OvmfPkg/XenBusDxe/TestAndClearBit.c

   1 /** @file
   2   Implementation of TestAndClearBit using compare-exchange primitive
   3
   4   Copyright (C) 2015, Linaro Ltd.
   5
   6   This program and the accompanying materials
   7   are licensed and made available under the terms and conditions of the BSD License
   8   which accompanies this distribution.  The full text of the license may be found at
   9   http://opensource.org/licenses/bsd-license.php
  10
  11   THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
  12   WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
  13
  14 **/
  15
  16 #include <Base.h>
  17 #include <Library/SynchronizationLib.h>
  18
  19 INT32
  20 EFIAPI
  21 TestAndClearBit (
  22   IN INT32            Bit,
  23   IN VOID             *Address
  24   )
  25 {
  26   UINT16    Word, Read;
  27   UINT16    Mask;
  28
  29   //
  30   // Calculate the effective address relative to 'Address' based on the
  31   // higher order bits of 'Bit'. Use signed shift instead of division to
  32   // ensure we round towards -Inf, and end up with a positive shift in
  33   // 'Bit', even if 'Bit' itself is negative.
  34   //
  35   Address += (Bit >> 4) * sizeof(UINT16);
  36   Mask = 1U << (Bit & 15);
  37
  38   for (Word = *(UINT16 *) Address; Word & Mask; Word = Read) {
  39     Read = InterlockedCompareExchange16 (Address, Word, Word & ~Mask);
  40     if (Read == Word) {
  41       return 1;
  42     }
  43   }
  44   return 0;
  45 }