linux-kernel - Re: [RFC PULL] remove arch/h8300

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Date:   Sun, 10 Apr 2022 02:26:45 -0500
From:   Rob Landley <rob@...dley.net>
To:     Greg Ungerer <gerg@...ux-m68k.org>,
        Finn Thain <fthain@...ux-m68k.org>
Cc:     Daniel Palmer <daniel@...f.com>, Arnd Bergmann <arnd@...db.de>,
        Christoph Hellwig <hch@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        "moderated list:H8/300 ARCHITECTURE" 
        <uclinux-h8-devel@...ts.sourceforge.jp>,
        "open list:TENSILICA XTENSA PORT (xtensa)" 
        <linux-xtensa@...ux-xtensa.org>, Max Filippov <jcmvbkbc@...il.com>,
        Linux-sh list <linux-sh@...r.kernel.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Damien Le Moal <damien.lemoal@....com>,
        linux-riscv <linux-riscv@...ts.infradead.org>
Subject: Re: [RFC PULL] remove arch/h8300

On 4/8/22 23:18, Greg Ungerer wrote:
> 
> On 9/4/22 11:59, Finn Thain wrote:
>> On Fri, 8 Apr 2022, Rob Landley wrote:
>> 
>>> On 4/5/22 08:07, Greg Ungerer wrote:
>>>> On 5/4/22 13:23, Daniel Palmer wrote:
>>>>> On Mon, 4 Apr 2022 at 22:42, Greg Ungerer <gerg@...ux-m68k.org> wrote:
>>>>>> But we could consider the Dragonball support for removal. I keep it
>>>>>> compiling, but I don't use it and can't test that it actually works.
>>>>>> Not sure that it has been used for a very long time now. And I
>>>>>> didn't even realize but its serial driver (68328serial.c) was
>>>>>> removed in 2015. No one seems too have noticed and complained.
>>>>>
>>>>> I noticed this and I am working on fixing it up for a new Dragonball
>>>>> homebrew machine. I'm trying to add a 68000 machine to QEMU to make
>>>>> the development easier because I'm currently waiting an hour or more
>>>>> for a kernel to load over serial. It might be a few months.
>>>
>>> I've been booting Linux on qemu-system-m68k -M q800 for a couple years
>>> now? (The CROSS=m68k target of mkroot in toybox?)
>>>
>>> # cat /proc/cpuinfo
>>> CPU:		68040
>>> MMU:		68040
>>> FPU:		68040
>>> Clocking:	1261.9MHz
>>> BogoMips:	841.31
>>> Calibration:	4206592 loops
>>>
>>> It certainly THINKS it's got m68000...
>>>
>> 
>> Most 68040 processor variants have a built-in MMU and the m68k "nommu"
>> Linux port doesn't support them. The nommu port covers processors like
>> 68000, Dragonball etc. whereas the m68k "mmu" port covers 680x0 where x is
>> one of 2,3,4,6 with MMU.

In theory you can switch the MMU off. (Or at least give it a NOP page table that
maps all the physical memory into one big contiguous block 1:1 with the physical
address and leave it there.)

Doesn't mean anybody's bothered to implement and add a config option to stub
that out in the kernel yet. But presumably you could have a bootloader shim do it...

>>> (I'd love to get an m68k nommu system working but never sat down and
>>> worked out a kernel .config qemu agreed to run, plus compiler and libc.
>>> Musl added m68k support but I dunno if that includes coldfire?)
>>>
>> 
>> I could never figure out how to boot a coldfire machine in qemu either.
>> There was no documentation about that back when I attempted it but maybe
>> things have improved since.
> 
> FWIW this will do it:
> 
>      qemu-system-m68k -nographic -machine mcf5208evb -kernel vmlinux
> 
> That will boot an m5208evb_defconfig generated vmlinux.
> But you will need a user space to get a full boot to login/shell.

No FDPIC support. :(

I had a binflt toolchain working with uClibc in 2015 or so, but I end of lifed
https://landley.net/aboriginal in 2017 (five years ago now). Multiple reasons,
but one was the old "last GPLv2 release" toolchain was getting painful to force
the kernel to build with.

These days there's articles on lwn.net about yanking a.out support, which fdpic
is a buggy variant of that didn't actually have a maintained elf2flt repository
when I was assembling my toolchain. (I vaguely recall I poked enough people that
somebody picked it up and stuck a repository on github, but Jeff Dionne
explained some fundamental design flaw that had been introduced having to do
with register offsets being calculated in the wrong framework or something?

I don't remember, I lost interest because it's _conceptually_ obsolete. FDPIC is
ELF with a little extra header info, it's clean and potentially even useful on
with-MMU systems as extra ASLR. BINFLT is a.out run through a postprocessing
tool that nominally converts ELF files into the new format but actually needs .o
files from earlier in the process and is kind of an alternate linker except it
doesn't replace the linker... It's layers of ugly.

> Regards
> Greg

Rob