Fixing vkCmdDrawIndexedIndirect with index buffer offset for llvmpipe

[Timur Kristóf]

Hello Lukas,

I'm not a llmpipe developer, but I think the place for this discussion is
on the Mesa gitlab. Feel free to open an issue about this there or better
yet, submit a merge request with your proposed fix.

Best regards,
Timur

<lists at luckyxxl.de> ezt írta (időpont: 2023. febr. 19., Vas 12:10):

> Hello everyone,
>
> newbie to the Mesa codebase here.
>
> I found that in the current main branch the llvmpipe Vulkan driver does
> not produce correct results for `vkCmdDrawIndexedIndirect` when the
> preceding call to `vkCmdBindIndexBuffer` passed a non-zero value for
> `offset`. I poked around in the code a little bit, found that the
> `offset` parameter is just not used at all for indirect draws and came
> up with the following two solutions to fix my usecase:
>
>
> 1)
>
> https://gitlab.freedesktop.org/luckyxxl/mesa/-/commits/fix_indirect_draw_ib_offset1
>
> While this is a pretty minimal change I don't think it's a good one.
> Adding `index_offset` to `struct pipe_draw_indirect_info` feels like a
> hack. Also it will fail silently when `offset` is not a multiple of the
> index type size, which is valid according to the Vulkan specification.
>
> 2)
>
> https://gitlab.freedesktop.org/luckyxxl/mesa/-/commits/fix_indirect_draw_ib_offset2
>
> While this one adheres the Vulkan specification in that any value for
> `offset` is supported, I feel like it is a fairly large/risky change.
> `struct pipe_draw_info` is used throughout the codebase in various
> utility functions and drivers, and I can't judge how big the impact of
> the suggested change would be. Also, it seems to me that the code of
> many drivers should be updated to handle the new parameter, though that
> might not be necessary given that the new behavior cannot be triggered
> via the OpenGL API (see below). How would such a global change be
> handled if it's worth proceeding-on?
>
>
> The issue can only be triggered using the Vulkan API, given that there's
> no way (at least none that I know of / could find) to use an offset into
> the index buffer for indirect draws with the OpenGL API. Therefore I
> think that some changes to core data-structures/algorithms are necessary
> in order to be able to implement the fix, unless I oversaw something. I
> would be glad if someone could advise on how to proceed: Does it look
> worth to you to try to upstream one of the two suggested fixes? In that
> case I'd be happy to write a proper PR. Or is the issue better worked-on
> by a developer who is experienced with the code, in that case I'll
> gladly file an issue and provide a repro. Also, as a final feedback, is
> this mailing-list the correct place to discuss such a matter, or would
> that better be done in a PR or via IRC?
>
> Thank you for your time :)
>
> Lukas
>
>
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