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Tweeted twitter.com/StackGameDev/status/731861264285241344
Added more information for context and removed some info about the integrated GPU because I didn't think it would be relevant.
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In the machine I'm running I have two GTX 980 Tis (and an Intel HD Graphics 530 but I'm not trying to utilize it with Vulkan). When I query the physical device count with vkEnumeratePhysicalDevices it always returns 1 despite having 2 physical discrete graphics cards and an integrated GPU. When I use vulkaninfo the only device listed is GPU0 which is a DISCRETE_GPU / GTX 980 Ti.

auto deviceCount = uint32_t{0}; instance.enumeratePhysicalDevices(&deviceCount, nullptr); // At this point deviceCount is 1

Also, to confirm that the OS sees the cards:

Windows 10:

C:\Users\matt>wmic path win32_VideoController get name Name NVIDIA GeForce GTX 980 Ti NVIDIA GeForce GTX 980 Ti

GPUs in Nvidia Driver Settings

Linux 4.5:

+------------------------------------------------------+ | NVIDIA-SMI 364.19 Driver Version: 364.19 | |-------------------------------+----------------------+----------------------+ | GPU Name Persistence-M| Bus-Id Disp.A | Volatile Uncorr. ECC | | Fan Temp Perf Pwr:Usage/Cap| Memory-Usage | GPU-Util Compute M. | |===============================+======================+======================| | 0 GeForce GTX 980 Ti Off | 0000:01:00.0 On | N/A | | 0% 56C P8 34W / 300W | 444MiB / 6140MiB | 0% Default | +-------------------------------+----------------------+----------------------+ | 1 GeForce GTX 980 Ti Off | 0000:02:00.0 Off | N/A | | 0% 43C P8 25W / 300W | 22MiB / 6143MiB | 0% Default | +-------------------------------+----------------------+----------------------+

GPUs in Nvidia XServer Settings

In the machine I'm running I have two GTX 980 Tis (and an Intel HD Graphics 530 but I'm not trying to utilize it with Vulkan). When I query the physical device count with vkEnumeratePhysicalDevices it always returns 1 despite having 2 physical discrete graphics cards and an integrated GPU. When I use vulkaninfo the only device listed is GPU0 which is a DISCRETE_GPU / GTX 980 Ti.

auto deviceCount = uint32_t{0}; instance.enumeratePhysicalDevices(&deviceCount, nullptr); // At this point deviceCount is 1

In the machine I'm running I have two GTX 980 Tis. When I query the physical device count with vkEnumeratePhysicalDevices it always returns 1 despite having 2 physical discrete graphics cards. When I use vulkaninfo the only device listed is GPU0 which is a DISCRETE_GPU / GTX 980 Ti.

auto deviceCount = uint32_t{0}; instance.enumeratePhysicalDevices(&deviceCount, nullptr); // At this point deviceCount is 1

Also, to confirm that the OS sees the cards:

Windows 10:

C:\Users\matt>wmic path win32_VideoController get name Name NVIDIA GeForce GTX 980 Ti NVIDIA GeForce GTX 980 Ti

GPUs in Nvidia Driver Settings

Linux 4.5:

+------------------------------------------------------+ | NVIDIA-SMI 364.19 Driver Version: 364.19 | |-------------------------------+----------------------+----------------------+ | GPU Name Persistence-M| Bus-Id Disp.A | Volatile Uncorr. ECC | | Fan Temp Perf Pwr:Usage/Cap| Memory-Usage | GPU-Util Compute M. | |===============================+======================+======================| | 0 GeForce GTX 980 Ti Off | 0000:01:00.0 On | N/A | | 0% 56C P8 34W / 300W | 444MiB / 6140MiB | 0% Default | +-------------------------------+----------------------+----------------------+ | 1 GeForce GTX 980 Ti Off | 0000:02:00.0 Off | N/A | | 0% 43C P8 25W / 300W | 22MiB / 6143MiB | 0% Default | +-------------------------------+----------------------+----------------------+

GPUs in Nvidia XServer Settings

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user5665
user5665

Vulkan vkEnumeratePhysicalDevices Returning Unexpected Device Count

In the machine I'm running I have two GTX 980 Tis (and an Intel HD Graphics 530 but I'm not trying to utilize it with Vulkan). When I query the physical device count with vkEnumeratePhysicalDevices it always returns 1 despite having 2 physical discrete graphics cards and an integrated GPU. When I use vulkaninfo the only device listed is GPU0 which is a DISCRETE_GPU / GTX 980 Ti.

This happens in Windows 10 as well as GNU/Linux 4.5 using Vulkan API 1.0.11 with proprietary Nvidia drivers.

Is this normal behavior and I'm not using the API correctly or is this a bug? I feel like I've made a glaring obvious mistake but I can't find anything in the Vulkan specification about it.

For clarity, this is exactly how I query physical device count:

auto deviceCount = uint32_t{0}; instance.enumeratePhysicalDevices(&deviceCount, nullptr); // At this point deviceCount is 1