[Mesa-dev] Implementation of VK_KHR_draw_indirect_count extension for anv

[Jason Ekstrand]

Danylo,

You're free to implement anything not already implemented.  Here are some
other (probably simpler) extensions that I think can be reasonably
implemented on Intel HW:

 - VK_EXT_conservative_rasterization
 - VK_EXT_conditional_render

As far as VK_KHR_draw_indirect_count go, I haven't implemented it yet
because the "proper" implementation is actually kind-of painful though not
impossible.  In general, there are two ways it can be done:

## 1. The cheap and easy way

The spec explicitly allows for the cheap and easy way by requiring the
caller to pass in a maxDrawCount.  The idea here would be to emit
maxDrawCount draw calls only have each one of them predicated on draw_id <
draw_count_from_buffer.  This one probably wouldn't take much to wire up
but it does mean doing maxDrawCount 3DPRIMITIVE commands no matter how many
of them are actually needed.

## 2. The hard but maybe more correct way

The Intel command streamer does have the ability, if used carefully, to
loop.  The difficulty here isn't in looping; that can be done fairly easily
on gen8+ by emitting a predicated MI_BATCH_BUFFER_START that's predicated
off of the looping condition which jumps to the top of the loop.  The real
difficult bit is taking your loop counter and using it to indirectly access
the array of draw information.  In order to do this, you have to have a
self-modifying batch buffer.  In short, you would emit MI commands which
read the draw information into registers and also emit MI commands (which
would probably come before the first set) which write the actual address
into the location in the batch where the first set of MI commands has their
address to read from.  This would be a painful to debug mess of GPU hangs
but could actually be kind-of fun to implement.

I hope I haven't scarred you away from working on anv; I just wanted to
make it clear what you're getting yourself into.  Both ways are totally
implementable and I think you'd pretty much have to do the first method on
gen7 if we really care about supporting it there.  The second is totally
doable, it'll just involve some headaches when it's broken.  If you want to
continue with this project after reading my scarry e-mail, I recommend
starting with method 1 to get your feet wet and then we can look into
method 2 once you have that working.

--Jason

On Wed, Sep 12, 2018 at 6:36 AM Danylo Piliaiev <danylo.piliaiev at gmail.com>
wrote:

> Hello everyone,
>
> I would like to try to implement one of the Vulkan extensions -
> VK_KHR_draw_indirect_count for anv,
> unless someone is already working on it.
>
> It's a relatively minor extension and I saw that the same functionality
> is already implemented
> for ARB_indirect_parameters in i965.
>
> Also I would appreciate any tips if there are any known possible tricky
> parts.
>
> - Danil
> _______________________________________________
> mesa-dev mailing list
> mesa-dev at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
>
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