diff --git a/12to11.conf b/12to11.conf
index cb7f974..220e48f 100644
--- a/12to11.conf
+++ b/12to11.conf
@@ -10,6 +10,7 @@
          XCBLIB = -lX11-xcb
          PIXMAN = -lpixman-1
             DRM = -ldrm
+     DRMFOURCCH = $(INCROOT)/drm/drm_fourcc.h
     DRMINCLUDES = -I$(INCROOT)/drm
  PIXMANINCLUDES = -I$(INCROOT)/pixman-1
     XPRESENTLIB = -lXpresent
diff --git a/12to11.man b/12to11.man
index dd7c83b..2a4fb56 100644
--- a/12to11.man
+++ b/12to11.man
@@ -160,6 +160,23 @@ the first process exits.
 The number of seconds the protocol translator waits between
 invocations of the command specified in \fBidleIntervalCommand\fP.
 Defaults to 60.
+.TP
+.B additionalModifiersOfScreen\fP\fINUMBER\fP (class \fBAdditionalModifiers\fP)
+A comma-separated list of additional DRM modifiers that can be applied
+to each format.  These modifiers must be specified by their names
+inside the `drm_fourcc.h' system header, and should be used if your X
+graphics driver fails to report which modifiers are available.
+\fINUMBER\fP should be replaced with the number of the screen on which
+the protocol translator is running.
+
+On 9th Generation Intel systems, for example, you may want to set this
+to:
+.IP
+.in +4
+.EX
+\fB12to11.AdditionalModifiers: I915_FORMAT_MOD_X_TILED,DRM_FORMAT_MOD_LINEAR\fP
+.EE
+.in
 .IP
 .SH ENVIRONMENT
 Several environment variables exist that modify the behavior of the
diff --git a/Imakefile b/Imakefile
index 59363eb..a6db4f9 100644
--- a/Imakefile
+++ b/Imakefile
@@ -39,7 +39,7 @@ OBJS = 12to11.o run.o alloc.o fns.o output.o compositor.o       \
   time.o relative_pointer.o keyboard_shortcuts_inhibit.o        \
   idle_inhibit.o process.o fence_ring.o
 
-GENHEADERS = transfer_atoms.h
+GENHEADERS = transfer_atoms.h drm_modifiers.h
 
 #ifdef HaveEglSupport
 
@@ -98,6 +98,9 @@ transfer_atoms.h: short_types.txt mime0.awk mime1.awk mime2.awk mime3.awk \
 	awk -f mime3.awk short_types.txt >> $@
 	awk -f mime4.awk short_types.txt >> $@
 
+drm_modifiers.h: modifiers.awk $(DRMFOURCCH)
+	awk -f modifiers.awk $(DRMFOURCCH) > $@
+
 /* Now, define generated files.  */
 
 #define ScannerTarget(name)                                   @@\
@@ -134,8 +137,7 @@ $(OBJS): $(GENHEADERS)
 depend:: $(GENHEADERS) $(SRCS)
 
 cleandir::
-	$(RM) $(GENHEADERS) $(GENSRCS) transfer_atoms.h short_types.txt \
-	      12to11.tar.gz
+	$(RM) $(GENHEADERS) $(GENSRCS) short_types.txt 12to11.tar.gz
 
 /* Undefine _BSD_SOURCE and _SVID_SOURCE, since both are deprecated
    and are also superseeded by _GNU_SOURCE.  */
diff --git a/compositor.h b/compositor.h
index 7916b37..b8ebee1 100644
--- a/compositor.h
+++ b/compositor.h
@@ -1695,6 +1695,11 @@ extern void MatrixExport (Matrix *, XTransform *);
 extern void MatrixRotate (Matrix *, float, float, float);
 extern void MatrixMirrorHorizontal (Matrix *, float);
 
+extern void ApplyInverseTransform (int, int, Matrix *,
+				   BufferTransform, Bool);
+extern void TransformBox (pixman_box32_t *, BufferTransform, int, int);
+extern BufferTransform InvertTransform (BufferTransform);
+
 /* Defined in wp_viewporter.c.  */
 
 extern void XLInitWpViewporter (void);
diff --git a/fns.c b/fns.c
index ca3e44d..0fad5e7 100644
--- a/fns.c
+++ b/fns.c
@@ -403,71 +403,6 @@ XLExtendRegion (pixman_region32_t *dst, pixman_region32_t *src,
     XLFree (dst_rects);
 }
 
