like its redundant(why 2 loaders)

 

Actually x-loaders need is more subtle

 

In OMAPx  the probale  reason for going with an x-loader was for

1) maximum  flexibility ,

2)ow cost(no boot flash required)  and yes

3) less than 64k of Internal RAM which also serves as stack and data area

 

u-boot for cost sensitive handheld device is different than that used

in a 25K COTS VME baord used to process radar to shoot down missles

 

 

 

[Jason Kridner]

On Dec 15, 2010, at 7:19 PM, aos...@gmail.com wrote:

> I tried this a few years ago. The documentation is shotty at best. Bear in mind that you're more or less running on metal. Try looking into toy operating systems, they'll touch on bootloaders at least a little.

Take a look at PuppyBits and x-loader for some minimalist examples.

[aos...@gmail.com]

Will do... Thanks!

-Alex

-----Original Message-----
From: Jason Kridner <jkri...@beagleboard.org>
Sender: beagl...@googlegroups.com
Date: Sat, 18 Dec 2010 15:24:17
To: beagl...@googlegroups.com<beagl...@googlegroups.com>
Reply-To: beagl...@googlegroups.com
Cc: beagl...@googlegroups.com<beagl...@googlegroups.com>
Subject: Re: [beagleboard] Writing a Bootloader???

[kiranbha...@gmail.com]

I have written an article after a long struggle with bootloader internals.

Here are the links.

It is written using GNU Assembler and C
http://www.codeproject.com/Articles/664165/Writing-a-boot-loader-in-Assembly-and-C-Part-1
http://www.codeproject.com/Articles/668422/Writing-a-boot-loader-in-Assembly-and-C-Part-2

Cheers :)

[Mark Lazarewicz]

You must understand how the chip boots have you read the boot rom section of the TRM?

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