[Intel-gfx] [PATCH] drm/i915: Restore GT performance in headless mode with DMC loaded

[Tvrtko Ursulin]

On 05/05/2017 15:49, Ville Syrjälä wrote:
> On Fri, May 05, 2017 at 12:43:21PM +0100, Tvrtko Ursulin wrote:
>> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>
>> It seems that the DMC likes to transition between the DC states
>> a lot when there are no connected displays (no active power
>> domains) during simple command submission.
>
> Is it trapping on some interrupt register accesses or what? And
> if so which registers are affected?

It looks like GT IIR or something along those lines it but I couldn't 
say with total confidence. It is just a guess. Firmware binary 
definitely "mentions" those registers as can be seen by inspecting it 
with a hex editor.

The data I collected at least seems to present a correlation between the 
batch frequency and DC state transition frequency:

tgt	DC		irqs	irqs/s	irq	batch/s	DC/s	DC/batch
submit	transitions			/
freq					batch
========================================================================
10000	20000		78300	7830.00	1.96	4000.00	2000.00	0.50
9901	14000		52855	7550.71	1.32	5714.29	2000.00	0.35
9524	13500		49100	7328.36	1.23	5970.15	2014.93	0.34
9091	13500		49200	7235.29	1.23	5882.35	1985.29	0.34
5000	16900		33290	3916.47	0.83	4705.88	1988.24	0.42
3333	27800		69550	4932.62	1.74	2836.88	1971.63	0.70
1667	57200		80200	2655.63	2.01	1324.50	1894.04	1.43
909	80000		80034	1482.11	2.00	740.74	1481.48	2.00
476	87000		80039	820.91	2.00	410.26	892.31	2.18
196	160000		80055	334.40	2.00	167.08	668.34	4.00

Submitted batches were ~100us long in all cases. So with low batch 
frequency it looks pretty believable. For example when we have 167.08 
batches/s, we have 334.40 irq/s - which is double - as expected for 
execlists. And we get again double that in terms of DC transitions per 
second. Each irq is one GT IIR write from the GPU side, and another from 
the CPU side.

This was actually Imre's suggestion btw. Before I was only looking at 
the irq/s to DC/s correlation which was confusing me a lot since there 
are more of the latter, so I thought it can't be the trigger. But once 
Imre mentioned the possibility that things are triggered by IIR register 
writes numbers started making more sense.

Then with higher batch frequencies the ratio starts falling, is it 
because DC transitions are too slow to keep up, or something else I am 
not sure.

Regards,

Tvrtko