x86: Hpet: Avoid the comparator readback penalty

x86: Hpet: Avoid the comparator readback penalty

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: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nix <nix@esperi.org.uk>
Tested-by: Artur Skawina <art.08.09@gmail.com>
Cc: Damien Wyart <damien.wyart@free.fr>
Tested-by: John 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: Borislav Petkov <borislav.petkov@amd.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
LKML-Reference: <alpine.LFD.2.00.1009151500060.2416@localhost6.localdomain6>