From 656ab7fd8e83c52137fe2a8ae7f6793e6d93b802 Mon Sep 17 00:00:00 2001
From: valaphee <32491319+valaphee@users.noreply.github.com>
Date: Sun, 15 Sep 2024 12:43:27 +0200
Subject: [PATCH] Implement priority hints for EGL

---
 glutin/src/api/egl/context.rs | 21 ++++++++++++++++++++-
 glutin/src/context.rs         | 31 +++++++++++++++++++++++++++++++
 glutin_egl_sys/build.rs       |  2 ++
 3 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/glutin/src/api/egl/context.rs b/glutin/src/api/egl/context.rs
index 7647e43347..0a48d8a815 100644
--- a/glutin/src/api/egl/context.rs
+++ b/glutin/src/api/egl/context.rs
@@ -9,7 +9,8 @@ use glutin_egl_sys::{egl, EGLContext};
 
 use crate::config::{Api, GetGlConfig};
 use crate::context::{
-    self, AsRawContext, ContextApi, ContextAttributes, GlProfile, RawContext, Robustness, Version,
+    self, AsRawContext, ContextApi, ContextAttributes, GlProfile, Priority, RawContext, Robustness,
+    Version,
 };
 use crate::display::{DisplayFeatures, GetGlDisplay};
 use crate::error::{ErrorKind, Result};
@@ -54,6 +55,24 @@ impl Display {
             },
         };
 
+        if context_attributes.priority != Priority::Medium
+            && self.inner.display_extensions.contains("EGL_IMG_context_priority")
+        {
+            attrs.push(egl::CONTEXT_PRIORITY_LEVEL_IMG as EGLint);
+            attrs.push(match context_attributes.priority {
+                Priority::Low => egl::CONTEXT_PRIORITY_LOW_IMG,
+                Priority::High => egl::CONTEXT_PRIORITY_HIGH_IMG,
+                Priority::Realtime => {
+                    if self.inner.display_extensions.contains("EGL_NV_context_priority_realtime") {
+                        egl::CONTEXT_PRIORITY_REALTIME_NV
+                    } else {
+                        egl::CONTEXT_PRIORITY_HIGH_IMG
+                    }
+                },
+                _ => unreachable!(),
+            } as EGLint);
+        }
+
         let is_one_five = self.inner.version >= Version::new(1, 5);
         if is_one_five || self.inner.display_extensions.contains("EGL_KHR_create_context") {
             let mut flags = 0;
diff --git a/glutin/src/context.rs b/glutin/src/context.rs
index 7d14582fef..1179c897b7 100644
--- a/glutin/src/context.rs
+++ b/glutin/src/context.rs
@@ -201,6 +201,19 @@ impl ContextAttributesBuilder {
         self
     }
 
+    /// Sets the priority hint, which might not be honored if the API does not support it,
+    /// if there are constraints on the number of high priority contexts available in the
+    /// system, or system policy limits access to high priority contexts to
+    /// appropriate system privilege level
+    ///
+    /// # Api-specific
+    ///
+    /// - Only EGL at the moment implements context priorities.
+    pub fn with_priority(mut self, priority: Priority) -> Self {
+        self.attributes.priority = priority;
+        self
+    }
+
     /// Build the context attributes.
     ///
     /// The `raw_window_handle` isn't required and here for WGL compatibility.
@@ -228,6 +241,8 @@ pub struct ContextAttributes {
 
     pub(crate) api: Option<ContextApi>,
 
+    pub(crate) priority: Priority,
+
     pub(crate) shared_context: Option<RawContext>,
 
     pub(crate) raw_window_handle: Option<RawWindowHandle>,
@@ -608,6 +623,22 @@ pub enum RawContext {
     Cgl(*const ffi::c_void),
 }
 
+/// Priority hint
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Default)]
+pub enum Priority {
+    /// Lowest priority, contexts using this priority give way for most other
+    /// contexts.
+    Low,
+    /// Default priority.
+    #[default]
+    Medium,
+    /// High priority is usually required for VR applications.
+    High,
+    /// Realtime priority contexts are executed immediately and preempt any
+    /// current context running. But will fallback to high if not supported.
+    Realtime,
+}
+
 /// Pick `GlProfile` and `Version` based on the provided params.
 #[cfg(any(egl_backend, glx_backend, wgl_backend))]
 pub(crate) fn pick_profile(
diff --git a/glutin_egl_sys/build.rs b/glutin_egl_sys/build.rs
index c8dbedf87a..3915585143 100644
--- a/glutin_egl_sys/build.rs
+++ b/glutin_egl_sys/build.rs
@@ -35,6 +35,7 @@ fn main() {
             "EGL_EXT_platform_wayland",
             "EGL_EXT_platform_x11",
             "EGL_EXT_swap_buffers_with_damage",
+            "EGL_IMG_context_priority",
             "EGL_KHR_create_context",
             "EGL_KHR_create_context_no_error",
             "EGL_KHR_display_reference",
@@ -47,6 +48,7 @@ fn main() {
             "EGL_KHR_swap_buffers_with_damage",
             "EGL_KHR_wait_sync",
             "EGL_MESA_platform_gbm",
+            "EGL_NV_context_priority_realtime",
         ]);
 
         if target.contains("ios") {