On Thu, Feb 4, 2021 at 9:39 PM Hector Martin [off-list ref] wrote:

This is a hack. I do not expect this to be merged as-is.

The problem: on Apple ARM platforms, SoC MMIO needs to use nGnRnE
mappings: writes using nGnRE are blackholed. This seems to be by design,
and there doesn't seem to be any fabric configuration or other bit we
can flip to make the problem go away.

Particularly tricky is that this affects earlycon, which uses fixmap,
which all gets initialized before any of the usual cpufeatures /
alternatives stuff. So we need to take care of fixmap very early.

Options I can think of:

(1) Unconditionally use nGnRnE on all platforms for fixmap IO. Maybe
    this is actually fine? I suspect it might break some PCI-based
    earlycons?

(2) Deal with this special case in the earlycon code, since that seems
    to be the only user that matters on these platforms. Since the
    IO mapping is done in earlycon.c, this will require some cooperation
    with samsung_tty.c so earlycon knows when it needs to do this. Note
    that doing it with DT properties will break cmdline-only earlycon
    config (which otherwise works fine on this driver).

(3) This patch, but do something saner, like use a specific DT flag to
    trigger this mode instead of a platform match.

Any other ideas?

I think we should decide the approach for the normal ioremap()
path first. Once we have solved that, there is likely an obvious
answer for this one as well.

My feeling is that we'll end up with (2).

        Arnd

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