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

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

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

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