svn commit: r306065 - in head/sys vs. PowerMacs: Nathan's trail patch included but inappropriate? [My hack vs. Apple G4's.]

[Mark Millard]

On 2016-Sep-21, at 3:01 PM, Peter Grehan <grehan at freebsd.org> wrote:

>> I've never had my hands on an eMac. I'm not sure if it has the same
>> SPRGx requirements as other G4's or not.
> 
> That's the machine that caused the sprg save/restore.
>     https://svnweb.freebsd.org/base?view=revision&revision=151891
> 
> later,
> 
> Peter.

My patch/hack as I have always had it for booting PowerMac G5's does not
change the content for G4's (such as eMac G4's). I made a separate case
for __powerpc64__ --and only for AIM-capable as well. My code has been:

# svnlite diff /usr/src/sys/powerpc/ofw/ofw_machdep.c 
Index: /usr/src/sys/powerpc/ofw/ofw_machdep.c
===================================================================
--- /usr/src/sys/powerpc/ofw/ofw_machdep.c      (revision 308247)
+++ /usr/src/sys/powerpc/ofw/ofw_machdep.c      (working copy)
@@ -111,6 +111,24 @@
         * Assume that interrupt are disabled at this point, or
         * SPRG1-3 could be trashed
         */
+#if defined(AIM) && defined(__powerpc64__)
+/* HACK: PowerMac G5 specific code to avoid demonstrated hangs in
+ *       the early boot time frame: avoid mtsprg0 use.
+ *       This would need a live test for PowerMac vs. not in order
+ *       to remove HACK status --but without calling into
+ *       OpenFirmware or the problem would be recreated.
+ */
+       if (1)
+               __asm __volatile("mfsprg0 %0\n\t"
+                                "mtsprg1 %1\n\t"
+                                "mtsprg2 %2\n\t"
+                                "mtsprg3 %3\n\t"
+                                : "=&r"(ofw_sprg0_save)
+                                : "r"(ofmsr[2]),
+                                "r"(ofmsr[3]),
+                                "r"(ofmsr[4]));
+       else
+#endif
        __asm __volatile("mfsprg0 %0\n\t"
                         "mtsprg0 %1\n\t"
                         "mtsprg1 %2\n\t"

(The comment does not reflect the modern discussion of why the
change works: the code has been around a long time.)

I do not know if there are any AIM-capable __powerpc64__
contexts for which the change is inappropriate. It would be
nice if it was always appropriate to that kind of context
and could be added to head and stable/11 and stable/10 .
(I've no experience before stable/10: so untested prior to
that.)

Jukka did experiments with eliminating more --and not just for
AIM-capable __powerpc64__ but some other PowerMac contexts as
well.

But I kept to the minimal generated-code change which appeared
to fix the problematical behavior.

===
Mark Millard
markmi at dsl-only.net