linux-kernel - Re: [PATCH] x86/PCI: Convert force_disable

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Date:   Wed, 25 Nov 2020 13:46:23 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Bjorn Helgaas <helgaas@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
        Feng Tang <feng.tang@...el.com>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>
Subject: Re: [PATCH] x86/PCI: Convert force_disable_hpet() to standard quirk

On Thu, Nov 19 2020 at 12:19, Bjorn Helgaas wrote:
> 62187910b0fc ("x86/intel: Add quirk to disable HPET for the Baytrail
> platform") implemented force_disable_hpet() as a special early quirk.
> These run before the PCI core is initialized and depend on the
> x86/pci/early.c accessors that use I/O ports 0xcf8 and 0xcfc.
>
> But force_disable_hpet() doesn't need to be one of these special early
> quirks.  It merely sets "boot_hpet_disable", which is tested by
> is_hpet_capable(), which is only used by hpet_enable() and hpet_disable().
> hpet_enable() is an fs_initcall(), so it runs after the PCI core is
> initialized.

hpet_enable() is not an fs_initcall(). hpet_late_init() is and that
invokes hpet_enable() only for the case that ACPI did not advertise it
and the force_hpet quirk provided a base address.

But hpet_enable() is also invoked via:

 start_kernel()
   late_time_init()
     x86_late_time_init()
       hpet_time_init()

which is way before the PCI core is available and we really don't want
to set it up there if it's known to be broken :)

Now the more interesting question is why this needs to be a PCI quirk in
the first place. Can't we just disable the HPET based on family/model
quirks?

e0748539e3d5 ("x86/intel: Disable HPET on Intel Ice Lake platforms")
f8edbde885bb ("x86/intel: Disable HPET on Intel Coffee Lake H platforms")
fc5db58539b4 ("x86/quirks: Disable HPET on Intel Coffe Lake platforms")
62187910b0fc ("x86/intel: Add quirk to disable HPET for the Baytrail platform")

I might be missing something here, but in general on anything modern
HPET is mostly useless.

Thanks,

        tglx