Re: [PATCH v2 1/1] drm/i915: Save PM interrupt register offsets in device info

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 25/10/2017 08:45, Jani Nikula wrote:
> On Tue, 24 Oct 2017, Tvrtko Ursulin <tursulin@ursulin.net> wrote:
>> On 24/10/17 18:48, Jani Nikula wrote:
>>> On Tue, 24 Oct 2017, Chris Wilson <chris@chris-wilson.co.uk> wrote:
>>>> Quoting Sagar Arun Kamble (2017-10-24 11:41:13)
>>>>> diff --git a/drivers/gpu/drm/i915/intel_device_info.c b/drivers/gpu/drm/i915/intel_device_info.c
>>>>> index 875d428..d1a4911 100644
>>>>> --- a/drivers/gpu/drm/i915/intel_device_info.c
>>>>> +++ b/drivers/gpu/drm/i915/intel_device_info.c
>>>>> @@ -462,4 +462,15 @@ void intel_device_info_runtime_init(struct drm_i915_private *dev_priv)
>>>>>                            info->sseu.has_subslice_pg ? "y" : "n");
>>>>>           DRM_DEBUG_DRIVER("has EU power gating: %s\n",
>>>>>                            info->sseu.has_eu_pg ? "y" : "n");
>>>>> +
>>>>> +       /* Initialize PM interrupt register offsets */
>>>>> +       if (INTEL_GEN(dev_priv) >= 8) {
>>>>> +               info->pm_iir_offset = GEN8_GT_IIR(2);
>>>>> +               info->pm_imr_offset = GEN8_GT_IMR(2);
>>>>> +               info->pm_ier_offset = GEN8_GT_IER(2);
>>>>> +       } else {
>>>>> +               info->pm_iir_offset = GEN6_PMIIR;
>>>>> +               info->pm_imr_offset = GEN6_PMIMR;
>>>>> +               info->pm_ier_offset = GEN6_PMIER;
>>>>> +       }
>>>>
>>>> If you are going to take another pass at this, move these into the
>>>> static tables in i915_pci.c
>>>>
>>>> Updating GEN6_FEATURES and GEN8_FEATURES will then percolate into
>>>> individual platform defines.
>>>
>>> Like I wrote in reply to v1, I'm not convinced we should do this at all.
>>>
>>> What makes *these* registers so important they must be in device info?
>>> What makes most of i915_reg.h so unimportant they don't deserve the same
>>> treatment? Where do you draw the line?
>>>
>>> I'd draw the line at, no registers at device info.
>>
>> I suggested to Sagar this change during review so feel responsible to
>> chime in.
>>
>> So in general I just find the amount of times our driver asks itself
>> what it's running on a bit tasteless. :(
>>
>> I did quick and dirty check by bumping a counter in all the
>> IS_this|or|that checks, all which can be known at driver probe time, and
>> wired it up to the PMU so I can check their frequency. The annotated
>> perf stat output:
>>
>> root@e31:~# perf stat -a -e i915/whoami/ -I 1000
>> #           time             counts unit events
>>
>> # idle system no X running
>>
>>        1.000298100                 10      i915/whoami/
>>
>>        2.000750955                  8      i915/whoami/
>>
>>        3.001104193                 10      i915/whoami/
>>
>>        4.001333433                 10      i915/whoami/
>>
>>        5.001703162                 10      i915/whoami/
>>
>>        6.002122721                 10      i915/whoami/
>>
>>
>> # starting X now..
>>
>>        7.002266228              2,203      i915/whoami/
>>
>>        8.002392598              4,682      i915/whoami/
>>
>>        9.002764398                  0      i915/whoami/
>>
>>       10.003027119                  0      i915/whoami/
>>
>>       11.003486048                 42      i915/whoami/
>>
>>
>> # X idling..
>>
>>       12.003854660                  0      i915/whoami/
>>
>>       13.004221680                  0      i915/whoami/
>>
>>       14.004622571                  0      i915/whoami/
>>
>>       15.004968110                  0      i915/whoami/
>>
>>       16.005372363                  0      i915/whoami/
>>
>>       17.005778034                  0      i915/whoami/
>>
>>       18.005941970                  0      i915/whoami/
>>
>>       19.006313427                  0      i915/whoami/
>>
>>       20.006676048                  0      i915/whoami/
>>
>>       21.007059927                  0      i915/whoami/
>>
>>       22.007507818                  0      i915/whoami/
>>
>>       23.007887628                  0      i915/whoami/
