• Thomas Gleixner's avatar
    x86/timer: Skip PIT initialization on modern chipsets · c8c40767
    Thomas Gleixner authored
    Recent Intel chipsets including Skylake and ApolloLake have a special
    ITSSPRC register which allows the 8254 PIT to be gated.  When gated, the
    8254 registers can still be programmed as normal, but there are no IRQ0
    timer interrupts.
    
    Some products such as the Connex L1430 and exone go Rugged E11 use this
    register to ship with the PIT gated by default. This causes Linux to fail
    to boot:
    
      Kernel panic - not syncing: IO-APIC + timer doesn't work! Boot with
      apic=debug and send a report.
    
    The panic happens before the framebuffer is initialized, so to the user, it
    appears as an early boot hang on a black screen.
    
    Affected products typically have a BIOS option that can be used to enable
    the 8254 and make Linux work (Chipset -> South Cluster Configuration ->
    Miscellaneous Configuration -> 8254 Clock Gating), however it would be best
    to make Linux support the no-8254 case.
    
    Modern sytems allow to discover the TSC and local APIC timer frequencies,
    so the calibration against the PIT is not required. These systems have
    always running timers and the local APIC timer works also in deep power
    states.
    
    So the setup of the PIT including the IO-APIC timer interrupt delivery
    checks are a pointless exercise.
    
    Skip the PIT setup and the IO-APIC timer interrupt checks on these systems,
    which avoids the panic caused by non ticking PITs and also speeds up the
    boot process.
    
    Thanks to Daniel for providing the changelog, initial analysis of the
    problem and testing against a variety of machines.
    Reported-by: default avatarDaniel Drake <drake@endlessm.com>
    Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Tested-by: default avatarDaniel Drake <drake@endlessm.com>
    Cc: bp@alien8.de
    Cc: hpa@zytor.com
    Cc: linux@endlessm.com
    Cc: rafael.j.wysocki@intel.com
    Cc: hdegoede@redhat.com
    Link: https://lkml.kernel.org/r/20190628072307.24678-1-drake@endlessm.com
    c8c40767
apic.h 13.5 KB