arm: Workaround incorrect HDLCD register order in kernel

Some versions of the kernel incorrectly swap the red and blue color
select registers. This changeset adds a workaround for that by
swapping them when instantiating a PixelConverter.

src/dev/arm/RealView.py

@@ -175,6 +175,8 @@ class HDLcd(AmbaDmaDevice):
     vnc = Param.VncInput(Parent.any, "Vnc server for remote frame buffer "
                                      "display")
     amba_id = 0x00141000
+    workaround_swap_rb = Param.Bool(True, "Workaround incorrect color "
+                                    "selector order in some kernels")
     enable_capture = Param.Bool(True, "capture frame to system.framebuffer.bmp")
 
 class RealView(Platform):

src/dev/arm/hdlcd.cc

@@ -74,7 +74,8 @@ HDLcd::HDLcd(const Params *p)
       fillPixelBufferEvent(this), intEvent(this),
       dmaDoneEventAll(MAX_OUTSTANDING_DMA_REQ_CAPACITY, this),
       dmaDoneEventFree(MAX_OUTSTANDING_DMA_REQ_CAPACITY),
-      enableCapture(p->enable_capture)
+      enableCapture(p->enable_capture),
+      workaround_swap_rb(p->workaround_swap_rb)
 {
     pioSize = 0xFFFF;
 
@@ -504,11 +505,24 @@ HDLcd::renderPixel()
 PixelConverter
 HDLcd::pixelConverter() const
 {
-    return PixelConverter(
-        bytesPerPixel(),
-        red_select.offset, green_select.offset, blue_select.offset,
-        red_select.size, green_select.size, blue_select.size,
+    ByteOrder byte_order(
         pixel_format.big_endian ? BigEndianByteOrder : LittleEndianByteOrder);
+
+    /* Some Linux kernels have a broken driver that swaps the red and
+     * blue color select registers. */
+    if (!workaround_swap_rb) {
+        return PixelConverter(
+            bytesPerPixel(),
+            red_select.offset, green_select.offset, blue_select.offset,
+            red_select.size, green_select.size, blue_select.size,
+            byte_order);
+    } else {
+        return PixelConverter(
+            bytesPerPixel(),
+            blue_select.offset, green_select.offset, red_select.offset,
+            blue_select.size, green_select.size, red_select.size,
+            byte_order);
+    }
 }
 
 void

src/dev/arm/hdlcd.hh

@@ -486,6 +486,8 @@ class HDLcd: public AmbaDmaDevice
 
     bool enableCapture;
 
+    const bool workaround_swap_rb;
+
   public:
     typedef HDLcdParams Params;