OvmfPkg/XenBusDxe/TestAndClearBit.c

   1 /** @file
   2   Implementation of TestAndClearBit using compare-exchange primitive
   3
   4   Copyright (C) 2015, Linaro Ltd.
   5   Copyright (c) 2015, Intel Corporation. All rights reserved.<BR>
   6
   7   This program and the accompanying materials
   8   are licensed and made available under the terms and conditions of the BSD License
   9   which accompanies this distribution.  The full text of the license may be found at
  10   http://opensource.org/licenses/bsd-license.php
  11
  12   THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
  13   WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
  14
  15 **/
  16
  17 #include <Base.h>
  18 #include <Library/SynchronizationLib.h>
  19
  20 INT32
  21 EFIAPI
  22 TestAndClearBit (
  23   IN INT32            Bit,
  24   IN VOID             *Address
  25   )
  26 {
  27   UINT16    Word, Read;
  28   UINT16    Mask;
  29
  30   //
  31   // Calculate the effective address relative to 'Address' based on the
  32   // higher order bits of 'Bit'. Use signed shift instead of division to
  33   // ensure we round towards -Inf, and end up with a positive shift in
  34   // 'Bit', even if 'Bit' itself is negative.
  35   //
  36   Address = (VOID*)((UINT8*) Address + ((Bit >> 4) * sizeof(UINT16)));
  37   Mask = 1U << (Bit & 15);
  38
  39   for (Word = *(UINT16 *) Address; Word & Mask; Word = Read) {
  40     Read = InterlockedCompareExchange16 (Address, Word, Word & ~Mask);
  41     if (Read == Word) {
  42       return 1;
  43     }
  44   }
  45   return 0;
  46 }