[Xen-devel] [PATCH] cpuidle: remove BM_RLD access from idle entry path

["Wei, Gang" <gang.wei@xxxxxxxxx>]

Port below linux patch into xen to reduce overhead from idle entry path.

commit 31878dd86b7df9a147f5e6cc6e07092b4308782b
Author: Len Brown <len.brown@xxxxxxxxx>
Date:   Wed Jan 28 18:28:09 2009 -0500

    ACPI: remove BM_RLD access from idle entry path

    It is true that BM_RLD needs to be set to enable
    bus master activity to wake an older chipset (eg PIIX4) from C3.

    This is contrary to the erroneous wording the ACPI 2.0, 3.0
    specifications that suggests that BM_RLD is an indicator
    rather than a control bit.

    ACPI 1.0's correct wording should be restored in ACPI 4.0:
    http://www.acpica.org/bugzilla/show_bug.cgi?id=689

    But the kernel should not have to clear BM_RLD
    when entering a non C3-type state just to set
    it again when entering a C3-type C-state.

    We should be able to set BM_RLD at boot time
    and leave it alone -- removing the overhead of
    accessing this IO register from the idle entry path.

    Signed-off-by: Len Brown <len.brown@xxxxxxxxx>

Signed-off-by: Wei Gang <gang.wei@xxxxxxxxx>

diff -r d035b66b5b4d xen/arch/x86/acpi/cpu_idle.c
--- a/xen/arch/x86/acpi/cpu_idle.c      Mon Mar 09 15:01:34 2009 +0000
+++ b/xen/arch/x86/acpi/cpu_idle.c      Tue Mar 10 22:29:01 2009 +0800
@@ -167,25 +167,6 @@ static void acpi_idle_do_entry(struct ac
     }
 }
 
-static inline void acpi_idle_update_bm_rld(struct acpi_processor_power *power,
-                                           struct acpi_processor_cx *target)
-{
-    if ( !power->flags.bm_check )
-        return;
-
-    if ( power->flags.bm_rld_set && target->type != ACPI_STATE_C3 )
-    {
-        acpi_set_register(ACPI_BITREG_BUS_MASTER_RLD, 0);
-        power->flags.bm_rld_set = 0;
-    }
-
-    if ( !power->flags.bm_rld_set && target->type == ACPI_STATE_C3 )
-    {
-        acpi_set_register(ACPI_BITREG_BUS_MASTER_RLD, 1);
-        power->flags.bm_rld_set = 1;
-    }
-}
-
 static int acpi_idle_bm_check(void)
 {
     u32 bm_status = 0;
@@ -258,8 +239,6 @@ static void acpi_processor_idle(void)
      * ------
      * Invoke the current Cx state to put the processor to sleep.
      */
-    acpi_idle_update_bm_rld(power, cx);
-
     switch ( cx->type )
     {
     case ACPI_STATE_C1:
@@ -579,6 +558,15 @@ static int check_cx(struct acpi_processo
                         "C3 support without BM control\n"));
                 }
             }
+            /*
+             * On older chipsets, BM_RLD needs to be set
+             * in order for Bus Master activity to wake the
+             * system from C3.  Newer chipsets handle DMA
+             * during C3 automatically and BM_RLD is a NOP.
+             * In either case, the proper way to
+             * handle BM_RLD is to set it and leave it set.
+             */
+            acpi_set_register(ACPI_BITREG_BUS_MASTER_RLD, 1);
         }
         else
         {

Attachment: BM_RLD_always_on.patch
Description: BM_RLD_always_on.patch

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