-static void
-TransformBox (pixman_box32_t *box, BufferTransform transform,
-	      int width, int height)
-{
-  pixman_box32_t work;
-
-  switch (transform)
-    {
-    case Normal:
-      work = *box;
-      break;
-
-    case CounterClockwise90:
-      work.x1 = height - box->y2;
-      work.y1 = box->x1;
-      work.x2 = height - box->y1;
-      work.y2 = box->x2;
-      break;
-
-    case CounterClockwise180:
-      work.x1 = width - box->x2;
-      work.y1 = height - box->y2;
-      work.x2 = width - box->x1;
-      work.y2 = height - box->y1;
-      break;
-
-    case CounterClockwise270:
-      work.x1 = box->y1;
-      work.y1 = width - box->x2;
-      work.x2 = box->y2;
-      work.y2 = width - box->x1;
-      break;
-
-    case Flipped:
-      work.x1 = width - box->x2;
-      work.y1 = box->y1;
-      work.x2 = width - box->x1;
-      work.y2 = box->y2;
-      break;
-
-    case Flipped90:
-      work.x1 = box->y1;
-      work.y1 = box->x1;
-      work.x2 = box->y2;
-      work.y2 = box->x2;
-      break;
-
-    case Flipped180:
-      work.x1 = box->x1;
-      work.y1 = height - box->y2;
-      work.x2 = box->x2;
-      work.y2 = height - box->y1;
-      break;
-
-    case Flipped270:
-      work.x1 = height - box->y2;
-      work.y1 = width - box->x2;
-      work.x2 = height - box->y1;
-      work.y2 = width - box->x1;
-      break;
-    }
-
-  *box = work;
-}
-
 void
 XLTransformRegion (pixman_region32_t *dst, pixman_region32_t *src,
 		   BufferTransform transform, int width, int height)
diff --git a/frame_clock.c b/frame_clock.c
index 70e92e1..48933a5 100644
--- a/frame_clock.c
+++ b/frame_clock.c
@@ -353,9 +353,9 @@ PostEndFrame (FrameClock *clock)
   /* The vertical blanking period itself can't actually be computed
      based on available data.  However, frame_delay must be inside the
      vertical blanking period for it to make any sense, so use it to
-     compute the deadline instead.  Add about 100 us to the frame
+     compute the deadline instead.  Add about 200 us to the frame
      delay to compensate for the roundtrip time.  */
-  target -= clock->frame_delay - 100;
+  target -= clock->frame_delay - 200;
 
   /* Add the remainder of now if it was probably truncated by the
      compositor.  */
diff --git a/picture_renderer.c b/picture_renderer.c
index b16cffd..8aafd29 100644
--- a/picture_renderer.c
+++ b/picture_renderer.c
@@ -26,6 +26,7 @@ along with 12to11.  If not, see <https://www.gnu.org/licenses/>.  */
 #include <drm_fourcc.h>
 
 #include "compositor.h"
+#include "drm_modifiers.h"
 
 #include <xcb/dri3.h>
 
@@ -37,6 +38,7 @@ along with 12to11.  If not, see <https://www.gnu.org/licenses/>.  */
 
 typedef struct _DrmFormatInfo DrmFormatInfo;
 typedef struct _DmaBufRecord DmaBufRecord;
+typedef struct _DrmModifierName DrmModifierName;
 
 typedef struct _PictureBuffer PictureBuffer;
 typedef struct _PictureTarget PictureTarget;
@@ -46,6 +48,15 @@ typedef struct _BufferActivityRecord BufferActivityRecord;
 typedef struct _IdleCallback IdleCallback;
 typedef struct _PresentCompletionCallback PresentCompletionCallback;
 
+struct _DrmModifierName
+{
+  /* The modifier name.  */
+  const char *name;
+
+  /* The modifier code.  */
+  uint64_t modifier;
+};
+
 /* Structure describing an expected PresentIdleNotify from the X
    server.  */
 
