[U-Boot] [PATCH] arm: mxs: Fix vectoring table crafting

[Marek Vasut]

Dear Otavio Salvador,

> On Fri, Apr 26, 2013 at 12:10 AM, Marek Vasut <marex at denx.de> wrote:
> > Dear Otavio Salvador,
> > 
> > [...]
> > 
> >> > +/*
> >> > + * This function will craft a jumptable at 0x0 which will redirect
> >> > interrupt + * vectoring to proper location of U-Boot in RAM.
> >> > + *
> >> > + * The structure of the jumptable will be as follows:
> >> > + *  ldr pc, [pc, #0x18] ..... for each vector, thus repeated 8 times
> >> > + *  <destination address> ... for each previous ldr, thus also
> >> > repeated 8 times + *
> >> > + * The "ldr pc, [pc, #0x18]" instruction above loads address from
> >> > memory at + * offset 0x18 from current value of PC register. Note
> >> > that PC is already + * incremented by 4 when computing the offset, so
> >> > the effective offset is + * actually 0x20, this the associated
> >> > <destination address>. Loading the PC + * register with an address
> >> > performs a jump to that address.
> >> > + */
> >> 
> >> I understood what you're doing but not the motivation for it. What
> >> problem you're fixing or feature this will allow?
> > 
> > In case the CPU core gets and interrupt (data abort, invalid
> > instruction...), the CPU will jump to 0x0. It is imperative for handling
> > code to be present at 0x0, but since U-Boot on MXS is started from RAM,
> > only remnants of SPL are present at 0x0. Any core interrupt will then
> > hit those remnants and cause undefined behavior. This patch puts
> > redirection table there which in turn jumps to U-Boot vectoring entry
> > points in RAM ... but this is all in the commit message already, so
> > dunno if that helps you.
> 
> Much clear now. Did you actually tested this in MX23 as well?

No, it is not needed, but you can test it if it'd make you feel better. The ARM 
core is the same in both, so is the memory map for DRAM and SRAM.

> How did
> you simulated the exception to test it?

Try "md.l 0x1" or "go 0x20", they'll cause either dabt or invalid instruction.


Best regards,
Marek Vasut