• Thomas Gleixner's avatar
    x86: Hpet: Avoid the comparator readback penalty · 995bd3bb
    Thomas Gleixner authored
    Due to the overly intelligent design of HPETs, we need to workaround
    the problem that the compare value which we write is already behind
    the actual counter value at the point where the value hits the real
    compare register. This happens for two reasons:
    
    1) We read out the counter, add the delta and write the result to the
       compare register. When a NMI or SMI hits between the read out and
       the write then the counter can be ahead of the event already
    
    2) The write to the compare register is delayed by up to two HPET
       cycles in certain chipsets.
    
    We worked around this by reading back the compare register to make
    sure that the written value has hit the hardware. For certain ICH9+
    chipsets this can require two readouts, as the first one can return
    the previous compare register value. That's bad performance wise for
    the normal case where the event is far enough in the future.
    
    As we already know that the write can be delayed by up to two cycles
    we can avoid the read back of the compare register completely if we
    make the decision whether the delta has elapsed already or not based
    on the following calculation:
    
      cmp = event - actual_count;
    
    If cmp is less than 8 HPET clock cycles, then we decide that the event
    has happened already and return -ETIME. That covers the above #1 and
    #2 problems which would cause a wait for HPET wraparound (~306
    seconds).
    Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Tested-by: default avatarNix <nix@esperi.org.uk>
    Tested-by: default avatarArtur Skawina <art.08.09@gmail.com>
    Cc: Damien Wyart <damien.wyart@free.fr>
    Tested-by: default avatarJohn Drescher <drescherjm@gmail.com>
    Cc: Venkatesh Pallipadi <venki@google.com>
    Cc: Arjan van de Ven <arjan@linux.intel.com>
    Cc: Andreas Herrmann <andreas.herrmann3@amd.com>
    Tested-by: default avatarBorislav Petkov <borislav.petkov@amd.com>
    Cc: Suresh Siddha <suresh.b.siddha@intel.com>
    LKML-Reference: <alpine.LFD.2.00.1009151500060.2416@localhost6.localdomain6>
    995bd3bb
hpet.c 28.6 KB