virtio: Formally reserve bits 28-31 to be 'transport' features.

We assign feature bits as required, but it makes sense to reserve some
for the particular transport, rather than the particular device.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 drivers/virtio/virtio.c       |    5 +++++
 include/linux/virtio_config.h |    6 ++++++
 2 files changed, 11 insertions(+)

diff -r d62421f0943e drivers/virtio/virtio.c
--- a/drivers/virtio/virtio.c	Thu Jun 26 01:14:50 2008 +1000
+++ b/drivers/virtio/virtio.c	Thu Jun 26 01:32:43 2008 +1000
@@ -113,6 +113,11 @@ static int virtio_dev_probe(struct devic
 			set_bit(f, dev->features);
 	}
 
+	/* Transport features are always preserved to pass to set_features. */
+	for (i = VIRTIO_TRANSPORT_F_START; i < VIRTIO_TRANSPORT_F_END; i++)
+		if (device_features & (1 << i))
+			set_bit(i, dev->features);
+
 	err = drv->probe(dev);
 	if (err)
 		add_status(dev, VIRTIO_CONFIG_S_FAILED);
diff -r d62421f0943e include/linux/virtio_config.h
--- a/include/linux/virtio_config.h	Thu Jun 26 01:14:50 2008 +1000
+++ b/include/linux/virtio_config.h	Thu Jun 26 01:32:43 2008 +1000
@@ -14,6 +14,12 @@
 #define VIRTIO_CONFIG_S_DRIVER_OK	4
 /* We've given up on this device. */
 #define VIRTIO_CONFIG_S_FAILED		0x80
+
+/* Some virtio feature bits (currently bits 28 through 31) are reserved for the
+ * transport being used (eg. virtio_ring), the rest are per-device feature
+ * bits. */
+#define VIRTIO_TRANSPORT_F_START	28
+#define VIRTIO_TRANSPORT_F_END		32
 
 /* Do we get callbacks when the ring is completely used, even if we've
  * suppressed them? */