[Mesa-dev] [RFC 1/3] tgsi: Add WORK_DIM System Value

[Ilia Mirkin]

On Wed, Apr 27, 2016 at 11:19 AM, Hans de Goede <hdegoede at redhat.com> wrote:
> Hi,
>
> On 27-04-16 16:49, Ilia Mirkin wrote:
>>
>> Please add this semantic to src/gallium/docs and explain what it
>> means.
>
>
> Ah, I was under the impression these were not documented, but I
> now see they are, will fix.
>
>> (I'm not even sure what this is, and the easily-found opencl
>>
>> docs helpfully indicate that get_work_dim returns the work_dim...)
>> This wasn't done for some of the other compute-specific semantics.
>
>
> When launching a grid through clEnqueueNDRangeKernel the user
> can tell how many dimensions of the pipe_grid_info->grid /
> pipe_grid_info->block equivalent the user is actually passing in
> this is done through a parameter to that function called work_dim:
>
> https://www.khronos.org/registry/cl/sdk/1.0/docs/man/xhtml/clEnqueueNDRangeKernel.html
>
> Clover pads the y and z values (if unused) to 1, so it always
> ends up giving a 3 dimensional grid and block sizes to the driver.
>
> But an opencl kernel can use the opencl builtin get_work_dim
> function, which returns how much dimensions where actually
> passed into clEnqueueNDRangeKernel.
>
> Note that passing in e.g. work_dim = 3 with a grid size of { 4, 1, 1 }
> is perfectly legal and then get_work_dim should actually return 3.
>
> So we need to store the clEnqueueNDRangeKernel workdim parameter
> somewhere, and allow getting it from a kernel. Using a system-value
> seems the best solution for this to me.
>
> I hope this helps to explain things, if not keep asking :)
>
> Also if you've a better solution, I'm all ears.

Sounds like there's no real way to distinguish it from the kernel, the
parameters given to the hw are all the same, so it's just a thing that
needs to be passed along. I think that what you have is a fine
solution, although I question the usefulness of this being an OpenCL
builtin. But we're not about to change that now.

An alternative would be for clover to stick it into a uniform and have
the kernel read it out from there. If there are going to be a ton of
these, that might be preferable. If it's just the one (or maybe two),
not worth it.

  -ilia