virtio: explicit advertisement of driver features

A recent proposed feature addition to the virtio block driver revealed
some flaws in the API: in particular, we assume that feature
negotiation is complete once a driver's probe function returns.

There is nothing in the API to require this, however, and even I
didn't notice when it was violated.

So instead, we require the driver to specify what features it supports
in a table, we can then move the feature negotiation into the virtio
core.  The intersection of device and driver features are presented in
a new 'features' bitmap in the struct virtio_device.

Note that this highlights the difference between Linux unsigned-long
bitmaps where each unsigned long is in native endian, and a
straight-forward little-endian array of bytes.

Drivers can still remove feature bits in their probe routine if they
really have to.

API changes:
- dev->config->feature() no longer gets and acks a feature.
- drivers should advertise their features in the 'feature_table' field
- use virtio_has_feature() for extra sanity when checking feature bits

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 36 insertions, 2 deletions

diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index b535483..1386678 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c
@@ -80,19 +80,51 @@ static void add_status(struct virtio_device *dev, unsigned status)
 	dev->config->set_status(dev, dev->config->get_status(dev) | status);
 }
 
+void virtio_check_driver_offered_feature(const struct virtio_device *vdev,
+					 unsigned int fbit)
+{
+	unsigned int i;
+	struct virtio_driver *drv = container_of(vdev->dev.driver,
+						 struct virtio_driver, driver);
+
+	for (i = 0; i < drv->feature_table_size; i++)
+		if (drv->feature_table[i] == fbit)
+			return;
+	BUG();
+}
+EXPORT_SYMBOL_GPL(virtio_check_driver_offered_feature);
+
 static int virtio_dev_probe(struct device *_d)
 {
-	int err;
+	int err, i;
 	struct virtio_device *dev = container_of(_d,struct virtio_device,dev);
 	struct virtio_driver *drv = container_of(dev->dev.driver,
 						 struct virtio_driver, driver);
+	u32 device_features;
 
+	/* We have a driver! */
 	add_status(dev, VIRTIO_CONFIG_S_DRIVER);
+
+	/* Figure out what features the device supports. */
+	device_features = dev->config->get_features(dev);
+
+	/* Features supported by both device and driver into dev->features. */
+	memset(dev->features, 0, sizeof(dev->features));
+	for (i = 0; i < drv->feature_table_size; i++) {
+		unsigned int f = drv->feature_table[i];
+		BUG_ON(f >= 32);
+		if (device_features & (1 << f))
+			set_bit(f, dev->features);
+	}
+
 	err = drv->probe(dev);
 	if (err)
 		add_status(dev, VIRTIO_CONFIG_S_FAILED);
-	else
+	else {
 		add_status(dev, VIRTIO_CONFIG_S_DRIVER_OK);
+		/* They should never have set feature bits beyond 32 */
+		dev->config->set_features(dev, dev->features[0]);
+	}
 	return err;
 }
 
@@ -114,6 +146,8 @@ static int virtio_dev_remove(struct device *_d)
 
 int register_virtio_driver(struct virtio_driver *driver)
 {
+	/* Catch this early. */
+	BUG_ON(driver->feature_table_size && !driver->feature_table);
 	driver->driver.bus = &virtio_bus;
 	driver->driver.probe = virtio_dev_probe;
 	driver->driver.remove = virtio_dev_remove;