Re: [coreboot] question about post_cache_as_ram() function

[Fengwei Zhang]

> DCACHE_RAM_BASE is the start address of the memory addresses that are used for
> CAR.
>
> DCACHE_RAM_SIZE is the size of that region.

I checked the value from Kconfig under /mainboard/asus/m2v-mx_se/
DCACHE_RAM_BASE is 0xcc000
DCACHE_RAM_SIZE is 0x4000
Why does they set to these value? Does them change?
Do they setup by hardware?
> RAMTOP is the highest address of the memory region that coreboot will use in
> RAM.
> > In addition, why do we need to set new esp here?
> >    	/* set new esp */ /* before CONFIG_RAMBASE */
> >    	"subl   %0, %%esp\n\t"
> > ::"a"( (CONFIG_DCACHE_RAM_BASE + CONFIG_DCACHE_RAM_SIZE)-
> >
> > (CONFIG_RAMTOP) )
>
> This moves the stack from CAR area to RAM.
>
> While in CAR, the stack is laid out at the top end of CAR, growing downwards
> (ie. starting at DCACHE_RAM_BASE + DCACHE_RAM_SIZE).
> While in RAM, it starts at the top end of the ram area used for coreboot
> (RAMTOP).

I see
> Before that line the stack is copied from CAR to RAM, then esp is setup so it
> points to the same place in RAM that it pointed to in CAR (that's why we use
> sub, not mov).

i see the copying from CAR to RAM. but I still confused about the value 
of esp. Why is this euqation:
esp = esp - (CONFIG_DCACHE_RAM_BASE + CONFIG_DCACHE_RAM_SIZE)- 
(CONFIG_RAMTOP))

I thought the esp should be: CONFIG_RAMTOP - CONFIG_DCACHE_RAM_SIZE

Thanks!

Best,
Fengwei

question about post_cache_as_ram() function

[Patrick Georgi]

Am Freitag, 17. Dezember 2010, um 17:18:38 schrieb Fengwei Zhang:
> I checked the value from Kconfig under /mainboard/asus/m2v-mx_se/
> DCACHE_RAM_BASE is 0xcc000
> DCACHE_RAM_SIZE is 0x4000
> Why does they set to these value? Does them change?
> Do they setup by hardware?

They're mostly read out from vendor BIOS or taken from the bios developer 
manuals (which usually amounts to the same).

The values depend somewhat on the characterics of the CPU cache, so we simply 
treat them as opaque numbers for the most part.

What makes you so curious about the details of CAR setup?
> > Before that line the stack is copied from CAR to RAM, then esp is setup
> > so it points to the same place in RAM that it pointed to in CAR (that's
> > why we use sub, not mov).
>
> i see the copying from CAR to RAM. but I still confused about the value
> of esp. Why is this euqation:
> esp = esp - (CONFIG_DCACHE_RAM_BASE + CONFIG_DCACHE_RAM_SIZE)-
> (CONFIG_RAMTOP))

Let's define
DCACHE_TOP := DCACHE_RAM_BASE + DCACHE_RAM_SIZE
esp = DCACHE_TOP - SB with
SB := "number of bytes used on the stack" (stack grows downwards)

Then it follows that
esp' = (DCACHE_TOP - SB) - (DCACHE_TOP - RAMTOP)
= DCACHE_TOP - SB - DCACHE_TOP + RAMTOP
= DCACHE_TOP - DCACHE_TOP + RAMTOP - SB
= RAMTOP - SB

So the new stack pointer points to the same location inside the copy of the 
stack.

Patrick