[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] RE: [Xen-devel] [PATCH 2/3] Fix acpi_set_register
acpi_set_register() has already done a read before switch-case block, this read with in switch-case is a unnecessary repeat. The non-continuous PIO/MMIO access will spent far longer time than imagination because of the chipset link ASPM features. One single PIO/MMIO read may spent up to tens of thousands cpu cycles, although the fastest read may only cause less than one thousand cycles. I tried to measure the cost via TSC. Jimmy On Wednesday, September 03, 2008 4:43 PM, Keir Fraser wrote: > acpi_set_register() is doing a read-modify-write of PM2_CONTROL. It's only > safe to skip the read if the modification is going to obliterate all old > bits. Is the read really that expensive (and equally, in patch 3, is the > write of ARB_DIS really that expensive)? > > -- Keir > > On 3/9/08 02:52, "Wei, Gang" <gang.wei@xxxxxxxxx> wrote: > >> ACPI: Remove a redundant call to acpi_hw_register_read(). >> >> Signed-off-by: Wei Gang <gang.wei@xxxxxxxxx> >> >> diff -r fc0b0c64246d xen/drivers/acpi/hwregs.c >> --- a/xen/drivers/acpi/hwregs.c Thu Aug 21 16:15:30 2008 +0800 >> +++ b/xen/drivers/acpi/hwregs.c Mon Aug 25 15:24:32 2008 +0800 >> @@ -238,12 +238,6 @@ acpi_status acpi_set_register(u32 regist >> break; >> >> case ACPI_REGISTER_PM2_CONTROL: >> - >> - status = acpi_hw_register_read(ACPI_REGISTER_PM2_CONTROL, >> - ®ister_value); >> - if (ACPI_FAILURE(status)) { >> - goto unlock_and_exit; >> - } >> >> ACPI_DEBUG_PRINT((ACPI_DB_IO, >> "PM2 control: Read %X from %8.8X%8.8X\n", >> _______________________________________________ >> Xen-devel mailing list >> Xen-devel@xxxxxxxxxxxxxxxxxxx >> http://lists.xensource.com/xen-devel _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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