linux-kernel - Re: No 8254 PIT & no HPET on new Intel N3350 platforms causes kernel panic during early boot

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Date:   Tue, 16 Apr 2019 13:21:22 +0800
From:   Daniel Drake <drake@...lessm.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Linux Kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Hans de Goede <hdegoede@...hat.com>,
        david.e.box@...ux.intel.com,
        Endless Linux Upstreaming Team <linux@...lessm.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>, x86@...nel.org,
        Len Brown <lenb@...nel.org>
Subject: Re: No 8254 PIT & no HPET on new Intel N3350 platforms causes kernel
 panic during early boot

On Wed, Apr 10, 2019 at 8:54 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> On Tue, 9 Apr 2019, Daniel Drake wrote:
> > On Wed, Apr 3, 2019 at 7:21 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> > >          - Prevent the TSC calibration code from touching PIT/HPET. It
> > >            should do that already when the TSC frequency can be retrieved
> > >            via CPUID or MSR. Should work, emphasis on should ...
> >
> > >From above, this seems to be working acceptably already. It does touch
> > the PIT, but ultimately ignores the information that it provided.
>
> Yes, but we might actually be smarter than that.

Do you have anything specific in mind?

You originally laid out this idea in the context of doing this if the
PIT/HPET is not working. However, I can't immediately see how to judge
that because:
- According to the analysis in my last mail, the PIT is actually
ticking even when it is gated in the BIOS. The BIOS setting just seems
to make it tick 4 times slower and not generate any IRQ0 interrupts.
- TSC calibration code runs really early during boot. To make it
detect this situation we could make it check if IRQ0 is working,
however setup_default_timer_irq() only happens a lot later, so I'm not
sure how this could be checked at such an early stage.

I think I'm now fairly clear on the other suggestions you have made so
I'll see if I can come up with some patches.

Thanks!
Daniel