Touching the initrd before paging_init

[Jon Masters]

Hi Folks,

Just a few notes that might be useful to someone else.

I mentioned before that we'll want to see ACPI_INITRD_TABLE_OVERRIDE
support on arm64 in due course for use during bringup of new systems. It
is commonly used on x86 to provide handy updated ACPI tables via a
prepended initrd cpio that is attached to the regular initrd image,
which is infinitely better than respinning firmware each time you want
to provide updated tables (yes yes, we all know the "devicetree"
directive in GRUB does this with DT and that's great and wonderful). I
know that someone is working on it, but I had a need for an interim
solution over the weekend for some hardware I am bringing up here.

The problem is that there's no mapping for the pages containing the
ramdisk data prior to paging_init. x86 does a bit of hoop jumping (and I
think they have certain assumptions about linear mapping too) and splits
the initrd setup into a couple of different phases, which we may well
need to do also if we want to be able to call something like:

#if defined(CONFIG_ACPI) && defined(CONFIG_BLK_DEV_INITRD)
        acpi_initrd_override((void *)initrd_start, initrd_end -
initrd_start);
#endif

prior to acpi_boot_table_init in arch/arm64/kernel/setup.c. That routine
will correctly map memory for data it copies out of the cpio, but it
assumes that the initrd is already mapped. For the very nasty hack I
want over the weekend, I added some extra fixmaps and then do an
early_memremap on the physical address of first page of the initrd (the
cpio is tiny, and is always prepended and this is very nasty). Viz:

[    0.000000] JCM: using fixmap for first page of ramdisk...
[    0.000000] JCM: first physical page of ramdisk: [mem REDACTED]
[    0.000000] JCM: first virtual page of ramdisk: [mem REDACTED]
[    0.000000] XXXX ACPI table found in initrd
[kernel/firmware/acpi/xxxx.aml][0xxxx]
[    0.000000] XXXX ACPI table found in initrd
[kernel/firmware/acpi/xxxx.aml][0xxx]
[    0.000000] XXXX ACPI table found in initrd
[kernel/firmware/acpi/xxxx.aml][0xxxx]

So maybe I'll get what I want working, but a head's up to whoever from
Linaro ends up looking into the broader implementation that I think
you'll need to rework initrd mapping as part of that. Unless of course
I'm missing something. Which is more than possible.

Jon.