kernel since 5.6 do not boot anymore on Apple PowerBook

[Giuseppe Sacco]

Il giorno gio, 27/08/2020 alle 00.28 +0200, Giuseppe Sacco ha scritto:
> Hello Christophe,
> 
> Il giorno mer, 26/08/2020 alle 15.53 +0200, Christophe Leroy ha
> scritto:
> [...]
> > If there is no warning, then the issue is something else, bad luck.
> > 
> > Could you increase the loglevel and try again both with and without 
> > VMAP_STACK ? Maybe we'll get more information on where it stops.
> 
> The problem is related to the CPU frequency changes. This is where the
> system stop: cpufreq get the CPU frequency limits and then start the
> default governor (performance) and then calls function
> cpufreq_gov_performance_limits() that never returns.
> 
> Rebuilding after enabling pr_debug for cpufreq.c, I've got some more
> lines of output:
> 
> cpufreq: setting new policy for CPU 0: 667000 - 867000 kHz
> cpufreq: new min and max freqs are 667000 - 867000 kHz
> cpufreq: governor switch
> cpufreq: cpufreq_init_governor: for CPU 0
> cpufreq: cpufreq_start_governor: for CPU 0
> cpufreq: target for CPU 0: 867000 kHz, relation 1, requested 867000 kHz
> cpufreq: __target_index: cpu: 0, oldfreq: 667000, new freq: 867000
> cpufreq: notification 0 of frequency transition to 867000 kHz
> cpufreq: saving 133328 as reference value for loops_per_jiffy; freq is 667000 kHz
> 
> no more lines are printed. I think this output only refers to the
> notification sent prior to the frequency change.
> 
> I was thinking that selecting a governor that would run at 667mHz would
> probably skip the problem. I added cpufreq.default_governor=powersave
> to the command line parameters but it did not work: the selected
> governor was still performance and the system crashed.

I kept following the thread and found that CPU frequency is changed in
function pmu_set_cpu_speed() in file drivers/cpufreq/pmac32-cpufreq.c.
As first thing, the function calls the macro preempt_disable() and this
is where it stops.

Bye,
Giuseppe