@@ -277,6 +288,12 @@ struct _DmaBufRecord
   short width, height;
 };
 
+/* Number of format modifiers specified by the user.  */
+static int num_specified_modifiers;
+
+/* Array of user-specified format modifiers.  */
+static uint64_t *user_specified_modifiers;
+
 /* Hash table mapping between presentation windows and targets.  */
 static XLAssocTable *xid_table;
 
@@ -360,6 +377,13 @@ static DrmFormatInfo all_formats[] =
     },
   };
 
+/* Array of all known DRM modifier names.  */
+static DrmModifierName known_modifiers[] =
+  {
+    /* Generated from drm_fourcc.h.  */
+    DrmModifiersList
+  };
+
 /* DRM formats reported to the caller.  */
 static DrmFormat *drm_formats;
 
@@ -741,6 +765,112 @@ InitSynchronizedPresentation (void)
     use_direct_presentation = True;
 }
 
+static void
+AddAdditionalModifier (const char *name)
+{
+  int i, j;
+
+  for (i = 0; i < ArrayElements (known_modifiers) - 1; ++i)
+    {
+      if (!strcmp (known_modifiers[i].name, name))
+	{
+	  /* The modifier was found.  See if it already exists.  */
+	  for (j = 0; j < num_specified_modifiers; ++j)
+	    {
+	      if (user_specified_modifiers[j]
+		  == known_modifiers[i].modifier)
+		/* The modifier was already specified.  */
+		return;
+	    }
+
+	  /* Otherwise, increment num_specified_modifiers.  */
+	  num_specified_modifiers++;
+
+	  /* Make user_specified_modifiers big enough.  */
+	  user_specified_modifiers
+	    = XLRealloc (user_specified_modifiers,
+			 num_specified_modifiers
+			 * sizeof *user_specified_modifiers);
+
+	  /* And add the modifier.  */
+	  user_specified_modifiers[num_specified_modifiers - 1]
+	    = known_modifiers[i].modifier;
+	  return;
+	}
+    }
+
+  fprintf (stderr, "Unknown buffer format modifier: %s\n", name);
+}
+
+static void
+ParseAdditionalModifiers (const char *string)
+{
+  const char *end, *sep;
+  char *buffer;
+
+  end = string + strlen (string);
+
+  while (string < end)
+    {
+      /* Find the next comma.  */
+      sep = strchr (string, ',');
+
+      if (!sep)
+	sep = end;
+
+      /* Copy the text between string and sep into buffer.  */
+      buffer = alloca (sep - string + 1);
+      memcpy (buffer, string, sep - string);
+      buffer[sep - string] = '\0';
+
+      /* Add this modifier.  */
+      AddAdditionalModifier (buffer);
+
+      string = sep + 1;
+    }
+}
+
+static void
+InitAdditionalModifiers (void)
+{
+  XrmDatabase rdb;
+  XrmName namelist[3];
+  XrmClass classlist[3];
+  XrmValue value;
+  XrmRepresentation type;
+  char *name;
+
+  rdb = XrmGetDatabase (compositor.display);
+
+  if (!rdb)
+    return;
+
+  if (!asprintf (&name, "additionalModifiersOfScreen%d",
+		 DefaultScreen (compositor.display)))
+    return;
+
+  namelist[1] = XrmStringToQuark (name);
+  free (name);
+
+  namelist[0] = app_quark;
+  namelist[2] = NULLQUARK;
+
+  classlist[1] = XrmStringToQuark ("AdditionalModifiers");
+  classlist[0] = resource_quark;
+  classlist[2] = NULLQUARK;
+
+  /* Enable the use of direct presentation if
+     *.UseDirectPresentation.*.useDirectPresentation is true.  This is
+     still incomplete, as the features necessary for it to play nice
+     with frame synchronization have not yet been implemented in the X
+     server.  */
+
+  if (XrmQGetResource (rdb, namelist, classlist,
+		       &type, &value)
+      && type == QString)
+    ParseAdditionalModifiers ((const char *) value.addr);
+}
+
 /* Forward declaration.  */
 static void AddRenderFlag (int);
 
@@ -769,6 +899,9 @@ InitRenderFuncs (void)
      presentation.  */
   InitSynchronizedPresentation ();
 
