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

[Jani Nikula <jani.nikula@linux.intel.com>]

On Thu, 26 Oct 2017, Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> wrote:
> 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.

Any change would have to be piecemeal anyway. I don't have a dislike for
that per se. I'm just saying that adding some registers to some data
structures on a whim leads to an ugly inconsistent end result, and it
gets cargo-culted to more and more places, uncontrolled. The driver will
become harder to maintain. The changes must be done piecemeal, but there
needs to be a plan where we want to take all this in the long term.

And that plan is going to be an awful bikeshed fest.

> 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.

Even with cached register offsets you'll anyway be doing 5k fetches of
the cached offsets per second. Sure you'll save a branch and couple of
immediates in code, but I can't imagine it being a huge penalty.

> 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?

This is not a non-issue, but, to be quite honest, I'd rather see people
go to bugzilla and fix a dozen actual issues that are hitting CI or end
users out there, today.


BR,
Jani.

-- 
Jani Nikula, Intel Open Source Technology Center
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