>>
>>       24.008207035                  0      i915/whoami/
>>
>>       25.008580496                  0      i915/whoami/
>>
>> #           time             counts unit events
>>       26.008949236                  0      i915/whoami/
>>
>>       27.009433473                  0      i915/whoami/
>>
>>
>> # gfxbench trex starting up
>>
>>       28.009677600              2,605      i915/whoami/
>>
>>       29.009941972                716      i915/whoami/
>>
>>       30.010127588              2,190      i915/whoami/
>>
>>       31.010249535                 46      i915/whoami/
>>
>>       32.010383565                 36      i915/whoami/
>>
>>       33.010527674                  0      i915/whoami/
>>
>>
>> # trex running
>>
>>       34.010760584              4,709      i915/whoami/
>>
>>       35.011079891              5,381      i915/whoami/
>>
>>       36.011280234              5,306      i915/whoami/
>>
>>       37.011719986              5,505      i915/whoami/
>>
>>       38.012017531              5,386      i915/whoami/
>>
>>       39.012529241              5,687      i915/whoami/
>>
>>       40.012922986              6,009      i915/whoami/
>>
>>       41.013120143              5,791      i915/whoami/
>>
>>       42.013399982              5,296      i915/whoami/
>>
>>       43.013712979              5,349      i915/whoami/
>>
>>       44.014107375              5,127      i915/whoami/
>>
>>       45.014553950              5,387      i915/whoami/
>>
>>       46.014953020              5,364      i915/whoami/
>>
>>       47.015243748              4,738      i915/whoami/
>>
>>       48.015560460              4,788      i915/whoami/
>>
>>       49.015867395              4,927      i915/whoami/
>>
>>       50.016152690              4,886      i915/whoami/
>>
>>
>> So.. I am not saying these particular registers are mega important, and
>> not even saying that these 5k/s conditionals are measurable (either as
>> branches or increased code size effect), but overall the situation is a
>> bit of.. bleurgh from the elegance point of view. :(
>>
>> If we have register sets which are 100% mutually exclusive, then I see
>> them as candidates to put them in some object at probe time. It doesn't
>> have to be device_info but I don't see why we wouldn't do it. It is just
>> a different flavour of the vfunc approach after all.
> 
> I think to fix something that is inelegant, you have to have a plan to
> actually improve things in the long run. IMO adding a few random
> registers to device info without a plan is less elegant and less
> consistent than the status quo.
> 
> We currently have at least three ways to index pipe/port/transcoder/etc
> based registers. Combine that with storing some register offsets in
> device info, you'll have six ways. There's a chance we'll end up adding
> the register offsets to device info both statically and
> dynamically. We're already struggling with guiding new contributors to
> defining registers in the existing schemes.
> 
> Now, I'm sure we could spend weeks on end devising a plan how to move
> register offsets to device info or another structure, working out the
> details and bikeshedding. After that, we could do weeks and weeks of
> busywork converting registers, causing conflicts in all the work in our
> internal trees and developers' own branches, not to mention making bug
> fix and feature backports more painful.
> 
> I have a pretty strong feeling this is not a good use of our time.

I can only read here a dislike of a big rework (which I did not suggest 
to start with), and dislike of the piecemeal changes. So basically 
preference for a status quo. And there will be more and more of such 
checks. So today it is 5k/sec, in a year it might be more.

So to clarify. Do you actually oppose some subsystem/area moving some 
registers to any data structure, or just to device info?

Do you have a suggestion on what we could do? Or you simply think this 
is a complete non-issue?

Regards,

Tvrtko
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