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Commit b488f021 authored by Andreas Mohr's avatar Andreas Mohr Committed by Len Brown
Browse files

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: default avatarAndreas Mohr <andi@lisas.de>
Signed-off-by: default avatarLen Brown <len.brown@intel.com>
parent 95b38b3f

drivers/acpi/processor_idle.c

+4 −2
Original line number Original line Diff line number Diff line
@@ -365,7 +365,9 @@ static void acpi_processor_idle(void) 
		t1 = inl(acpi_fadt.xpm_tmr_blk.address);

		t1 = inl(acpi_fadt.xpm_tmr_blk.address);

		/* Invoke C2 */

		/* Invoke C2 */

		inb(cx->address);

		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);

		t2 = inl(acpi_fadt.xpm_tmr_blk.address);

		/* Get end time (ticks) */

		/* Get end time (ticks) */

		t2 = inl(acpi_fadt.xpm_tmr_blk.address);

		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);

		t1 = inl(acpi_fadt.xpm_tmr_blk.address);

		/* Invoke C3 */

		/* Invoke C3 */

		inb(cx->address);

		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);

		t2 = inl(acpi_fadt.xpm_tmr_blk.address);

		/* Get end time (ticks) */

		/* Get end time (ticks) */

		t2 = inl(acpi_fadt.xpm_tmr_blk.address);

		t2 = inl(acpi_fadt.xpm_tmr_blk.address);