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

[Marek Vasut]

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.

Best regards,
Marek Vasut