• Huacai Chen's avatar
    MIPS: hpet: Choose a safe value for the ETIME check · e76f1004
    Huacai Chen authored
    commit 5610b125 upstream.
    
    This patch is borrowed from x86 hpet driver and explaind below:
    
    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 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 AMD chipsets.
    
    We can work around this by reading back the compare register to make
    sure that the written value has hit the hardware. But that is 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 64 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 avatarHuacai Chen <chenhc@lemote.com>
    Cc: Aurelien Jarno <aurelien@aurel32.net>
    Cc: Steven J. Hill <Steven.Hill@imgtec.com>
    Cc: Fuxin Zhang <zhangfx@lemote.com>
    Cc: Zhangjin Wu <wuzhangjin@gmail.com>
    Cc: Huacai Chen <chenhc@lemote.com>
    Cc: linux-mips@linux-mips.org
    Patchwork: https://patchwork.linux-mips.org/patch/12162/Signed-off-by: default avatarRalf Baechle <ralf@linux-mips.org>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    e76f1004
hpet.c 6.19 KB