Move get_swap_chain_preferred_format from device to adapter and check formats

Squashed commit of the following:

commit 16e9c89dbccda3e37d05f7548dc8b9ddfb1d8613
Author: Niklas Korz <niklas@niklaskorz.de>
Date:   Mon Jan 11 19:15:43 2021 +0100

    format

commit 8c69a70ec22427b6a030915e06654bb0fc9b3c69
Author: Niklas Korz <niklas@niklaskorz.de>
Date:   Mon Jan 11 19:12:24 2021 +0100

    Implement Global::adapter_get_swap_chain_preferred_format

commit 63efae1cff508d060eaa6ccafe80c02af6af21f4
Author: Niklas Korz <niklas@niklaskorz.de>
Date:   Mon Jan 11 17:25:42 2021 +0100

    Move get_swap_chain_preferred_format to adapter

commit 1e72a876994820aec9b18f9dac7f1a9ae82d7cee
Author: Niklas Korz <niklas@niklaskorz.de>
Date:   Mon Jan 11 15:49:10 2021 +0100

    Add fallback to device_get_swap_chain_preferred_format

wgpu-core/src/device/mod.rs

@@ -8,7 +8,7 @@ use crate::{
     hub::{
         GfxBackend, Global, GlobalIdentityHandlerFactory, Hub, Input, InvalidId, Storage, Token,
     },
-    id, pipeline, resource, span, swap_chain,
+    id, instance, pipeline, resource, span, swap_chain,
     track::{BufferState, TextureSelector, TextureState, TrackerSet},
     validation::{self, check_buffer_usage, check_texture_usage},
     FastHashMap, FastHashSet, Label, LabelHelpers, LifeGuard, MultiRefCount, PrivateFeatures,
@@ -2379,6 +2379,28 @@ pub struct ImplicitPipelineIds<'a, G: GlobalIdentityHandlerFactory> {
 }
 
 impl<G: GlobalIdentityHandlerFactory> Global<G> {
+    pub fn adapter_get_swap_chain_preferred_format<B: GfxBackend>(
+        &self,
+        adapter_id: id::AdapterId,
+        surface_id: id::SurfaceId,
+    ) -> Result<TextureFormat, instance::GetSwapChainPreferredFormatError> {
+        span!(_guard, INFO, "Adapter::get_swap_chain_preferred_format");
+
+        let hub = B::hub(self);
+        let mut token = Token::root();
+
+        let (mut surface_guard, mut token) = self.surfaces.write(&mut token);
+        let (adapter_guard, mut _token) = hub.adapters.read(&mut token);
+        let adapter = adapter_guard
+            .get(adapter_id)
+            .map_err(|_| instance::GetSwapChainPreferredFormatError::InvalidAdapter)?;
+        let surface = surface_guard
+            .get_mut(surface_id)
+            .map_err(|_| instance::GetSwapChainPreferredFormatError::InvalidSurface)?;
+
+        adapter.get_swap_chain_preferred_format(surface)
+    }
+
     pub fn device_features<B: GfxBackend>(
         &self,
         device_id: id::DeviceId,
@@ -3885,17 +3907,6 @@ impl<G: GlobalIdentityHandlerFactory> Global<G> {
             .push(layout_id);
     }
 
-    pub fn device_get_swap_chain_preferred_format<B: GfxBackend>(
-        &self,
-        _device_id: id::DeviceId,
-    ) -> Result<TextureFormat, InvalidDevice> {
-        span!(_guard, INFO, "Device::get_swap_chain_preferred_format");
-        //TODO: we can query the formats like done in `device_create_swapchain`,
-        // but its not clear which format in the list to return.
-        // For now, return `Bgra8UnormSrgb` that we know is supported everywhere.
-        Ok(TextureFormat::Bgra8UnormSrgb)
-    }
-
     pub fn device_create_swap_chain<B: GfxBackend>(
         &self,
         device_id: id::DeviceId,

wgpu-core/src/instance.rs

@@ -250,6 +250,44 @@ impl<B: GfxBackend> Adapter<B> {
         }
     }
 
+    pub fn get_swap_chain_preferred_format(
+        &self,
+        surface: &mut Surface,
+    ) -> Result<wgt::TextureFormat, GetSwapChainPreferredFormatError> {
+        span!(_guard, INFO, "Adapter::get_swap_chain_preferred_format");
+
+        let formats = {
+            let surface = B::get_surface_mut(surface);
+            let queue_family = &self.raw.queue_families[0];
+            if !surface.supports_queue_family(queue_family) {
+                return Err(GetSwapChainPreferredFormatError::UnsupportedQueueFamily);
+            }
+            surface.supported_formats(&self.raw.physical_device)
+        };
+        if let Some(formats) = formats {
+            // Check the four formats mentioned in the WebGPU spec:
+            // Bgra8UnormSrgb, Rgba8UnormSrgb, Bgra8Unorm, Rgba8Unorm
+            // Also, prefer sRGB over linear as it is better in
+            // representing perceived colors.
+            if formats.contains(&hal::format::Format::Bgra8Srgb) {
+                return Ok(wgt::TextureFormat::Bgra8UnormSrgb);
+            }
+            if formats.contains(&hal::format::Format::Rgba8Srgb) {
+                return Ok(wgt::TextureFormat::Rgba8UnormSrgb);
+            }
+            if formats.contains(&hal::format::Format::Bgra8Unorm) {
+                return Ok(wgt::TextureFormat::Bgra8Unorm);
+            }
+            if formats.contains(&hal::format::Format::Rgba8Unorm) {
+                return Ok(wgt::TextureFormat::Rgba8Unorm);
+            }
+            return Err(GetSwapChainPreferredFormatError::NotFound);
+        }
+
+        // If no formats were returned, use Bgra8UnormSrgb
+        Ok(wgt::TextureFormat::Bgra8UnormSrgb)
+    }
+
     pub(crate) fn get_texture_format_features(
         &self,
         format: wgt::TextureFormat,
@@ -448,6 +486,18 @@ impl<B: hal::Backend> crate::hub::Resource for Adapter<B> {
     }
 }
 
+#[derive(Clone, Debug, Error)]
+pub enum GetSwapChainPreferredFormatError {
+    #[error("no suitable format found")]
+    NotFound,
+    #[error("invalid adapter")]
+    InvalidAdapter,
+    #[error("invalid surface")]
+    InvalidSurface,
+    #[error("surface does not support the adapter's queue family")]
+    UnsupportedQueueFamily,
+}
+
 #[derive(Clone, Debug, Error)]
 /// Error when requesting a device from the adaptor
 pub enum RequestDeviceError {