LWN: Comments on "Gentoo dropping support of SPARC"

Gentoo dropping support of SPARC

nix — Fri, 16 Jun 2017 17:32:45 +0000

I think it was the first, or nearly the first, widely-available UltraSPARC. I wasn't suggesting that it was 32-bit: I was noting that even some UltraSPARC stuff is weird and ancient and has little justification for existence any more (and, by extension, that nobody not running a computer museum will miss the ability to boot 32-bit kernels on their SPARC Classic).

Gentoo dropping support of SPARC

jch — Wed, 14 Jun 2017 22:16:43 +0000

> SPARC32 kernel support is gone, but there is still code in the kernel -- and weird uncommented workarounds in the kernel -- to support things like the Spitfire, released in 1995

Isn't the Spitfire a SPARCv9 (a.k.a. UltraSparc or SPARC64)?

Gentoo dropping support of SPARC

glaubitz — Wed, 07 Jun 2017 13:54:58 +0000

> It's worth mentioning that the oldest UltraSPARC/sun4u machines are over two decades old now.

Exactly. And all of these are fully supported by the sparc64 ports of Debian and OpenBSD. They're not
supported by Oracle's own distributions Linux for SPARC and Oracle Linux though as these require
"sun4v". But Oracle fully supports the development of sparc64.

They are working on: golang, the kernel, GRUB, Ocaml, gcc, binutils and much more.

People interested in Linux on SPARC should join #sparc on Freenode.

Gentoo dropping support of SPARC

pizza — Wed, 07 Jun 2017 13:48:39 +0000

It's worth mentioning that the oldest UltraSPARC/sun4u machines are over two decades old now. (And a modern smartphone is more than an order of magnitude more capable, even before the GPU is factored in..)

Gentoo dropping support of SPARC

glaubitz — Wed, 07 Jun 2017 13:34:21 +0000

> Well sure, if their goal is to exclude using 32bit sparc systems with Gentoo. But perhaps that is not their goal.

sparcv9 *is* a 64-bit architecture. There is currently no usable 32-bit SPARC support in Linux.

What Gentoo (and Debian's "sparc" port used) is using is "sparc64" with 32-bit pointers. Gentoo for
SPARC requires at least an UltraSPARC machine (sun4u) and all of these are 64-bit.

If you boot a current version of Gentoo on a SPARC and run "uname -a" it will say "sparc64", it's
just the userland that is 32-bit. And Oracle is currently investing all resources into sparc64
which is why the best idea would be to fully switch to sparc64 like Debian and OpenBSD already
did.

Gentoo dropping support of SPARC

jond — Wed, 07 Jun 2017 13:17:45 +0000

> What you should do first is switch the default from sparcv9 (32 bits) to sparcv9 (64 bits) which is what we did in Debian.

Well sure, if their goal is to exclude using 32bit sparc systems with Gentoo. But perhaps that is not their goal.

Gentoo dropping support of SPARC

glaubitz — Wed, 07 Jun 2017 08:58:11 +0000

> Is SPARC one of those architectures where you have sparc, sparc64, sparcv5, supersparc, ultrasparc, sparcman or whatever and some of them are really ancient stuff, so you can 'drop' one of them but it doesn't affect any hardware sold in the last twenty years?

sparc64 is fully supported by Oracle:

> http://www.oracle.com/technetwork/server-storage/linux/do...
> https://oss.oracle.com/projects/linux-sparc/

A variant of sparcv8 called LEON is still supported, too, because ESA uses it:

> https://en.wikipedia.org/wiki/LEON

Gentoo dropping support of SPARC

glaubitz — Wed, 07 Jun 2017 08:54:33 +0000

> Ya, in fact we are looking into getting more sparc hardware for testing.

What you should do first is switch the default from sparcv9 (32 bits) to sparcv9 (64 bits) which is what we did in Debian.

> https://lists.debian.org/debian-sparc/2016/06/msg00126.html

Also, Oracle itself is fully investing the 64-bit port themselves. In fact, they offer two Linux distributions for sparc64:

> http://www.oracle.com/technetwork/server-storage/linux/do...
> https://oss.oracle.com/projects/linux-sparc/

But since Oracle's distributions require at least sun4v machines, many people will be better off running Debian sparc64
which also has many more and up-to-date packages.

Adrian

Gentoo dropping support of SPARC

nix — Tue, 06 Jun 2017 20:42:30 +0000

Quite. SPARC32 kernel support is gone, but there is still code in the kernel -- and weird uncommented workarounds in the kernel -- to support things like the Spitfire, released in 1995, an artifact which is actually literally in the Computer History Museum.

The artifact I ran into half an hour ago was code in arch/sparc/kernel/time_64.c:tick_get_tick():

static unsigned long long tick_get_tick(void)
{
        unsigned long ret;

        __asm__ __volatile__("rd        %%tick, %0\n\t"
                             "mov       %0, %0"
                             : "=r" (ret));

        return ret & ~TICK_PRIV_BIT;
}

What's that weird mov for? Well, there was a comment before this code was split up in 2003:

   * Just to be anal we add a workaround for Spitfire
   * Errata 50 by preventing pipeline bypasses on the
   * final read of the %tick register into a compare
   * instruction.  The Errata 50 description states
   * that %tick is not prone to this bug, but I am not
   * taking any chances.

So we have a weird workaround with a long-lost comment in a frequently-called path to handle bugs that, uh, are believed not to affect actual museum artifacts, and certainly don't affect anything more recent (though anything in actual use these days won't use this code path at all because it will have the %stick register).

Gentoo dropping support of SPARC

epa — Tue, 06 Jun 2017 20:35:16 +0000

Is SPARC one of those architectures where you have sparc, sparc64, sparcv5, supersparc, ultrasparc, sparcman or whatever and some of them are really ancient stuff, so you can 'drop' one of them but it doesn't affect any hardware sold in the last twenty years?

Gentoo dropping support of SPARC

prometheanfire — Tue, 06 Jun 2017 20:22:09 +0000

Ya, in fact we are looking into getting more sparc hardware for testing.

Gentoo dropping support of SPARC

mageta — Tue, 06 Jun 2017 17:46:48 +0000

The headline is misleading. The consequences are written in the mail-text, and they don't seem to imply that SPARC is dropped as architecture from Gentoo as a whole.