linux-kernel - Re: [PATCH 2/2] drm/i915: Limit the busy wait on requests to 2us not 10ms!

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Message-ID: <5649D5A0.3080907@linux.intel.com>
Date:	Mon, 16 Nov 2015 13:09:52 +0000
From:	Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
To:	Chris Wilson <chris@...is-wilson.co.uk>,
	Jens Axboe <axboe@...nel.dk>, intel-gfx@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Eero Tamminen <eero.t.tamminen@...el.com>,
	"Rantala, Valtteri" <valtteri.rantala@...el.com>,
	stable@...nel.vger.org
Subject: Re: [PATCH 2/2] drm/i915: Limit the busy wait on requests to 2us not
 10ms!


On 16/11/15 12:55, Chris Wilson wrote:
> On Mon, Nov 16, 2015 at 12:08:08PM +0000, Tvrtko Ursulin wrote:
>>
>> On 16/11/15 11:12, Chris Wilson wrote:
>>> On Mon, Nov 16, 2015 at 10:24:45AM +0000, Tvrtko Ursulin wrote:
>>>> On 15/11/15 13:32, Chris Wilson wrote:
>>>>> +static u64 local_clock_us(unsigned *cpu)
>>>>> +{
>>>>> +	u64 t;
>>>>> +
>>>>> +	*cpu = get_cpu();
>>>>> +	t = local_clock() >> 10;
>>>>
>>>> Needs comment I think to explicitly mention the approximation, or
>>>> maybe drop the _us suffix?
>>>
>>> I did consider _approx_us but thought that was overkill. A comment along
>>> the lines of
>>> /* Approximately convert ns to us - the error is less than the
>>>   * truncation!
>>>   */
>>
>> And the result is not used in subsequent calculations apart from
>> comparing against an approximate timeout?
>
> Exactly, the timeout is fairly arbitrary and defined in the same units.
> That we truncate is a much bigger cause for concern in terms of spinning
> accurately for a definite length of time.

Bah sorry that was not supposed to be a question but a suggestion to add 
to the comment. Must had mistyped the question mark. :)

Regards,

Tvrtko
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