Re: [PATCH 12/13] drm/i915: Consolidate legacy semaphore initialization

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

On 19/07/16 19:38, Dave Gordon wrote:
> On 15/07/16 14:13, Tvrtko Ursulin wrote:
>>
>> On 29/06/16 17:00, Chris Wilson wrote:
>>> On Wed, Jun 29, 2016 at 04:41:58PM +0100, Tvrtko Ursulin wrote:
>>>>
>>>> On 29/06/16 16:34, Chris Wilson wrote:
>>>>> On Wed, Jun 29, 2016 at 04:09:31PM +0100, Tvrtko Ursulin wrote:
>>>>>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>>>>
>>>>>> Replace per-engine initialization with a common half-programatic,
>>>>>> half-data driven code for ease of maintenance and compactness.
>>>>>>
>>>>>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>>>
>>>>> This is the biggest pill to swallow (since our 5x5 table is only
>>>>> sparsely populated), but it looks correct, and more importantly
>>>>> easier to
>>>>> read.
>>>>
>>>> Yeah I was out of ideas on how to improve it. Fresh mind needed to
>>>> try and spot a pattern in how MI_SEMAPHORE_SYNC_* and GEN6_*SYNC map
>>>> to bits and registers respectively, and write it as a function.
>>>
>>> It's actually a very simple cyclic function based on register
>>> offset = base + (signaler hw_id - waiter hw_id - 1) % num_rings.
>>>
>>> (The only real challenge is picking the direction.)
>>>
>>> commit c8c99b0f0dea1ced5d0e10cdb9143356cc16b484
>>> Author: Ben Widawsky <ben@bwidawsk.net>
>>> Date:   Wed Sep 14 20:32:47 2011 -0700
>>>
>>>      drm/i915: Dumb down the semaphore logic
>>>
>>>      While I think the previous code is correct, it was hard to follow
>>> and
>>>      hard to debug. Since we already have a ring abstraction, might as
>>> well
>>>      use it to handle the semaphore updates and compares.
>>
>> Doesn't seem to fit, or I just can't figure it out. Needs two functions
>> to get rid of the table:
>>
>> f1(0, 1) = 2
>> f1(0, 2) = 0
>> f1(0, 3) = 2
>> f1(1, 0) = 0
>> f1(1, 2) = 2
>> f1(1, 3) = 1
>> f1(2, 0) = 2
>> f1(2, 1) = 0
>> f1(2, 3) = 0
>> f1(3, 0) = 1
>> f1(3, 1) = 1
>> f1(3, 2) = 1
>>
>> and:
>>
>> f2(0, 1) = 1
>> f2(0, 2) = 0
>> f2(0, 3) = 1
>> f2(1, 0) = 0
>> f2(1, 2) = 1
>> f2(1, 3) = 2
>> f2(2, 0) = 1
>> f2(2, 1) = 0
>> f2(2, 3) = 0
>> f2(3, 0) = 2
>> f2(3, 1) = 2
>> f2(3, 2) = 2
>>
>> A weekend math puzzle for someone? :)
>>
>> Regards,
>> Tvrtko
>
> Here's the APL expression for (the transpose of) f2, with -1's filled in
> along the leading diagonal (you need ⎕io←0 so the ⍳-vectors are in
> origin 0)
>
>        {¯1+(⍵≠⍳4)⍀(2|⍵)⌽(⌽⍣(1=⍵))1+⍳3}¨⍳4
>
> ┌────────┬────────┬────────┬────────┐
> │¯1 0 1 2│1 ¯1 0 2│0 1 ¯1 2│1 2 0 ¯1│
> └────────┴────────┴────────┴────────┘
>
> or transposed back so that the first argument is the row index and the
> second is the column index:
>
>        ⍉↑{¯1+(⍵≠⍳4)⍀(2|⍵)⌽(⌽⍣(1=⍵))1+⍳3}¨⍳4
>
> ¯1  1  0  1
>   0 ¯1  1  2
>   1  0 ¯1  0
>   2  2  2 ¯1
>
> http://tryapl.org/?a=%u2349%u2191%7B%AF1+%28%u2375%u2260%u23734%29%u2340%282%7C%u2375%29%u233D%28%u233D%u2363%281%3D%u2375%29%291+%u23733%7D%A8%u23734&run

  :-C ! How to convert that to C ? :)

> f1 is trivially derived from this by the observation that f1 is just f2
> with the 1's and 2's interchanged.

Ah yes, nicely spotted.

Regards,

Tvrtko


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