diff --git a/kms/connector.py b/kms/connector.py
index 1246fc8..51a7284 100644
--- a/kms/connector.py
+++ b/kms/connector.py
@@ -68,6 +68,21 @@ def __init__(self, card: Card, id, idx) -> None:
     def connected(self):
         return self.connector_res.connection in (kms.uapi.DRM_MODE_CONNECTED, kms.uapi.DRM_MODE_UNKNOWNCONNECTION)
 
+    def refresh_modes(self):
+        res = kms.uapi.drm_mode_get_connector(connector_id=self.id)
+
+        fcntl.ioctl(self.card.fd, kms.uapi.DRM_IOCTL_MODE_GETCONNECTOR, res, True)
+
+        modes = (kms.uapi.drm_mode_modeinfo * res.count_modes)()
+        res.modes_ptr = ctypes.addressof(modes)
+
+        res.count_props = 0
+        res.count_encoders = 0
+
+        fcntl.ioctl(self.card.fd, kms.uapi.DRM_IOCTL_MODE_GETCONNECTOR, res, True)
+
+        self.modes = [kms.VideoMode(m) for m in modes]
+
     def get_default_mode(self):
         return self.modes[0]
 
diff --git a/kms/framebuffer.py b/kms/framebuffer.py
index 2c4c71b..b321173 100644
--- a/kms/framebuffer.py
+++ b/kms/framebuffer.py
@@ -5,6 +5,7 @@
 import mmap
 import os
 import weakref
+from math import ceil
 
 from typing import TYPE_CHECKING
 
@@ -64,13 +65,21 @@ class DumbFramebuffer(Framebuffer):
     def __init__(self, card: Card, width: int, height: int, format: kms.PixelFormat) -> None:
         planes = []
 
-        assert width % format.pixelspergroup == 0
+        assert width % format.pixelspergroup[0] == 0
+
+        # DRM_IOCTL_MODE_CREATE_DUMB takes a 'bpp' (bits-per-pixel) argument,
+        # which is then used with the width and height to allocate the buffer.
+        # This doesn't work for pixel formats where the average bits-per-pixel
+        # is not an integer (e.g. XV15)
+        #
+        # So, we instead use the number of bits per (horizontal) pixel group as
+        # the 'bpp' argument, and adjust the width accordingly.
 
         for pi in format.planes:
             creq = kms.uapi.drm_mode_create_dumb()
-            creq.width = width
-            creq.height = height // pi.verticalsubsampling
-            creq.bpp = pi.bytespergroup * 8 // format.pixelspergroup
+            creq.width = int(ceil(width / format.pixelspergroup[0]))
+            creq.height = int(ceil(height / format.pixelspergroup[1])) * pi.linespergroup
+            creq.bpp = pi.bytespergroup * 8
 
             fcntl.ioctl(card.fd, kms.uapi.DRM_IOCTL_MODE_CREATE_DUMB, creq, True)
 
diff --git a/utils/hpd.py b/utils/hpd.py
new file mode 100755
index 0000000..a53cce9
--- /dev/null
+++ b/utils/hpd.py
@@ -0,0 +1,25 @@
+#!/usr/bin/python3
+
+import pprint
+import pyudev
+
+import kms
+
+card = kms.Card()
+connectors = card.connectors
+
+context = pyudev.Context()
+
+dev = pyudev.Devices.from_name(context, 'drm', 'card0')
+
+monitor = pyudev.Monitor.from_netlink(context)
+monitor.filter_by('drm')
+
+for device in iter(monitor.poll, None):
+    if 'HOTPLUG' in device:
+        print('== HPD ==')
+        for conn in connectors:
+            conn.refresh_modes()
+            strs = (conn.fullname,
+                    ['{}x{}'.format(m.hdisplay, m.vdisplay) for m in conn.modes])
+            pprint.pprint(strs, compact=True, width=120)
diff --git a/utils/kmstest.py b/utils/kmstest.py
index 5db6218..b7c976a 100755
--- a/utils/kmstest.py
+++ b/utils/kmstest.py
@@ -40,6 +40,7 @@ def main():
     parser = argparse.ArgumentParser()
     parser.add_argument('-c', '--connector', default='')
     parser.add_argument('--dmabuf', nargs='?', const='reserved', metavar='HEAP', help='use dmabuf')
+    parser.add_argument('-f', '--format', default='XRGB8888')
     args = parser.parse_args()
 
     card = kms.Card()
@@ -54,7 +55,8 @@ def main():
 
     modeb = mode.to_blob(card)
 
-    fmt = kms.PixelFormats.XRGB8888
+    fmt = kms.PixelFormats.find_by_name(args.format)
+
     width = mode.hdisplay
     height = mode.vdisplay
 
@@ -70,12 +72,13 @@ def main():
     else:
         fb = kms.DumbFramebuffer(card, width, height, fmt)
 
-    ts1 = time.perf_counter()
-    fb.begin_cpu_access('w')
-    draw_test_pattern(fb)
-    fb.end_cpu_access()
-    ts2 = time.perf_counter()
-    print(f'Drawing took {(ts2 - ts1) * 1000:.4f} ms')
+    if fmt == kms.PixelFormats.XRGB8888:
+        ts1 = time.perf_counter()
+        fb.begin_cpu_access('w')
+        draw_test_pattern(fb)
+        fb.end_cpu_access()
+        ts2 = time.perf_counter()
+        print(f'Drawing took {(ts2 - ts1) * 1000:.4f} ms')
 
     req = kms.AtomicReq(card)