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

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

Good suggestion. I will try it.

Jimmy

On Wednesday, September 03, 2008 7:44 PM, Keir Fraser wrote:
> Agreed. Linux has the same double-read by the way, as recently as
> 2.6.26-rc4. I hope it's not for a particular reason, and is just a typo. You
> might send your patch upstream to Linux if it is still not fixed there.
>
>  -- Keir
>
> On 3/9/08 11:44, "Wei, Gang" <gang.wei@xxxxxxxxx> wrote:
>
>> 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",
>>>> _______________________________________________
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>>>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>>>> http://lists.xensource.com/xen-devel

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