panic in display_plane_supported_displays

[timglauertadder]

vulkano: 0.34.1

Calling display_plane_supported_displays can cause a panic by calling unwrap on None:

thread 'main' panicked at 'called Option::unwrap() on a None value', /home/tim/.cargo/registry/src/index.crates.io-6f17d22bba15001f/vulkano-0.34.1/src/device/physical.rs:925:66

This is physical.rs lines 921 onwards:

if let Some(display) = self.display_properties.get(&display_vk) {
      display
  } else {
      self.display_properties_unchecked()?;
      self.display_properties.get(&display_vk).unwrap()
  },

The else branch is executed when self.display_properties.get(&display_vk) returns None but the branch calls it again and unwraps it, causing a panic. Presumably self.display_properties_unchecked() is supposed to fix this, but it doesn't on my system.

I found this as part of a wider problem where none of the values returned by display_plane_properties had a current_display that was not None. I may not be initialising something properly, but the code should handle this cleanly rather than calling panic.

[Rua]

What that code is attempting to do, is figure out which display object belongs with the display handle that was returned from the vkGetDisplayPlaneSupportedDisplaysKHR API call. First it looks in its cache of objects it knows about, but this fails, so then it calls display_properties_unchecked, which is then supposed to put all the objects returned from that call into the cache. The final line, then, makes the assumption that the previously missing display handle was returned by vkGetPhysicalDeviceDisplayPropertiesKHR, had an object constructed, and that object must now be in the cache.

The fact that it isn't in the cache is quite strange. That means that vkGetDisplayPlaneSupportedDisplaysKHR returned a display handle that didn't appear in the result of a subsequent vkGetPhysicalDeviceDisplayPropertiesKHR call. I can't really fathom why this would happen.

[Rua]

As a way to confirm this, can you run your code with the Vulkan API dump layer enabled (via vkconfig or directly in your program)? Then specifically find the calls to these two functions, and see what they are returning.

[timglauertadder]

Is there a vulkano call for this? Not able to get the tools onto the platform I am using.

[Rua]

No, it's part of the Vulkan SDK.

[timglauertadder]

The BestPractices validation layer is giving this warning:

UNASSIGNED-BestPractices-vkGetDisplayPlaneSupportedDisplaysKHR-properties-not-retrieved` validation warning: Validation Warning: [ UNASSIGNED-BestPractices-vkGetDisplayPlaneSupportedDisplaysKHR-properties-not-retrieved ] Object 0: handle = 0x55ee0f169b30, type = VK_OBJECT_TYPE_PHYSICAL_DEVICE; | MessageID = 0xe3d810d8 | Potential problem with calling vkGetDisplayPlaneCapabilities2KHR() without first retrieving properties from vkGetPhysicalDeviceDisplayPlanePropertiesKHR or vkGetPhysicalDeviceDisplayPlaneProperties2KHR.

However I am calling device.display_properties() and device.display_plane_properties() before this message appears, and before calling display_plane_capabilities(0) on the first mode of the display. Really not sure what is going on.

[timglauertadder]

It looks like this is a lifetime issue in my code.

This code fails:

let displays = device.display_properties().expect("display properties");
for display in displays {
   ...
   device.display_plane_supported_displays(p_num).unwrap();

Using an explicit .iter() fixes the problem:

for display in displays.iter() {

I don't know enough about rust or vulkano to understand exactly why this makes a difference, but it does at least prevent the panic. (Unfortunately the current_display for each surface is still None and I still don't get any pixels)

[Rua]

An API dump of what your program is calling would be very helpful.

[timglauertadder]

Finally got dump working and have attached the file of a failed run (before the fix I mentioned above).
I have a solution to the immediate problem (the panic) but I was hoping that it might shed light on why there are no displays associated with any of the surfaces.

api_dump.txt

[Rua]

Ok, distilling out the important bits, the dump contains this sequence of calls related to displays:

• vkGetPhysicalDeviceDisplayProperties2KHR. Returns 2 display handles 0xfab64d0000000002 and 0xfa21a40000000003.
• vkGetPhysicalDeviceDisplayPlaneProperties2KHR. Returns 6 display planes, but all of them with display handle 0 as the current display.
• vkGetDisplayModeProperties2KHR with display handle 0xfab64d0000000002. Returns 38 display mode handles.
• vkGetDisplayPlaneSupportedDisplaysKHR with plane index 0. Returns 1 display handle 0xfab64d0000000002.
• vkGetDisplayPlaneCapabilities2KHR with display mode handle 0xf56c9b0000000004 and plane index 0. Call succeeds.
• vkCreateDisplayPlaneSurfaceKHR with display mode handle 0xf56c9b0000000004 and plane index 0. Call succeeds.
• vkGetDisplayModeProperties2KHR with display handle 0xfa21a40000000003. Returns 39 display mode handles (one more than before), but none of them are the handle 0xf56c9b0000000004 that was used before.
• vkGetDisplayPlaneSupportedDisplaysKHR with plane index 0. Returns 1 display handle 0xfab64d0000000002, same as before.
• vkGetPhysicalDeviceDisplayProperties2KHR. Returns 2 display handles 0xfab64d0000000002 and 0xfa21a40000000003, same as before.

After this point, objects are destroyed and the dump ends, so I assume the crash happens here.

It looks like getting None as the current_display is something the Vulkan driver itself is doing, so at least that part is not a Vulkano bug. Other than that, at first glance, nothing seems out of the ordinary as far as the function calls go. It is a bit strange that, after the display surface is created, there's three more calls to display-related functions. I'm not really sure what those are there for.

[timglauertadder]

The code is trying to create a surface on each display, so it looks like the code after vkCreateDisplayPlaneSurfaceKHR is reading data for the second display. The final call to vkGetPhysicalDeviceDisplayProperties2KHR is the one in display_plane_supported_displays but for some reason it doesn't populate the display_properties field which is why there is the panic.

I really think this is a resource lifetime issue in my code when I wasn't using .iter() in the outer loop, so not really worth persuing further. As you say, the problem with a Null value for current_display is a driver problem, not a vulkano problem.