[Mesa-dev] [PATCH] main/shaderimage: image unit invalid if texture is incomplete, independently of the level

[Francisco Jerez]

Alejandro Piñeiro <apinheiro at igalia.com> writes:

> On 14/07/16 21:24, Francisco Jerez wrote:
>> Alejandro Piñeiro <apinheiro at igalia.com> writes:
>>
>>> Without this commit, a image is considered valid if the level of the
>>> texture bound to the image is complete, something we can check as mesa
>>> save independently if it is "base incomplete" of "mipmap incomplete".
>>>
>>> But, from the OpenGL 4.3 Core Specification, section 8.25 ("Texture
>>> Image Loads and Stores"):
>>>
>>>   "An access is considered invalid if:
>>>     the texture bound to the selected image unit is incomplete;"
>>>
>>> This implies that the access to the image unit is invalid if the
>>> texture is incomplete, no mattering details about the specific texture
>>> level bound to the image.
>>>
>>> This fixes:
>>> GL44-CTS.shader_image_load_store.incomplete_textures
>>> ---
>>>
>>> Current piglit test is not testing what this commit tries to fix. I
>>> will send a patch to piglit in short.
>>>
>>>  src/mesa/main/shaderimage.c | 14 +++++++++++---
>>>  1 file changed, 11 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/src/mesa/main/shaderimage.c b/src/mesa/main/shaderimage.c
>>> index 90643c4..d20cd90 100644
>>> --- a/src/mesa/main/shaderimage.c
>>> +++ b/src/mesa/main/shaderimage.c
>>> @@ -469,10 +469,18 @@ _mesa_is_image_unit_valid(struct gl_context *ctx, struct gl_image_unit *u)
>>>     if (!t->_BaseComplete && !t->_MipmapComplete)
>>>         _mesa_test_texobj_completeness(ctx, t);
>>>  
>>> +   /* From the OpenGL 4.3 Core Specification, Chapter 8.25, Texture Image
>>> +    * Loads and Stores:
>>> +    *
>>> +    *  "An access is considered invalid if:
>>> +    *    the texture bound to the selected image unit is incomplete;"
>>> +    */
>>> +   if (!t->_BaseComplete ||
>>> +       !t->_MipmapComplete)
>>> +      return GL_FALSE;
>> I don't think this is correct, AFAIUI a texture having _MipmapComplete
>> equal to false doesn't imply that the texture as a whole would be
>> considered incomplete according to the GL's definition of completeness.
>> Whether or not it's considered complete usually depends on the sampler
>> state while you're doing regular texture sampling: If the sampler a
>> texture object is used with has any of the mipmap filtering modes
>> enabled you need to check _MipmapComplete, otherwise you need to check
>> _BaseComplete.  The problem when you attempt to carry over this
>> definition to shader images (as the spec implies) is that image units
>> have no sampler state as such, and that they can only ever access one
>> specified level of the texture at a time (potentially a texture level
>> other than the base).  This patch makes image units behave like a
>> sampler unit with mipmap filtering enabled for the purpose of texture
>> completeness validation, which is almost definitely too strong.
>
> Yes, I didn't realize that _BaseComplete and _MipmapComplete were not
> checking the state at all. Thanks for pointing it.
>
>> An alternative would be to do something along the lines of:
>>
>> | if (!_mesa_is_texture_complete(t, &t->Sampler))
>> |    return GL_FALSE;
>
> Yes, that is what I wanted, to return false if the texture is incomplete.
>
>>
>> The problem is that you would then run into problems when some of the
>> non-base mipmap levels are missing but the sampler state baked into the
>> gl_texture_object says that you aren't mipmapping, so the GL spec would
>> normally consider the texture to be complete and
>> _mesa_is_texture_complete would return true accordingly, but still you
>> wouldn't be able to use any of the missing texture levels as shader
>> image if the application tried to bind them to an image unit (that's the
>> reason for the u->Level vs t->BaseLevel checks below you're removing).
>
> Ok, then if I understand correctly, the solution is not about replacing
> the level checks for _mesa_is_texture_complete, but keeping current
> checks, and add a _mesa_is_texture_complete check. Just checked and
> everything seems to work fine (except that now the behaviour is more
> strict, see below). I will send a patch in short.
>

Yeah, that would likely work and get the CTS test to pass, but it would
still be more strict than the spec says and consider cases that are OK
according to the spec to be incomplete, so I was reluctant to call it a
solution.

I think the ideal solution would be for the state of an image unit to be
independent from the filtering and sampling state, and depend on the
completeness of the bound level *only*.  Any idea if this CTS (or your
equivalent piglit test) passes on other GL implementations that support
image load/store (e.g. nVidia's -- I would be surprised if it does).

>> Honestly I think the current definition of image unit completeness is
>> broken, because it considers the image unit state to be complete in
>> cases where the image unit state is unusable (because the target image
>> level may be missing even though the texture object itself is considered
>> complete), and because it considers it to be incomplete in cases where
>> you really have no reason to care (e.g. why should you care what the
>> filtering mode from the sampler state says if you aren't doing any
>> filtering at all, as long as the one level you've bound to the image
>> unit is present and complete).
>
> Well, yes in general the spec is being perhaps too strict. But I assume
> that if we want to follow the spec, we don't have any other option.
>
I think the rule is not only unnecessarily strict...  If my reading of
the spec is correct it's also impossible to implement to the letter,
because the spec expects you to consider the state of an image unit to
be image-complete just because the bound texture object is
sampler-complete even though the bound texture level may be missing.  It
might be worth filing a GL bug at Khronos.

> FWIW, when testing the patch I mentioned before, some of the piglit
> tests start to fail. The reason is that with that patch everything is
> more strict, so the piglit tests need to set the filtering to NEAREST in
> order to not fail. I will send a piglit patch soon too.
>>
>>> +
>>>     if (u->Level < t->BaseLevel ||
>>> -       u->Level > t->_MaxLevel ||
>>> -       (u->Level == t->BaseLevel && !t->_BaseComplete) ||
>>> -       (u->Level != t->BaseLevel && !t->_MipmapComplete))
>>> +       u->Level > t->_MaxLevel)
>>>        return GL_FALSE;
>>>  
>>>     if (_mesa_tex_target_is_layered(t->Target) &&
>>> -- 
>>> 2.7.4
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