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ACPI: restore comment justifying 'extra' P_LVLx access

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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>
This commit is contained in:
Andreas Mohr 2006-06-26 15:58:00 -04:00 committed by Len Brown
parent 95b38b3f45
commit b488f02156
1 changed files with 4 additions and 2 deletions
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6
drivers/acpi/processor_idle.c
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@ -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);