[Mesa-dev] memoryBarrier + SSBO

[Ilia Mirkin]

On Sat, Sep 26, 2015 at 6:12 PM, Albert Freeman
<albertwdfreeman at gmail.com> wrote:
> On 26 September 2015 at 03:32, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
>> Hi Ian (and other spec experts),
>>
>> The ARB_ssbo spec mentions the following:
>>
>>     OpenGL 4.0 (either core or compatibility profile) is required.
>>
>> ...
>>
>>     Additionally, the shading language provides the memoryBarrier() function
>>     to control the relative order of memory accesses within individual shader
>>     invocations and provides various memory qualifiers controlling how the
>>     memory corresponding to individual variables is accessed.
>>
>> However the memoryBarrier() function only becomes available in GLSL
>> 4.20 [along with the glMemoryBarrier() function that the spec also
>> refers to] or with ARB_shader_image_load_store.
>>
>> Is the implication that such functionality should be auto-exposed in
>> image-less drivers that support ssbo? Or that this functionality
>> should just not exist unless images are supported?
>>
>> This is relevant to me as I plan on adding ssbo support to gallium
>> before images. Also, is there really anything in there that would
>> prevent this from being exposed on a GL 3.1 driver (I'm thinking of
>> freedreno which doesn't support GS yet and is my initial target for
>> this, since I can make atomics actually work on there, unlike NVIDIA
>> where the damn things just won't budge without some magic
>> yet-to-be-found make-it-work bit).
>>
>> Cheers,
>>
>>   -ilia
>
> I think that is an error in the ARB_ssbo spec (as that spec also
> mentions: "This extension is written against the OpenGL 4.2
> (Compatibility Profile) Specification."). I believe this occurred as
> ARB_ssbo was adapted from NV_ssbo; NV_ssbo does not mention
> MemoryBarrier (i.e. someone forgot to update the version when NV_ssbo
> was enhanced for ARB).

Many extension specs are like this though... written as a diff against
OpenGL version X, but require OpenGL version Y, s.t. Y < X.

What are you saying is the spec error, by the way? The minimum version
requirement? Or the fact that the interaction isn't listed?

  -ilia