[Mesa-dev] [PATCH 01/18] i965: Query whether we have kernel support for the TIMESTAMP register once

[Martin Peres]

On 06/07/15 19:12, Chris Wilson wrote:
> On Mon, Jul 06, 2015 at 04:19:36PM +0300, Martin Peres wrote:
>>
>> On 06/07/15 16:15, Martin Peres wrote:
>>> On 06/07/15 16:13, Chris Wilson wrote:
>>>> On Mon, Jul 06, 2015 at 03:10:48PM +0300, Martin Peres wrote:
>>>>> On 06/07/15 13:33, Chris Wilson wrote:
>>>>>> Move the query for the TIMESTAMP register from context init to the
>>>>>> screen, so that it is only queried once for all contexts.
>>>>>>
>>>>>> On 32bit systems, some old kernels trigger a hw bug resulting in the
>>>>>> TIMESTAMP register being shifted and the low bits always zero. Detect
>>>>>> this by repeating the read a few times and check the register is
>>>>>> incrementing.
>>>>> You do not do the latter. You only check for the low bits.
>>>>>
>>>>> I guess the counter is supposed to be monotonically increasing and
>>>>> with a resolution of a few microseconds which would make this
>>>>> perfectly valid. Could you confirm and make sure to add this
>>>>> information in the commit message please?
>>>> The counter should increment every 80ns. What's misleading in what I
>>>> wrote? It describes the hw bug and how to detect it.
>>> Well, it is not misleading, it just lacks this information.
>>>
>>> If it incremented every seconds, the patch would be stupid because
>>> the timestamp could be at 0 and polling 10 times at a few us of
>>> interval would always yield the same result. That's all :)
>> Oh, forgot to say: With this information added in the commit message
>> and the commit message duplicated as a comment in
>> intel_detect_timestamp(), the patch is:
> How about:
>
> On 32bit systems, some old kernels trigger a hw bug resulting in the
> TIMESTAMP register being shifted and the low 32bits always zero. Detect
> this by repeating the read a few times and check the register is
> incrementing every 80ns as expected and not stuck on zero (as would be
> the case with the buggy kernel/hw.).
> -Chris
Perfect!