+  /* Find out what additional modifiers the user wants.  */
+  InitAdditionalModifiers ();
+
   if (use_direct_presentation)
     AddRenderFlag (SupportsDirectPresent);
 
@@ -1166,82 +1299,6 @@ CompareStretch (DrawParams *params, DrawParams *other)
   return True;
 }
 
-static void
-ApplyInverseTransform (PictureBuffer *buffer, Matrix *matrix,
-		       BufferTransform transform)
-{
-  float width, height;
-
-  /* Note that the transform is applied in reverse, meaning that a
-     counterclockwise rotation is done clockwise, etc, as TRANSFORM
-     transforms destination coordinates to source ones.  */
-
-  width = buffer->width;
-  height = buffer->height;
-
-  switch (transform)
-    {
-    case Normal:
-      break;
-
-    case CounterClockwise90:
-      /* Apply clockwise 270 degree rotation around the origin.  */
-      MatrixRotate (matrix, M_PI * 1.5, 0, 0);
-
-      /* Translate y by the width.  */
-      MatrixTranslate (matrix, 0, -width);
-      break;
-
-    case CounterClockwise180:
-      /* Apply clockwise 180 degree rotation around the center.  */
-      MatrixRotate (matrix, M_PI, width / 2.0f, height / 2.0f);
-      break;
-
-    case CounterClockwise270:
-      /* Apply clockwise 90 degree rotation around the origin.  */
-      MatrixRotate (matrix, M_PI * 0.5, 0, 0);
-
-      /* Translate by the height.  */
-      MatrixTranslate (matrix, -height, 0);
-      break;
-
-    case Flipped:
-      /* Apply horizontal flip.  */
-      MatrixMirrorHorizontal (matrix, width);
-      break;
-
-    case Flipped90:
-      /* Apply horizontal flip.  */
-      MatrixMirrorHorizontal (matrix, width);
-
-      /* Apply clockwise 90 degree rotation around the origin.  */
-      MatrixRotate (matrix, M_PI * 0.5, 0, 0);
-
-      /* Translate by the height.  */
-      MatrixTranslate (matrix, -height, 0);
-      break;
-
-    case Flipped180:
-      /* Apply horizontal flip.  */
-      MatrixMirrorHorizontal (matrix, width);
-
-      /* Apply clockwise 180 degree rotation around the center.  */
-      MatrixRotate (matrix, M_PI, width / 2.0f, height / 2.0f);
-      break;
-
-    case Flipped270:
-      /* Apply horizontal flip.  */
-      MatrixMirrorHorizontal (matrix, width);
-
-      /* Apply clockwise 270 degree rotation around the origin.  */
-      MatrixRotate (matrix, M_PI * 1.5, 0, 0);
-
-      /* Translate y by the width.  */
-      MatrixTranslate (matrix, 0, -width);
-      break;
-    }
-}
-
 static void
 MaybeApplyTransform (PictureBuffer *buffer, DrawParams *params)
 {
@@ -1268,8 +1325,9 @@ MaybeApplyTransform (PictureBuffer *buffer, DrawParams *params)
       MatrixIdentity (&ftransform);
 
       if (params->flags & TransformSet)
-	ApplyInverseTransform (buffer, &ftransform,
-			       params->transform);
+	ApplyInverseTransform (buffer->width, buffer->height,
+			       &ftransform, params->transform,
+			       False);
 
       /* Note that these must be applied in the right order.  First,
 	 the scale is applied.  Then, the offset, and finally the
@@ -1796,8 +1854,9 @@ FindSupportedModifiers (int *pair_count_return)
 
 	  /* pair_count is the number of format-modifier pairs that
 	     will be returned.  First, add one for each implicit
-	     modifier, and another one for the linear modifier.  */
-	  pair_count += 2;
+	     modifier, and another one for each manually specified
+	     modifier.  */
+	  pair_count += 1 + num_specified_modifiers;
 	}
     }
 
@@ -1844,7 +1903,7 @@ FindSupportedModifiers (int *pair_count_return)
 static void
 InitDrmFormats (void)
 {
-  int pair_count, i, j, n;
+  int pair_count, i, j, n, k;
 
