ACPICA: Clear PM register write-only bits on reading

Affects PM1 Control register only. When reading the register, zero
the write-only bits as per the ACPI spec.  ACPICA BZ 443. Lin Ming.

http://www.acpica.org/bugzilla/show_bug.cgi?id=443

Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/acpica/aclocal.h

@@ -781,6 +781,10 @@ struct acpi_bit_register_info {
  */
 #define ACPI_PM1_STATUS_PRESERVED_BITS          0x0800	/* Bit 11 */
 
+/* Write-only bits must be zeroed by software */
+
+#define ACPI_PM1_CONTROL_WRITEONLY_BITS         0x2004	/* Bits 13, 2 */
+
 /* For control registers, both ignored and reserved bits must be preserved */
 
 #define ACPI_PM1_CONTROL_IGNORED_BITS           0x0201	/* Bits 9, 0(SCI_EN) */

drivers/acpi/acpica/hwregs.c

@@ -207,6 +207,13 @@ acpi_hw_register_read(u32 register_id, u32 * return_value)
 					       xpm1a_control_block,
 					       &acpi_gbl_FADT.
 					       xpm1b_control_block);
+
+		/*
+		 * Zero the write-only bits. From the ACPI specification, "Hardware
+		 * Write-Only Bits": "Upon reads to registers with write-only bits,
+		 * software masks out all write-only bits."
+		 */
+		value &= ~ACPI_PM1_CONTROL_WRITEONLY_BITS;
 		break;
 
 	case ACPI_REGISTER_PM2_CONTROL:	/* 8-bit access */