Re: [RFC] ARM: mmu: Do not try to pick early TTB up

[Sascha Hauer <s.hauer@pengutronix.de>]

On Fri, May 25, 2018 at 10:09:52AM -0700, Andrey Smirnov wrote:
> On Fri, May 25, 2018 at 2:01 AM, Sascha Hauer <s.hauer@pengutronix.de> wrote:
> > On Thu, May 24, 2018 at 08:14:40PM -0700, Andrey Smirnov wrote:
> >> On Wed, May 23, 2018 at 1:43 AM, Sascha Hauer <s.hauer@pengutronix.de> wrote:
> >> > On Tue, May 22, 2018 at 10:10:12PM -0700, Andrey Smirnov wrote:
> >> >> The call to create_flat_mapping() in mmu.c will change both memory
> >> >> type and shareability of all RAM in use by barebox while MMU is on
> >> >> when done in conjunction with CONFIG_MMU_EARLY.
> >> >
> >> > I notice that with MMU_EARLY enabled we call create_flat_mapping()
> >> > twice, once in the early MMU code and once when setting up the MMU for
> >> > real. In between we remap the the SDRAM cached which then is reverted
> >> > during the second call to create_flat_mapping().
> >> >
> >> > This seems unnecessary. Does the following help you?
> >>
> >> Yeah, this, disabling MMU before or having a tlb_invalidate() after
> >> all seem to help. Your patch works fine, but it has a slight weirdness
> >> in my case because early MMU code would mark OCRAM as cached and
> >> regular MMU code wouldn't undo it without the call to
> >> create_flat_mapping(), so I'd end up with slightly different memory
> >> configuration depending on if EARLY_MMU is enabled or not. Other than
> >> that it should work fine.
> >>
> >> The main reason I chose to go "disable MMU" route is because that
> >> follows what ARMv8 MMU code does, but I am perfectly happy with either
> >> solution.
> >
> > Disabling the MMU probably has a performance impact (I would have to
> > remeasure, maybe this is not true at all), that's why I would prefer
> > keeping it enabled.
> >
> 
> OK, sure. Where do we go from here? Do you want to just take your
> patch or should I update mine, with its War And Peace of a commit
> message, and incorporate what you proposed?

I applied my patch for now. We can still disable the MMU between PBL and
barebox later should we have to.

Sascha

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