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>

2 files changed, 11 insertions, 0 deletions

diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index fc85cba64578..baf103361e3a 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c
@@ -113,6 +113,11 @@ static int virtio_dev_probe(struct device *_d)
                         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 --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
index 7eb4b34d13bb..5a30cfb7934b 100644
--- a/include/linux/virtio_config.h
+++ b/include/linux/virtio_config.h
@@ -18,6 +18,12 @@
 /* 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? */
 #define VIRTIO_F_NOTIFY_ON_EMPTY        24