[Mesa-dev] memoryBarrier + SSBO

[Albert Freeman]

On 27 September 2015 at 08:32, Albert Freeman <albertwdfreeman at gmail.com> wrote:
> On 27 September 2015 at 08:30, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
>> 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
> The minimum version requirement, it is GL 4.0 on NV_ssbo and wasen't
> changed when that was "enhanced" (e.g. Memory Barrier stuff was added)
> for ARB_ssbo.
Actually, it could be either, probably better for the ARB to decide.