[Mesa-dev] Implementation of VK_KHR_draw_indirect_count extension for anv

Mon Sep 17 14:34:07 UTC 2018

On Mon, Sep 17, 2018 at 8:34 AM Danylo Piliaiev <danylo.piliaiev at gmail.com>
wrote:

> Hi Jason,
>
> I have implemented the extension and it works, however before sending the
> patch I decided to see how it can interact with other extension -
> VK_EXT_conditional_render
> and got confused:
>
> From the spec it is not disallowed to call functions of
> VK_KHR_draw_indirect_count in conditional rendering block. So let's say
> that predicate of conditional rendering
> will result in FALSE, we call vkCmdDrawIndirectCountKHR which sees that
> there is already a predicate emitted and it should be taken into account,
> since it will be FALSE
> all next predicates should result in FALSE. The issue is that I don't see
> an easy way to do this.
>
> My current implementation uses the next predicate (it is same as in GL
> implementation):
>
>        /* While draw_index < maxDrawCount the predicate's result will be
>         *  (draw_index == maxDrawCount) ^ TRUE = TRUE
>         * When draw_index == maxDrawCount the result is
>         *  (TRUE) ^ TRUE = FALSE
>         * After this all results will be:
>         *  (FALSE) ^ FALSE = FALSE
>         */
>        anv_batch_emit(&cmd_buffer->batch, GENX(MI_PREDICATE), mip) {
>           mip.LoadOperation    = LOAD_LOAD;
>           mip.CombineOperation = COMBINE_XOR;
>           mip.CompareOperation = COMPARE_SRCS_EQUAL;
>        }
>
> But if the initial predicate state is FALSE then when draw_index equals
> maxDrawCount the result will be
>   (FALSE) ^ TRUE = TRUE
> Which isn't something we want. But without "not equal" operation or
> without MI_MATH I don't see how to fix this.
>

First off, thanks for looking into the combination of these two features.
Getting them to work together nicely is half of the difficulty of these two
extensions.

On platforms which support MI_MATH, I think we're probably better off just
using it.  For Ivy Bridge, the only thing I could think to do when both are
in use would be to do two MI_PREDICATEs for every draw call.  The first
would be what you describe above and the second would be the MI_PREDICATE
for the conditional render with COMBINE_AND.  When the condition is true,
the AND would have no effect and you would get the behavior above.  If the
condition is false, the above logic for implementing draw_indirect_count
wouldn't matter because it would get ANDed with false.  On Haswell and
later, it's likely more efficient to just use MI_MATH and avoid re-loading
the draw count and condition on every draw call.  (We could just leave the
draw count in CS_GPR0, for instance.)  Does that work?

Since you're already looking at it, it may be best to implement the two
extensions together as one patch series so we can be sure we have the
interactions right.  If we can't get them to play nicely together, we may
have to disable one of them on Ivy Bridge and I'd rather not enable an
extension and then take the functionality away later.

> I don't see anything related in Vulkan or GL specs neither I see anything
> in Piglit and CTS tests.
> Maybe I'm missing something obvious, could you help me here?
>

There's nothing preventing the two from being used together.  If we don't
have piglit tests that exercise the GL versions together, that would be
bad.  Have you found good Vulkan CTS tests for either of those two
extensions?  VK_KHR_indirect_count should have tests since it's a KHR
extension but we may need to write the tests for EXT_conditional_render.

--Jason

> You can find current implementation in
> https://gitlab.freedesktop.org/GL/mesa/commit/9d1c7ae0db618c6f7281d5f667c96612ff0bb2c2
>
> - Danil
>
> On 9/12/18 6:30 PM, Danylo Piliaiev wrote:
>
> Hi,
>
> Thank you for the directions!
>
> On 9/12/18 6:13 PM, Jason Ekstrand wrote:
>
> 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
>
> Didn't see them, will take closer look later.
>
>
> 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.
>
> I saw such implementation for i965, looked straightforward and I thought
> it will easily translate into Vulkan implementation. Didn't know that it's
> possible to do it other way on Intel.
>
>
> ## 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.
>
> The correct way looks interesting, I'll need some time to understand
> details.
>
>
> 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.
>
> I'll follow your recommendation and will start from the first method.
>
> - Danil
>
>
> --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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