Issues with the first PowerPC updates for the kernel 6.1

[Christian Zigotzky]

On 29 October 2022 at 5:33 pm, Segher Boessenkool wrote:
> On Mon, Oct 17, 2022 at 09:53:04AM +0200, Christian Zigotzky wrote:
>>> On 17. Oct 2022, at 02:43, Michael Ellerman <mpe at ellerman.id.au> wrote:
>>> Previously BIG_ENDIAN && GENERIC_CPU would use -mcpu=power5, now it uses
>>> -mcpu=power4.
>> Maybe this is the issue. We will wait and not release the RC1 for testing because it is a risk for our testers to test these new kernels because of this issue.
>>
>> It is really important do not to rewrite code, that is well worked before.
>> Bugfixing and adding some new features is ok but rewriting of good code is expensive and doesn’t make any sense.
> It was just a bugfix, and a (partial) revert.
>
> 471d7ff8b51b says it removed ISA 2.00 support (original power4, "GP").
> Support for ISA 2.01 was retained it says.  That is power4+, "GQ", but
> also 970 (Apple G5).  That patch actually switched to ISA 2.02 though,
> unintendedly, and code generated for ISA 2.02 will not run on systems
> like 970, in principle.  It is just one uncommon instruction that is
> problematical, namely popcntb, because the kernel does not use floating
> point at all, so that is why we got away with it for so long (most code
> that does use fp will fall flat on its face in no time).  It still is a
> bug fix though!
>
> PA6T is ISA 2.04, it's not clear how this (bugfix, and revert!) change
> made code not run on PA6T anymore.  Smells a lot like something indirect
> (or triply indirect), a separate bug, something that was introduced in
> the last two years maybe, but I'll even bet it is something *exposed* in
> that time, a bug that has been here for longer!
>
>
> Segher
Unfortunately my FSL P5040 system is also affected.

-- Christian