Invalid memory address failures due to copy_rom_font

[Julio Merino <julio%meroh.net@localhost>]

Hi all,

I've been trying to get NetBSD (any version really...) to run on this
old PowerBook6,8 (G4, 12") and every single attempt has failed. After
some digging, I found the two blockers for booting. One is the need to
define FIRMWORKSBUGS, which I'll skip for now but I'm mentioning for
completeness. The other is described below, and is the cause behind
all "Invalid memory access" and "Decrementer exception" problems I
encountered at early boot time. With these, I've gotten 9.99.36 to
work!

The problem comes from the copy_rom_font functionality in
ofw_rascons.c. Attempting to use the ROM font results in the "Invalid
memory access" errors above. Disabling this functionality lets the
kernel boot just fine.

Now, I'm wondering how this has ever worked, and what a good solution would be.

What this code does is construct a wsfont descriptor for the font in
the ROM and points that descriptor the memory address where the font
lives. Now... the code has a comment like: "Convert to physical
address.  We cannot access to Open Firmware's virtual address space."
which I think is where the root of the problems live.

* Using the font-adr virtual address as returned from OpenFirmware,
without conversion to a physical address does "work". If I disable the
translate call, the kernel is able to print some messages until it
crashes, which I suspect happens when the kernel takes over the page
table because it knows nothing about this area (?).

* I've tried adding pmap_enter for these addresses (inspired by what
ofwea_machdep.c does for the framebuffer) in an attempt to make the
above work... but I really don't know what I'm doing here.

* Accessing memory through the physical address right after the
conversion via OF's translate results in an immediate "Invalid memory
address" exception. So I'm wondering... how has this ever worked or
how does this work in some machines but not others?

Here are some thoughts for "fixes":

* I've tried modifying copy_rom_font to copy the font data out of the
ROM and into our own static buffer -- and things work.

* But soon after boot, the kernel ignores the copied font, clears the
screen, and uses its own font. Which makes me wonder if it's worth
going that path at all. Maybe copy_rom_font can be ifdefed out? On
macppc?

* It'd also be nice to register the ROM font with wsfont_add() so that
it'd remain usable throughout the boot process. Right? But wsfont_add
is unusable that early in the boot process due to its use of malloc()
as I understand it.

Any ideas for a good fix (that's not one more ifdef)?

Thanks

-- 
jmmv.dev