[Intel-gfx] [RFC] drm/i915: Emit to ringbuffer directly

[Tvrtko Ursulin]

On 09/09/16 14:45, Chris Wilson wrote:
> On Fri, Sep 09, 2016 at 09:32:50AM +0100, Tvrtko Ursulin wrote:
>>
>> On 08/09/16 17:40, Chris Wilson wrote:
>>> On Thu, Sep 08, 2016 at 04:12:55PM +0100, Tvrtko Ursulin wrote:
>>>> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>>>
>>>> This removes the usage of intel_ring_emit in favour of
>>>> directly writing to the ring buffer.
>>>
>>> I have the same patch! But I called it out, for historical reasons.
>>
>> Yes I know we talked about it in the past but I did not think you
>> will find time to actually write it amongst all the other things.
>>
>>> Oh, except mine uses out[0]...out[N] because gcc prefers that over
>>> *out++ = ...
>>
>> It copes just fine with the latter here, for example:
>>
>> 	*rbuf++ = cmd;
>> 	*rbuf++ = I915_GEM_HWS_SCRATCH_ADDR | MI_FLUSH_DW_USE_GTT;
>> 	*rbuf++ = 0; /* upper addr */
>> 	*rbuf++ = 0; /* value */
>>
>> Is:
>>
>>       3e9:       89 10                   mov    %edx,(%rax)
>>       3eb:       c7 40 04 04 01 00 00    movl   $0x104,0x4(%rax)
>>       3f2:       c7 40 08 00 00 00 00    movl   $0x0,0x8(%rax)
>>       3f9:       c7 40 0c 00 00 00 00    movl   $0x0,0xc(%rax)
>
> Last time Dave suggested using something like
>
> i915_gem_request_emit(req, (struct cmd_packet){ dw0, dw1, dw2 });
>
> I tried mocking something up, but just found gcc was constructing the
> struct on the stack and then copying across, and generating far more
> code than the sequence above. Worth seeing if that is better (or if my
> mockup was just bad).

Not sure that I like that. It would be a bit ugly in cases where batches 
are built dynamically, no? Perhaps I am misunderstanding the idea?

Regards,

Tvrtko