[FBDEV] Reuse the same memory region for the back buffer

src/Linux/Avalonia.LinuxFramebuffer/LockedFramebuffer.cs

@@ -1,46 +0,0 @@
-using System;
-using System.Runtime.InteropServices;
-using Avalonia.Platform;
-
-namespace Avalonia.LinuxFramebuffer
-{
-    unsafe class LockedFramebuffer : ILockedFramebuffer
-    {
-        private readonly int _fb;
-        private readonly fb_fix_screeninfo _fixedInfo;
-        private fb_var_screeninfo _varInfo;
-        private readonly IntPtr _address;
-
-        public LockedFramebuffer(int fb, fb_fix_screeninfo fixedInfo, fb_var_screeninfo varInfo, IntPtr address, Vector dpi)
-        {
-            _fb = fb;
-            _fixedInfo = fixedInfo;
-            _varInfo = varInfo;
-            _address = address;
-            Dpi = dpi;
-            //Use double buffering to avoid flicker
-            Address = Marshal.AllocHGlobal(RowBytes * Size.Height);
-        }
-
-
-        void VSync()
-        {
-            NativeUnsafeMethods.ioctl(_fb, FbIoCtl.FBIO_WAITFORVSYNC, null);
-        }
-
-        public void Dispose()
-        {
-            VSync();
-            NativeUnsafeMethods.memcpy(_address, Address, new IntPtr(RowBytes * Size.Height));
-
-            Marshal.FreeHGlobal(Address);
-            Address = IntPtr.Zero;
-        }
-
-        public IntPtr Address { get; private set; }
-        public PixelSize Size => new PixelSize((int)_varInfo.xres, (int) _varInfo.yres);
-        public int RowBytes => (int) _fixedInfo.line_length;
-        public Vector Dpi { get; }
-        public PixelFormat Format => _varInfo.bits_per_pixel == 16 ? PixelFormat.Rgb565 : _varInfo.blue.offset == 16 ? PixelFormat.Rgba8888 : PixelFormat.Bgra8888;
-    }
-}

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

@@ -0,0 +1,70 @@
+using System;
+using System.Runtime.InteropServices;
+using System.Threading;
+using Avalonia.Platform;
+
+namespace Avalonia.LinuxFramebuffer.Output
+{
+    internal unsafe class FbDevBackBuffer : IDisposable
+    {
+        private readonly int _fb;
+        private readonly fb_fix_screeninfo _fixedInfo;
+        private readonly fb_var_screeninfo _varInfo;
+        private readonly IntPtr _targetAddress;
+        private readonly object _lock = new object();
+
+        public FbDevBackBuffer(int fb, fb_fix_screeninfo fixedInfo, fb_var_screeninfo varInfo, IntPtr targetAddress)
+        {
+            _fb = fb;
+            _fixedInfo = fixedInfo;
+            _varInfo = varInfo;
+            _targetAddress = targetAddress;
+            Address = Marshal.AllocHGlobal(RowBytes * Size.Height);
+        }
+        
+
+        public void Dispose()
+        {
+            if (Address != IntPtr.Zero)
+            {
+                Marshal.FreeHGlobal(Address);
+                Address = IntPtr.Zero;
+            }
+        }
+
+        public ILockedFramebuffer Lock(Vector dpi)
+        {
+            Monitor.Enter(_lock);
+            try
+            {
+                return new LockedFramebuffer(Address,
+                    new PixelSize((int)_varInfo.xres, (int)_varInfo.yres),
+                    (int)_fixedInfo.line_length, dpi,
+                    _varInfo.bits_per_pixel == 16 ? PixelFormat.Rgb565
+                    : _varInfo.blue.offset == 16 ? PixelFormat.Rgba8888
+                    : PixelFormat.Bgra8888,
+                    () =>
+                    {
+                        try
+                        {
+                            NativeUnsafeMethods.ioctl(_fb, FbIoCtl.FBIO_WAITFORVSYNC, null);
+                            NativeUnsafeMethods.memcpy(_targetAddress, Address, new IntPtr(RowBytes * Size.Height));
+                        }
+                        finally
+                        {
+                            Monitor.Exit(_lock);
+                        }
+                    });
+            }
+            catch
+            {
+                Monitor.Exit(_lock);
+                throw;
+            }
+        }
+
+        public IntPtr Address { get; private set; }
+        public PixelSize Size => new PixelSize((int)_varInfo.xres, (int) _varInfo.yres);
+        public int RowBytes => (int) _fixedInfo.line_length;
+    }
+}

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

@@ -14,6 +14,7 @@ namespace Avalonia.LinuxFramebuffer
         private fb_var_screeninfo _varInfo;
         private IntPtr _mappedLength;
         private IntPtr _mappedAddress;
+        private FbDevBackBuffer _backBuffer;
         public double Scaling { get; set; }
 
         /// <summary>
@@ -146,7 +147,9 @@ namespace Avalonia.LinuxFramebuffer
         {
             if (_fd <= 0)
                 throw new ObjectDisposedException("LinuxFramebuffer");
-            return new LockedFramebuffer(_fd, _fixedInfo, _varInfo, _mappedAddress, new Vector(96, 96) * Scaling);
+            return (_backBuffer ??=
+                    new FbDevBackBuffer(_fd, _fixedInfo, _varInfo, _mappedAddress))
+                .Lock(new Vector(96, 96) * Scaling);
         }
 
 
@@ -165,6 +168,8 @@ namespace Avalonia.LinuxFramebuffer
 
         public void Dispose()
         {
+            _backBuffer?.Dispose();
+            _backBuffer = null;
             ReleaseUnmanagedResources();
             GC.SuppressFinalize(this);
         }