RE: [Xen-devel] [PATCH 2/3] Fix acpi_set_register

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

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,
>> -                                              &register_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