[fbdev] Added a mode that should not waste time in FBIO_WAITFORVSYNC (#17124)

* [fbdev] Added a mode that should not waste time in FBIO_WAITFORVSYNC

* Log

src/Linux/Avalonia.LinuxFramebuffer/Output/FbDevBackBuffer.cs

@@ -3,6 +3,7 @@
 using System;
 using System.Runtime.InteropServices;
 using System.Threading;
+using Avalonia.Logging;
 using Avalonia.Platform;
 
 namespace Avalonia.LinuxFramebuffer.Output
@@ -14,14 +15,76 @@ namespace Avalonia.LinuxFramebuffer.Output
         private readonly fb_var_screeninfo _varInfo;
         private readonly IntPtr _targetAddress;
         private readonly object _lock = new object();
+        private readonly AsyncFbBlitter? _asyncBlit;
 
-        public FbDevBackBuffer(int fb, fb_fix_screeninfo fixedInfo, fb_var_screeninfo varInfo, IntPtr targetAddress)
+        class AsyncFbBlitter : IDisposable
+        {
+            private readonly FbDevBackBuffer _fb;
+            private AutoResetEvent _signalToThread = new AutoResetEvent(false);
+            private ManualResetEvent _transferCompleted = new ManualResetEvent(true);
+            private Thread _thread;
+            private volatile bool _exit;
+
+            public AsyncFbBlitter(FbDevBackBuffer fb)
+            {
+                _fb = fb;
+                _thread = new Thread(Worker)
+                {
+                    IsBackground = true,
+                    Name = "FbDevBackBuffer::AsyncBlitter",
+                    Priority = ThreadPriority.Highest
+                };
+                _thread.Start();
+            }
+
+            private void Worker()
+            {
+                while (true)
+                {
+                    _signalToThread.WaitOne();
+                    if (_exit)
+                        return;
+                    try
+                    {
+                        _fb.BlitToDevice();
+                    }
+                    catch(Exception e)
+                    {
+                        Logger.TryGet(LogEventLevel.Fatal, "FBDEV")?.Log(this, "Unable to update framebuffer: " + e);
+                    }
+
+                    _transferCompleted.Set();
+                }
+            }
+            
+            public void Dispose()
+            {
+                _exit = true;
+                _signalToThread.Set();
+                _thread.Join();
+                _signalToThread.Dispose();
+                _transferCompleted.Dispose();
+            }
+
+            public void WaitForTransfer() => _transferCompleted.WaitOne();
+
+            public void BeginBlit()
+            {
+                _transferCompleted.Reset();
+                _signalToThread.Set();
+            }
+        }
+
+        public FbDevBackBuffer(int fb, fb_fix_screeninfo fixedInfo, fb_var_screeninfo varInfo, IntPtr targetAddress,
+            bool asyncBlit)
         {
             _fb = fb;
             _fixedInfo = fixedInfo;
             _varInfo = varInfo;
             _targetAddress = targetAddress;
             Address = Marshal.AllocHGlobal(RowBytes * Size.Height);
+            if (asyncBlit)
+                _asyncBlit = new AsyncFbBlitter(this);
         }
         
 
@@ -32,6 +95,9 @@ namespace Avalonia.LinuxFramebuffer.Output
                 Marshal.FreeHGlobal(Address);
                 Address = IntPtr.Zero;
             }
+
+            if (_asyncBlit != null)
+                _asyncBlit.Dispose();
         }
 
         public static LockedFramebuffer LockFb(IntPtr address, fb_var_screeninfo varInfo,
@@ -45,8 +111,15 @@ namespace Avalonia.LinuxFramebuffer.Output
                 : PixelFormat.Bgra8888, dispose);
         }
 
+        private void BlitToDevice()
+        {
+            NativeUnsafeMethods.ioctl(_fb, FbIoCtl.FBIO_WAITFORVSYNC, null);
+            NativeUnsafeMethods.memcpy(_targetAddress, Address, new IntPtr(RowBytes * Size.Height));
+        }
+
         public ILockedFramebuffer Lock(Vector dpi)
         {
+            _asyncBlit?.WaitForTransfer();
             Monitor.Enter(_lock);
             try
             {
@@ -55,8 +128,10 @@ namespace Avalonia.LinuxFramebuffer.Output
                     {
                         try
                         {
-                            NativeUnsafeMethods.ioctl(_fb, FbIoCtl.FBIO_WAITFORVSYNC, null);
-                            NativeUnsafeMethods.memcpy(_targetAddress, Address, new IntPtr(RowBytes * Size.Height));
+                            if (_asyncBlit != null)
+                                _asyncBlit.BeginBlit();
+                            else
+                                BlitToDevice();
                         }
                         finally
                         {

src/Linux/Avalonia.LinuxFramebuffer/Output/FbDevOutputOptions.cs

@@ -29,4 +29,10 @@ public class FbDevOutputOptions
     /// The initial scale factor to use
     /// </summary>
     public double Scaling { get; set; } = 1;
+
+    /// <summary>
+    /// If set to true, FBIO_WAITFORVSYNC ioctl and following memcpy call will run on a dedicated thread
+    /// saving current one from doing nothing in a blocking call
+    /// </summary>
+    public bool? UseAsyncFrontBufferBlit { get; set; }
 }

src/Linux/Avalonia.LinuxFramebuffer/Output/FbdevOutput.cs

@@ -181,7 +181,7 @@ namespace Avalonia.LinuxFramebuffer
             _lockedAtLeastOnce = true;
             properties = new FramebufferLockProperties(retained);
             return (_backBuffer ??=
-                    new FbDevBackBuffer(_fd, _fixedInfo, _varInfo, _mappedAddress))
+                    new FbDevBackBuffer(_fd, _fixedInfo, _varInfo, _mappedAddress, _options.UseAsyncFrontBufferBlit == true))
                 .Lock(new Vector(96, 96) * Scaling);
         }