Re: ARM A53 and initial MMU mapping for EL0/1/2/3 ?

Joakim Tjernlund
2022-02-09T18:07:34Z

On Wed, 2022-02-09 at 13:13 +0000, Andre Przywara wrote:
> On Wed, 9 Feb 2022 14:05:57 +0100
> Michael Walle <michael@walle.cc> wrote:
>
> Hi,
>
> > > > The problem I have is that I boot a custom SOC into u-boot and when u-boot tries
> > > > to boot linux I get an error exception when u-boot calls armv8_switch_to_el2 to enter linux.
> > >
> > > So that means that U-Boot runs in EL3, is that the first and only firmware
> > > that you run? I think the EL3 part of U-Boot is not widely used and tested
> > > beyond the very few platforms that use it.
> >
> > FWIW, the u-boot on the kontron_sl28 (which is a A72 core) is running
> > in EL3. But it's a layerscape so it might be worth looking in
> > arch/arm/cpu/armv8/fsl-layerscale/ if there is anything special.
>
> Thanks Michael, that's what I meant: the generic EL3 code in
> arch/arm/cpu/armv8 might depend on more platform specific code to make it
> work.

Thanks guys, will look at these boards.
I have learnt that EL2 state in our design may be broken so I started to look at going directly to EL1
instead, is that a bad idea?

In u-boot I find:

#ifdef CONFIG_ARMV8_SWITCH_TO_EL1
static void switch_to_el1(void)
{
if ((IH_ARCH_DEFAULT == IH_ARCH_ARM64) &&
(images.os.arch == IH_ARCH_ARM))
armv8_switch_to_el1(0, (u64)gd->bd->bi_arch_number,
(u64)images.ft_addr, 0,
(u64)images.ep,
ES_TO_AARCH32);
else
armv8_switch_to_el1((u64)images.ft_addr, 0, 0, 0,
images.ep,
ES_TO_AARCH64);
}
#endif
....
#ifdef CONFIG_ARMV8_SWITCH_TO_EL1
armv8_switch_to_el2((u64)images->ft_addr, 0, 0, 0,
(u64)switch_to_el1, ES_TO_AARCH64);
#else
.....

which looks promising but when I look closer I see that this seems to do:
EL3 -> EL1 -> EL2
Really? What is the point ? Looks like I need to hack something myself to go EL3->EL1
or am I missing something ?

Jocke