   /* First, look up which formats are supported.  */
   if (!FindSupportedFormats ())
@@ -1872,24 +1931,35 @@ InitDrmFormats (void)
       drm_formats[n].drm_modifier = DRM_FORMAT_MOD_INVALID;
       n++;
 
-      /* Check n < pair_count.  */
-      XLAssert (n < pair_count);
+      /* And add all of the user-specified modifiers.  */
+      for (j = 0; j < num_specified_modifiers; ++j)
+	{
+	  /* Assert that n < pair_count.  */
+	  XLAssert (n < pair_count);
 
-      /* And the linear modifier.  */
-      drm_formats[n].drm_format = all_formats[i].format_code;
-      drm_formats[n].drm_modifier = DRM_FORMAT_MOD_LINEAR;
-      n++;
+	  drm_formats[n].drm_format = all_formats[i].format_code;
+	  drm_formats[n].drm_modifier = user_specified_modifiers[j];
+	  n++;
+	}
 
       /* Now add every supported explicit modifier.  */
       for (j = 0; j < all_formats[i].n_supported_modifiers; ++i)
 	{
+	  /* Ignore previously specified modifiers.  */
+
 	  if ((all_formats[i].supported_modifiers[j]
-	       == DRM_FORMAT_MOD_INVALID)
-	      || (all_formats[i].supported_modifiers[j]
-		  == DRM_FORMAT_MOD_LINEAR))
-	    /* Ignore previously specified modifiers.  */
+	       == DRM_FORMAT_MOD_INVALID))
 	    continue;
 
+	  /* Ignore user-specified modifiers.  */
+
+	  for (k = 0; k < num_specified_modifiers; ++k)
+	    {
+	      if (user_specified_modifiers[k]
+		  == all_formats[i].supported_modifiers[j])
+		continue;
+	    }
+
 	  /* Check n < pair_count.  */
 	  XLAssert (n < pair_count);
 
diff --git a/subcompositor.c b/subcompositor.c
index 0d7e151..be55614 100644
--- a/subcompositor.c
+++ b/subcompositor.c
@@ -1632,22 +1632,6 @@ GetContentScale (int scale)
   return -scale + 1;
 }
 
-static BufferTransform
-InvertTransform (BufferTransform transform)
-{
-  switch (transform)
-    {
-    case CounterClockwise270:
-      return CounterClockwise90;
-
-    case CounterClockwise90:
-      return CounterClockwise270;
-
-    default:
-      return transform;
-    }
-}
-
 static int
 BufferWidthAfterTransform (View *view)
 {
diff --git a/svnignore.txt b/svnignore.txt
index 3d21340..905b3bd 100644
--- a/svnignore.txt
+++ b/svnignore.txt
@@ -13,3 +13,4 @@ Makefile
 vgcore*
 massif*
 callgrind*
+drm_modifiers.h
diff --git a/transform.c b/transform.c
index a11493e..6e3b268 100644
--- a/transform.c
+++ b/transform.c
@@ -227,3 +227,192 @@ MatrixExport (Matrix *transform, XTransform *xtransform)
 
