svn commit: r306065 - in head/sys vs. PowerMacs: Nathan's trail patch included but inappropriate?

[Mark Millard]

On 2016-Sep-21, at 11:15 AM, Jukka A. Ukkonen <jau789 at gmail.com> wrote:

> On 09/21/16 20:28, Justin Hibbits wrote:
>> On Sep 21, 2016, at 1:46 AM, Mark Millard wrote:
>> 
>>> The following from
>>> 
>>> https://lists.freebsd.org/pipermail/svn-src-head/2016-September/091934.html
>>> 
>>> 
>>> seems in include a patch that Nathan made for testing on
>>> PowerMac/iMac/Xserve G5's that failed the PowerMac7,3 test that was
>>> tried by Jukka A. Ukkonen.
>>> 
>>> . . .
>> 
>> Crap, I got extra stuff in my diff.  Going to revert this part tonight,
>> until everything is known good.
>> 
>> - Justin
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> 
> 
> The least that will have to be changed to make the code boot on
> PowerMac G5 is to change the following ...
> 
>        __asm __volatile("mfsprg0 %0\n\t"
>                         "mtsprg0 %1\n\t"
>                         "mtsprg1 %2\n\t"
>                         "mtsprg2 %3\n\t"
>                         "mtsprg3 %4\n\t"
>                         : "=&r"(ofw_sprg0_save)
>                         : "r"(ofmsr[1]),
>                         "r"(ofmsr[2]),
>                         "r"(ofmsr[3]),
>                         "r"(ofmsr[4]));
> 
> in function ofw_sprg_prepare() in sys/powerpc/ofw/ofw_machdep.c
> to this ...
> 
>        __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]));
> 
> This only removes "mtsprg0" and its corresponding parameter.
> 
> Alternatively one could use the absolutely minimalistic ...
> 
>        __asm __volatile("mfsprg0 %0\n\t"
>                         : "=&r"(ofw_sprg0_save)
>                         :
>                         );
> 
> on a G5. I have been running exactly this since my testing sessions
> with no ill effects at all. The downside is that nobody has tested
> this minimalistic approach on any other ppc variant but my G5.
> I have had a pending plan to test the same approach on my 32-bit G4,
> but the task being somewhat time consuming it is still pending. ;-)
> 
> Anyhow the old code will not boot on a G5 without a change, neither
> will it boot with Nathan's patch included. Releasing 11.0-stable in
> a state that will not actually boot on one of the supported platforms
> will not look awfully bright either. I guess this is an issue for
> the re team to decide.
> 
> --jau

Nathan Whitehorn at one point wrote:

> I believe that OF's SPRG0 is maintained only for compatibility with some G4
> Apple hardware, the eMac in particular, but will check . . .


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.

Your testing has shown that the PowerMac7,3 G5 you have has some SPRG0 value save/restore activity that is required: the activity associated with ofw_sprg0_save but not the other SPRG0 activity. My prior G5 activity and your recent testing both showed that the "other" SPRG0 activity associated with ofmsr[1] actually causes observable problems for at least a range of PowerMac G5 contexts. Your testing also showed that the PowerMac7,3 G5 did not require SPRG1, SPRG2, or SPRG3 to be saved and restored.

My powerpc/powerpc64 examples have only been:

PowerMac11,3 G5      ["Quad Core": 2 dual-core processors] (2 of these)
PowerMac7,2  G5      [dual-processor, each single core]
PowerMac3,6  G4      FW800 [dual-processor, each single core) (2 of these)
PowerMac3,5  G4      Dual Drive Door [dual-processor, each single core]
PowerMac3,4  G4      [single-processor, single core] (multiple speeds)
PowerMac4,1  iMac G3 [single-processor, single core]

I only put my mtsprg0 removal hack (so: leaving the FreeBSD SPRG0 value in place) in my G5 builds. I've never tested any G4's or the G3 with such changes that I remember: I left the working context alone. No amount of testing by me would cover contexts I never had access to.

I've never had access to a non-Apple example of a powerpc/powerpc64 context or Open Firmware context. So I have no evidence of what that brings to the compatibility issues for the powerpc or powerpc64 Open Firmware related code in the FreeBSD kernel versions.

I'm not sure of the current status but for a long time the G4-capable FreeBSD builds also worked in the G5's (but ignored "extra" memory). No hack/change needed.

FreeBSD stopped booting the iMac G3 a long time ago (2015-Mar or before, at least for 11.0). Last I tried and recorded the result it got:

> [Thread pid 0 tid 100037]
> Stopped at pmap_activate+0x7c lwz r11,r1,0x0

Later attempts were similar as I remember. But I've never tried hard to track down why it was failing or what changed that made the difference.

One PowerMac3,4 G4 FreeBSD has never managed to boot for any FreeBSD version that I tried. It was a 733 MHz model. (Mac OS X 10.4 and lubuntu 14.04 had no such problem in the same system --and they operate fine after booting as well.) But I've never tried hard to track down why or where it was failing.

So that is the scope of my relevant testing.


[I have various experiments to do with the above systems once I have access again.]

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