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

[Martin Peres]

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 :)