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>

1 files changed, 7 insertions, 0 deletions

diff --git a/drivers/acpi/acpica/hwregs.c b/drivers/acpi/acpica/hwregs.c
index f8ee0a7fd44d..7b2fb602b5cb 100644
--- a/drivers/acpi/acpica/hwregs.c
+++ b/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 */