[U-Boot-Users] how to make a core uboot? (Comments about AT91 BMS)

[Ken.Fuchs at bench.com]

Andreas Schweigstill wrote:

> On certain platforms (e.g. Atmel AT91) even U-Boot will just 
> be used as a second or third level bootloader. The basic CPU
> setup has been done before starting U-Boot.

Qualification: The following is true for all AT91SAM9 processors.
It probably also applies to AT91CAP9 processors.  I'm not sure it
applies to other AT91 processors though.

This is only true of Atmel AT91 processors with BMS set to 1.
Execution starts in the Atmel AT91's internal ROM.  The ROM
loads code from a SPI NOR flash (Dataflash) into internal SRAM
or external SDRAM.  (If the SRAM step is skipped, U-Boot would
be the second stage bootloader.)  With an SRAM bootloader stage
(AT91Bootstrap) as the second stage bootloader, that would
make U-Boot the third stage bootloader.

With BMS=0, execution of AT91 processors will start with the
EBI chip select 0 device, usually (EBI) NOR flash.  The AT91's
internal ROM is completely bypassed.

It just happens that official U-Boot for AT91 uses the AT91's
internal ROM to boot via SPI NOR flash.  There is a patch by
Rubini to Atmel's U-Boot 1.1.5 version that runs U-Boot from
an AT91SAM9263-EK board with BMS=0.  It is technically a two
stage bootloader with AT91Bootstrap code being loaded into
SRAM and U-Boot loaded into SDRAM by AT91Bootstrap.

Does the new AT91 code in U-Boot handle BMS=0 properly?

If so, ignore the following paragraph:

I've not heard of a single stage U-Boot for Atmel AT91, but
it is just as easy to do as any other single stage U-Boot
for any other processor.  It should be easier to do than a
U-Boot port to a new architecture.

Sincerely,

Ken Fuchs

> -----Original Message-----
> From: u-boot-users-bounces at lists.sourceforge.net 
> [mailto:u-boot-users-bounces at lists.sourceforge.net] On Behalf 
> Of Andreas Schweigstill
> Sent: Monday, April 28, 2008 04:51
> To: u-boot-users at lists.sourceforge.net
> Subject: Re: [U-Boot-Users] how to make a core uboot?
> 
> 
> Hello!
> 
> loody mil schrieb:
> > I intend to make a pure uboot, without including any driver 
> but only 
> > basic cpu setting and uboot commands supported. But I 
> cannot see any 
> > config option in Makefile.
> 
> If you need a really minimalistic bootloader U-Boot will not be the
> first choice. The smallest possible U-Boot will be around 100KByte in
> size and contain much more than just basic CPU initialisation.
> 
> > I know I may choose any config in boards and truncate those 
> things I 
> > don't need at all.
> 
> That's the right way if you want to use U-Boot.
> 
> > But I send this mail and want to know is there already a 
> config option 
> > or some fast way I can meet my requirement?
> 
> Nobody knows what are you requirements. If you need a really
> minimalistic bootloader you should take a look at some sample
> initialization code from the processor manufacturer.
> 
> On certain platforms (e.g. Atmel AT91) even U-Boot will just 
> be used as
> a second or third level bootloader. The basic CPU setup has been done
> before starting U-Boot.
> 
> With best regards
> Andreas Schweigstill
> 
> -- 
> Dipl.-Phys. Andreas Schweigstill
> Schweigstill IT | Embedded Systems
> Schauenburgerstraße 116, D-24118 Kiel, Germany
> Phone: (+49) 431 5606-435, Fax: (+49) 431 5606-436
> Mobile: (+49) 171 6921973, Web: http://www.schweigstill.de/
> 
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