 #undef Export
 }
+
+/* Various routines shared between renderers.  */
+
+void
+ApplyInverseTransform (int buffer_width, int buffer_height, Matrix *matrix,
+		       BufferTransform transform, Bool cartesian)
+{
+  float width, height;
+
+  /* Note that the transform is applied in reverse, meaning that a
+     counterclockwise rotation is done clockwise, etc, as TRANSFORM
+     transforms destination coordinates to source ones.  CARTESIAN
+     specifies whether or not an actual cartesian coordinate system is
+     being used.  */
+
+  width = buffer_width;
+  height = buffer_height;
+
+  switch (transform)
+    {
+    case Normal:
+      break;
+
+    case CounterClockwise90:
+
+      if (!cartesian)
+	{
+	  /* Apply clockwise 270 degree rotation around the
+	     origin.  */
+	  MatrixRotate (matrix, M_PI * 1.5, 0, 0);
+
+	  /* Translate y by the width.  */
+	  MatrixTranslate (matrix, 0, -width);
+	}
+      else
+	{
+	  /* Apply clockwise 270 degree rotation around the
+	     origin.  */
+	  MatrixRotate (matrix, M_PI * 0.5, 0, 1);
+
+	  /* Translate by the width.  */
+	  MatrixTranslate (matrix, 0, width);
+	}
+
+      break;
+
+    case CounterClockwise180:
+      /* Apply clockwise 180 degree rotation around the center.  */
+      MatrixRotate (matrix, M_PI, width / 2.0f, height / 2.0f);
+      break;
+
+    case CounterClockwise270:
+
+      if (!cartesian)
+	{
+	  /* Apply clockwise 90 degree rotation around the origin.  */
+	  MatrixRotate (matrix, M_PI * 0.5, 0, 0);
+
+	  /* Translate by the height.  */
+	  MatrixTranslate (matrix, -height, 0);
+	}
+      else
+	{
+	  /* Apply clockwise 90 degree rotation around the origin.  */
+	  MatrixRotate (matrix, M_PI * 1.5, 0, 1);
+
+	  /* Translate by the height.  */
+	  MatrixTranslate (matrix, -height, 0);
+	}
+
+      break;
+
+    case Flipped:
+      /* Apply horizontal flip.  */
+      MatrixMirrorHorizontal (matrix, width);
+      break;
+
+    case Flipped90:
+      /* Apply horizontal flip.  */
+      MatrixMirrorHorizontal (matrix, width);
+
+      /* Apply clockwise 90 degree rotation around the origin.  */
+      MatrixRotate (matrix, M_PI * 0.5, 0, 0);
+
+      /* Translate by the height.  */
+      MatrixTranslate (matrix, -height, 0);
+      break;
+
+    case Flipped180:
+      /* Apply horizontal flip.  */
+      MatrixMirrorHorizontal (matrix, width);
+
+      /* Apply clockwise 180 degree rotation around the center.  */
+      MatrixRotate (matrix, M_PI, width / 2.0f, height / 2.0f);
+      break;
+
+    case Flipped270:
+      /* Apply horizontal flip.  */
+      MatrixMirrorHorizontal (matrix, width);
+
+      /* Apply clockwise 270 degree rotation around the origin.  */
+      MatrixRotate (matrix, M_PI * 1.5, 0, 0);
+
+      /* Translate y by the width.  */
+      MatrixTranslate (matrix, 0, -width);
+      break;
+    }
+}
+
+void
+TransformBox (pixman_box32_t *box, BufferTransform transform,
+	      int width, int height)
+{
+  pixman_box32_t work;
+
+  switch (transform)
+    {
+    case Normal:
+      work = *box;
+      break;
+
+    case CounterClockwise90:
+      work.x1 = height - box->y2;
+      work.y1 = box->x1;
+      work.x2 = height - box->y1;
+      work.y2 = box->x2;
+      break;
+
+    case CounterClockwise180:
+      work.x1 = width - box->x2;
+      work.y1 = height - box->y2;
+      work.x2 = width - box->x1;
+      work.y2 = height - box->y1;
+      break;
+
+    case CounterClockwise270:
+      work.x1 = box->y1;
+      work.y1 = width - box->x2;
+      work.x2 = box->y2;
+      work.y2 = width - box->x1;
+      break;
+
+    case Flipped:
+      work.x1 = width - box->x2;
+      work.y1 = box->y1;
+      work.x2 = width - box->x1;
+      work.y2 = box->y2;
+      break;
+
+    case Flipped90:
+      work.x1 = box->y1;
+      work.y1 = box->x1;
+      work.x2 = box->y2;
+      work.y2 = box->x2;
+      break;
+
+    case Flipped180:
+      work.x1 = box->x1;
+      work.y1 = height - box->y2;
+      work.x2 = box->x2;
+      work.y2 = height - box->y1;
+      break;
+
+    case Flipped270:
+      work.x1 = height - box->y2;
+      work.y1 = width - box->x2;
+      work.x2 = height - box->y1;
+      work.y2 = width - box->x1;
+      break;
+    }
+
+  *box = work;
+}
+
+BufferTransform
+InvertTransform (BufferTransform transform)
+{
+  switch (transform)
+    {
+    case CounterClockwise270:
+      return CounterClockwise90;
+
+    case CounterClockwise90:
+      return CounterClockwise270;
+
+    default:
+      return transform;
+    }
+}