feat: power down dGPUs on hybrid graphics laptops

kernel/src/Timekeeping/ApicTimer.cpp

@@ -365,6 +365,35 @@ namespace Timekeeping {
         Hal::HaltWithInterruptsDisabled();
     }
 
+    void BusyWaitUs(uint64_t us) {
+        // An I/O write to port 0x80 is an ISA-bus cycle of roughly a
+        // microsecond on every chipset since the 486, and never shorter.
+        for (uint64_t i = 0; i < us; i++)
+            Io::IoPortWait();
+    }
+
+    void BusyWaitMs(uint64_t ms) {
+        if (ms == 0) return;
+
+        uint64_t flags;
+        asm volatile("pushfq; pop %0" : "=r"(flags));
+        bool ticking = g_ticksPerMs != 0 && (flags & (1 << 9));
+
+        // Either clock alone is long enough, so stop at whichever says so
+        // first. The tick is the accurate one; the port writes bound the wait
+        // when the tick is not running yet (or is masked), so this can never
+        // hang on a timer that does not fire. One extra tick of slack because
+        // the first may already be nearly over.
+        uint64_t start = g_tickCount.load(std::memory_order_relaxed);
+        uint64_t portBudget = (ms + 1) * 1000;
+        for (uint64_t spins = 0; spins < portBudget; spins++) {
+            if (ticking &&
+                g_tickCount.load(std::memory_order_relaxed) - start >= ms + 1)
+                return;
+            Io::IoPortWait();
+        }
+    }
+
     void Sleep(uint64_t ms) {
         if (ms == 0) return;
         // Use BlockForSleep to properly deschedule the process instead