[U-Boot] more U-boot for Pentium

[Raleigh Boulware]

Thanks for your help I will study up on this more and get back with more questions next week! thanks for your help..
 
BTW I just bought an eprom programer off EBAY it will take quite some time to ship it from china, so i am making baby steps.. :)
 

> Date: Thu, 30 Jun 2011 12:17:46 +1000
> Subject: Re: more U-boot for Pentium
> From: graeme.russ at gmail.com
> To: r_boulware at hotmail.com
> CC: u-boot at lists.denx.de
> 
> On Thu, Jun 30, 2011 at 12:08 PM, Raleigh Boulware
> <r_boulware at hotmail.com> wrote:
> >
> >
> >> Date: Thu, 30 Jun 2011 11:58:55 +1000
> >> Subject: Re: more U-boot for Pentium
> >> From: graeme.russ at gmail.com
> >> To: r_boulware at hotmail.com
> >> CC: u-boot at lists.denx.de
> >>
> >> On Thu, Jun 30, 2011 at 11:54 AM, Raleigh Boulware
> >> <r_boulware at hotmail.com> wrote:
> >> >
> >> >
> >> >> Hi Raleigh,
> >> >>
> >> >> On Thu, Jun 30, 2011 at 11:16 AM, Raleigh Boulware
> >> >> <r_boulware at hotmail.com> wrote:
> >> >> > I have been going through the code and I have stumbled accross a file
> >> >> > called
> >> >> > sc520_car.S that has something called Cache as ram. I am guessing
> >> >> > that
> >> >> > this
> >> >> > is something special for the sc520 correct? Why was there a need to
> >> >> > do
> >> >> > this?
> >> >> > Why was the Cache used like ram in the boot process?
> >> >>
> >> >> Cache-As-RAM allows the CPU cache to be used as regular RAM while
> >> >> U-Boot
> >> >> is running from Flash before the onboard SDRAM has been initialised.
> >> >> This
> >> >> allows a temporary stack, and therefore C runtime environment to be
> >> >> created. This this allows you to write the SDRAM initialisation code in
> >> >> C
> >> >> which is a lot easier than assembler.
> >> >>
> >> > So this is pnly used if we want to setup the SDRAM controller in C code
> >>
> >> More that just that - It is used for the duration of board_init_f() which
> >> may include all manner of peripheral initialisation prior to relocating
> >> U-Boot into RAM.
> > Is it possible to just set up the DDR controller early in the process in
> > assembler to avoid doing a Cache as ram? The reason I ask is because I might
> > have trouble getting other mother boards to do this.
> 
> Yes, you can setup the SDRAM controller in assembler. This would occur in
> place of the Cache-As-RAM init you see for the sc520. You also need to
> determine the memory size while you are at it (at a pinch, you can hard-
> code the memory size initially)
> 
> >
> > Als I was looking at 2 rutines. board_init_r()  and board_init_f(). It
> > appears that the code goes from assmbler to this board_init_f() then back to
> > assembler then to board_init_r(). I am not sure of the reason for this.
> 
> board_init_f() runs from Flash - It is responsible for getting everything
> setup ready to run U-Boot from RAM. The last thing board_init_f does is
> copy U-Boot from Flash into RAM and perform the relocation fixups.
> board_init_f() then calls back into assembler at which point a new stack
> is created in RAM before jumping to the in-RAM copy of board_init_r().
> Once inside board_init_r(), the Flash is never again accessed unless you
> save the environment or erase it and write a new U-Boot binary (i.e. you
> can physically remove the Flash chip and U-Boot will still run fine)
> 
> Regards,
> 
> Graeme