Low maxVariableDescriptorCount being reported

[MennoVink]

I'm receiving a rather low variable descriptor count which i expected to be higher.

I'm on Intel Iris 6100 and these are my limits:

maxPerStageDescriptorSamplers   uint32_t    16
maxPerStageDescriptorUniformBuffers uint32_t    31
maxPerStageDescriptorStorageBuffers uint32_t    31
maxPerStageDescriptorSampledImages  uint32_t    128
maxPerStageDescriptorStorageImages  uint32_t    8
maxPerStageDescriptorInputAttachments   uint32_t    128
maxPerStageResources    uint32_t    159
maxDescriptorSetSamplers    uint32_t    80
maxDescriptorSetUniformBuffers  uint32_t    155
maxDescriptorSetUniformBuffersDynamic   uint32_t    155
maxDescriptorSetStorageBuffers  uint32_t    155
maxDescriptorSetStorageBuffersDynamic   uint32_t    155
maxDescriptorSetSampledImages   uint32_t    640
maxDescriptorSetStorageImages   uint32_t    40
maxDescriptorSetInputAttachments    uint32_t    640

The values important to me are 16 samplers and 128 sampled images (both per-stage). This is the layout for my descriptor set (glsl):

struct Constants
{
    mat4 world;
    int albedoIndex;
};
layout( std430, set = 0, binding = 0 ) readonly buffer buf
{
    Constants constants[];
};
layout( set = 0, binding = 1) uniform sampler albedoSampler;
layout( set = 0, binding = 2 ) uniform texture2D albedoImages[];

My layout bindings match this and i have bindFlags of 0, 0, 8 (variable only for the albedoImages).

Now when i call getDescriptorSetLayoutSupport attaching a DescriptorSetVariableDescriptorCountLayoutSupportEXT, i get the maximum variable count being 15.

Where does this 15 come from? How come it doesn't tell me that i can use 128 sampled images?

*edit: To add insult to injury, if i ask whether or not a layout using those 15 descriptors is supported it still tells me no. 14 is the highest amount that's reported as being supported. This seems to me like the same off-by-one issue i reported here https://github.com/KhronosGroup/MoltenVK/issues/1696.

[MennoVink]

I tried to get this to run in the cube example but it's configured to disallow debugging so not sure how to recreate it there.

Here's what i've got so far:

        VkDescriptorSetLayoutBinding bindings[ 3 ] = {};
        bindings[ 0 ].binding = 0;
        bindings[ 0 ].descriptorType = VK_DESCRIPTOR_TYPE_STORAGE_BUFFER;
        bindings[ 0 ].descriptorCount = 1;
        bindings[ 0 ].stageFlags = VK_SHADER_STAGE_VERTEX_BIT | VK_SHADER_STAGE_FRAGMENT_BIT;

        bindings[ 1 ].binding = 1;
        bindings[ 1 ].descriptorType = VK_DESCRIPTOR_TYPE_SAMPLER;
        bindings[ 1 ].descriptorCount = 1;
        bindings[ 1 ].stageFlags = VK_SHADER_STAGE_VERTEX_BIT | VK_SHADER_STAGE_FRAGMENT_BIT;

        bindings[ 2 ].binding = 2;
        bindings[ 2 ].descriptorType = VK_DESCRIPTOR_TYPE_SAMPLED_IMAGE;
        bindings[ 2 ].descriptorCount = 0;
        bindings[ 2 ].stageFlags = VK_SHADER_STAGE_VERTEX_BIT | VK_SHADER_STAGE_FRAGMENT_BIT;

        VkDescriptorSetLayoutCreateInfo setLayoutCreateInfo = {};
        setLayoutCreateInfo.sType = VK_STRUCTURE_TYPE_DESCRIPTOR_SET_LAYOUT_CREATE_INFO;
        setLayoutCreateInfo.bindingCount = 3;
        setLayoutCreateInfo.pBindings = bindings;

        VkDescriptorBindingFlags bindingFlags[ 3 ] = {};
        bindingFlags[ 2 ] = VK_DESCRIPTOR_BINDING_VARIABLE_DESCRIPTOR_COUNT_BIT;

        VkDescriptorSetLayoutBindingFlagsCreateInfo bindingFlagsCreateInfo = {};
        bindingFlagsCreateInfo.sType = VK_STRUCTURE_TYPE_DESCRIPTOR_SET_LAYOUT_BINDING_FLAGS_CREATE_INFO;
        bindingFlagsCreateInfo.bindingCount = 3;
        bindingFlagsCreateInfo.pBindingFlags = bindingFlags;
        setLayoutCreateInfo.pNext = &bindingFlagsCreateInfo;

        VkDescriptorSetVariableDescriptorCountLayoutSupport variableCountSupport = {};
        variableCountSupport.sType = VK_STRUCTURE_TYPE_DESCRIPTOR_SET_VARIABLE_DESCRIPTOR_COUNT_LAYOUT_SUPPORT;
        VkDescriptorSetLayoutSupport descriptorSupport = {};
        descriptorSupport.sType = VK_STRUCTURE_TYPE_DESCRIPTOR_SET_LAYOUT_SUPPORT;
        descriptorSupport.pNext = &variableCountSupport;

        vkGetDescriptorSetLayoutSupport( demo->device, &setLayoutCreateInfo, &descriptorSupport );
        assert( variableCountSupport.maxVariableDescriptorCount == 128 );

Obviously it also needs the indexing features:

    VkPhysicalDeviceDescriptorIndexingFeatures indexingFeatures = {};
    indexingFeatures.sType = VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_DESCRIPTOR_INDEXING_FEATURES;
    indexingFeatures.runtimeDescriptorArray = true;
    indexingFeatures.descriptorBindingVariableDescriptorCount = true;
    VkPhysicalDeviceFeatures2 enabledFeatures2 = {};
    enabledFeatures2.sType = VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_FEATURES_2;
    enabledFeatures2.pNext = &indexingFeatures;

And the extension:

        for (uint32_t i = 0; i < device_extension_count; i++) {
            if (!strcmp(VK_EXT_DESCRIPTOR_INDEXING_EXTENSION_NAME, device_extensions[i].extensionName)) {
                demo->extension_names[demo->enabled_extension_count++] = VK_EXT_DESCRIPTOR_INDEXING_EXTENSION_NAME;
            }
        }