diff options
| author | Andreas Mohr <[andi@rhlx01.fht-esslingen.de]> | 2006-06-26 15:58:00 -0400 |
|---|---|---|
| committer | Len Brown <len.brown@intel.com> | 2006-06-28 03:27:02 -0400 |
| commit | b488f02156d3deb08f5ad7816d565c370a8cc6f1 (patch) | |
| tree | 7ba4c86b66451780a39c28b618dcfa8037584cb2 /drivers/acpi/processor_idle.c | |
| parent | 95b38b3f453c16de0f8cddcde3e71050bbfb37b9 (diff) | |
ACPI: restore comment justifying 'extra' P_LVLx access
While trying to look for superfluous I/O accesses that can be optimized
away, I stumbled upon this ACPI sleep I/O access and couldn't figure out
why the hell this dummy op was necessary.
After more than one hour of internet research, I had collected a sufficient
number of documents (among those very old kernel versions) that finally
told me what this dummy read was about: STPCLK# doesn't get asserted in time
on (some) chipsets, which is why we need to have a dummy I/O read to delay
further instruction processing until the CPU is fully stopped.
Signed-off-by: Andreas Mohr <andi@lisas.de>
Signed-off-by: Len Brown <len.brown@intel.com>
Diffstat (limited to 'drivers/acpi/processor_idle.c')
| -rw-r--r-- | drivers/acpi/processor_idle.c | 6 |
1 files changed, 4 insertions, 2 deletions
diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c index 89d3fd4c3cd..5551bfbc47a 100644 --- a/drivers/acpi/processor_idle.c +++ b/drivers/acpi/processor_idle.c @@ -365,7 +365,9 @@ static void acpi_processor_idle(void) t1 = inl(acpi_fadt.xpm_tmr_blk.address); /* Invoke C2 */ inb(cx->address); - /* Dummy op - must do something useless after P_LVL2 read */ + /* Dummy wait op - must do something useless after P_LVL2 read + because chipsets cannot guarantee that STPCLK# signal + gets asserted in time to freeze execution properly. */ t2 = inl(acpi_fadt.xpm_tmr_blk.address); /* Get end time (ticks) */ t2 = inl(acpi_fadt.xpm_tmr_blk.address); @@ -403,7 +405,7 @@ static void acpi_processor_idle(void) t1 = inl(acpi_fadt.xpm_tmr_blk.address); /* Invoke C3 */ inb(cx->address); - /* Dummy op - must do something useless after P_LVL3 read */ + /* Dummy wait op (see above) */ t2 = inl(acpi_fadt.xpm_tmr_blk.address); /* Get end time (ticks) */ t2 = inl(acpi_fadt.xpm_tmr_blk.address); |