[Mesa-dev] [PATCH 2/2] i965: Implement ARB_indirect_parameters

[Manolova, Plamena]

Thank you for taking a look Jason!
Sorry my bad, I'll make sure to add some comments to v2 at least. Hopefully
it'll make editing this portion of
the i965 drawing logic a bit easier.


On Fri, Sep 8, 2017 at 4:48 AM, Jason Ekstrand <jason at jlekstrand.net> wrote:

> Hey Pam!
>
> A quick skim through this looks fairly good.  I'll be honest, I expected
> to have some quick comments on the v1 but I'm not seeing anything
> immediately.  That said, I've got to think about it fairly hard to convince
> myself that it's actually correct.  The i965 drawing logic isn't really my
> area.
>
> On Thu, Aug 31, 2017 at 11:26 AM, Ilia Mirkin <imirkin at alum.mit.edu>
> wrote:
>
>> On Thu, Aug 31, 2017 at 1:45 PM, Bas Nieuwenhuizen
>> <bas at basnieuwenhuizen.nl> wrote:
>> >
>> >
>> > On Wed, Aug 30, 2017, at 14:59, Ilia Mirkin wrote:
>> >> On Wed, Aug 30, 2017 at 3:17 AM, Bas Nieuwenhuizen
>> >> <bas at basnieuwenhuizen.nl> wrote:
>> >> > So as a random drive-by review, I think the risk in this
>> implementation
>> >> > is that apps just set maxdrawcount to some high value . If I'm
>> reading
>> >> > the spec correctly there is no real bound on the value except for the
>> >> > max-representable value for the integer.  Also AFAIK the AMD and
>> NVidia
>> >> > implementation don't really get slower if you specify maxdrawcount
>> very
>> >>
>> >> Can't speak for AMD, but this is actually extremely similar (in
>> >> principle) to how this is handled on NVIDIA. It's a little cleaner
>> >> since there's a nice macro, but since macros can't have a variable
>> >> number of arguments, you have to feed it maxdrawcount args. Admittedly
>> >> this is the nouveau impl, but I'm almost certain that the blob does it
>> >> in a similar way.
>> >
>> > I thought, you had a macro that looped, but seems that was a
>> > misunderstanding on my side.  If nvidia does it that way, then I suppose
>> > there is little risk of games expecting something more efficient. Never
>> > mind my comments then.
>>
>> There indeed is a macro that loops. However that macro takes a number
>> of parameters determined at the time of cmd stream generation. Also
>> the indirect draw arguments have to be added into the cmd stream (via
>> IB references). Here's how nouveau does it:
>>
>> https://cgit.freedesktop.org/mesa/mesa/tree/src/gallium/driv
>> ers/nouveau/nvc0/nvc0_vbo.c#n798
>>
>> I don't remember if I looked precisely at what NVIDIA does, but I
>> definitely can't think of another way. Even if you use the command
>> which takes the argument length as a separate word, that won't be
>> right since it's count * N arguments. macros can't read arbitrary
>> things either, it has to come out of the cmdstream.
>>
>
> I'll agree that this solution is suboptimal.  However, it's really the
> best we can do without getting crazy.  A "real" implementation would
> require a self-modifying command buffer which is a sketchy idea in the best
> of circumstances. :/
>
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