Re: [PATCH v3] drm/i915: Replace gen6 semaphore signal table with code

[Dave Gordon <david.s.gordon@intel.com>]

On 22/07/16 13:51, Tvrtko Ursulin wrote:
>
> On 22/07/16 13:42, Dave Gordon wrote:
>> On 21/07/16 14:46, Tvrtko Ursulin wrote:
>>> On 21/07/16 14:31, Chris Wilson wrote:
>>>> On Thu, Jul 21, 2016 at 02:16:22PM +0100, Tvrtko Ursulin wrote:
>>>>>
>>>>> On 21/07/16 13:59, Chris Wilson wrote:
>>>>>> On Thu, Jul 21, 2016 at 01:00:47PM +0100, Tvrtko Ursulin wrote:
>>>>>>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>>>>>
>>>>>>> Static table wastes space for invalid combinations and
>>>>>>> engines which are not supported by Gen6 (legacy semaphores).
>>>>>>>
>>>>>>> Replace it with a function devised by Dave Gordon.
>>>>>>>
>>>>>>> I have verified that it generates the same mappings between
>>>>>>> mbox selectors and signalling registers.
>>>>>>
>>>>>> So just how big was that table? How big are the functions replacing
>>>>>> it?
>>>>>
>>>>> With I915_NUM_ENGINES of 5 table is 5 * 5 * (2 * 4) = 200 bytes.
>>>>>
>>>>> With the patch .text grows by 144 bytes here and .rodata shrinks by
>>>>> 256. So a net gain of 112 bytes with my config. Conclusion is that
>>>>> as long as we got five engines it is not that interesting to get rid
>>>>> of the table.
>>
>> Since the semaphore matrix is only relevant to a specific gen, you could
>> remove it from the multi-generational engine-list and instead just have
>> it in the gen-specific code that needs it. That way it won't continue to
>> grow as new engines are added. The one gen that needs it is fixed at
>> 4x4, so it could just be a 16-byte lookup table, or 32 bits
>> (0b11001001_10110001_00101101_10010011) if you really want to save space
>> ;-)
>
> Not so much save space today as prevent from it exploding in the future.
>
> Rewriting the table to use hw_ids sounds like the best idea so far so
> I'll go with that.
>
> Regards,
>
> Tvrtko

+ * This distilled to integers looks like this:
+ *
+ *   |  0  |  1  |  2  |  3
+ * --+-----+-----+-----+-----
+ * 0 | -1  |  0  |  2  |  1
+ * 1 |  2  | -1  |  0  |  1
+ * 2 |  0  |  2  | -1  |  1
+ * 3 |  2  |  1  |  0  | -1

static unsigned int sem_wait_mbox(unsigned int x, unsigned int y)
{
	const unsigned int z = 0b11000110011110000100111001100011;

	return (z >> (8*y + 2*x)) & 3;
}

          x
       0 1 2 3
       -------
   0:  3 0 2 1
y 1:  2 3 0 1
   2:  0 2 3 1
   3:  2 1 0 3

No complicated code, and hardly any table :)

.Dave.
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