drm: Introduce per-device driver_features

We wish to control certain driver_features flags on a per-device basis
while still sharing a single drm_driver instance across all the
devices. To that end introduce device.driver_features. By default
it will be set to ~0 to not impose any limits beyond
driver.driver_features. Drivers can then clear specific flags
in the per-device bitmask to limit the capabilities of the device.

An alternative approach would be to copy the driver_features from
the driver into the device in drm_dev_init(), however that would
require verifying that no driver is currently changing
driver.driver_features after drm_dev_init(). Hence the ~0 apporach
was easier.

Ideally we'd also make drm_driver const but there is plenty of code
left that wants to mutate it (eg. various vfunc assignments). We'll
need to fix all that up before we can make it const.

And while at it fix up the type of the feature flag passed to
drm_core_check_feature().

v2: Streamline the && vs. & (Chris)
    s/int/u32/ in drm_core_check_feature() args

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180913131622.17690-1-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>

drivers/gpu/drm/drm_drv.c

@@ -506,6 +506,9 @@ int drm_dev_init(struct drm_device *dev,
 	dev->dev = parent;
 	dev->driver = driver;
 
+	/* no per-device feature limits by default */
+	dev->driver_features = ~0u;
+
 	INIT_LIST_HEAD(&dev->filelist);
 	INIT_LIST_HEAD(&dev->filelist_internal);
 	INIT_LIST_HEAD(&dev->clientlist);

include/drm/drm_device.h

@@ -45,6 +45,16 @@ struct drm_device {
 	/* currently active master for this device. Protected by master_mutex */
 	struct drm_master *master;
 
+	/**
+	 * @driver_features: per-device driver features
+	 *
+	 * Drivers can clear specific flags here to disallow
+	 * certain features on a per-device basis while still
+	 * sharing a single &struct drm_driver instance across
+	 * all devices.
+	 */
+	u32 driver_features;
+
 	/**
 	 * @unplugged:
 	 *

include/drm/drm_drv.h

@@ -653,14 +653,14 @@ static inline bool drm_dev_is_unplugged(struct drm_device *dev)
  * @dev: DRM device to check
  * @feature: feature flag
  *
- * This checks @dev for driver features, see &drm_driver.driver_features and the
- * various DRIVER_\* flags.
+ * This checks @dev for driver features, see &drm_driver.driver_features,
+ * &drm_device.driver_features, and the various DRIVER_\* flags.
  *
  * Returns true if the @feature is supported, false otherwise.
  */
-static inline bool drm_core_check_feature(struct drm_device *dev, int feature)
+static inline bool drm_core_check_feature(struct drm_device *dev, u32 feature)
 {
-	return dev->driver->driver_features & feature;
+	return dev->driver->driver_features & dev->driver_features & feature;
 }
 
 /**