devlink: support directly reading from region memory

To read from a region, user space must currently request a new snapshot of
the region and then read from that snapshot. This can sometimes be overkill
if user space only reads a tiny portion. They first create the snapshot,
then request a read, then destroy the snapshot.

For regions which have a single underlying "contents", it makes sense to
allow supporting direct reading of the region data.

Extend the DEVLINK_CMD_REGION_READ to allow direct reading from a region if
requested via the new DEVLINK_ATTR_REGION_DIRECT. If this attribute is set,
then perform a direct read instead of using a snapshot. Direct read is
mutually exclusive with DEVLINK_ATTR_REGION_SNAPSHOT_ID, and care is taken
to ensure that we reject commands which provide incorrect attributes.

Regions must enable support for direct read by implementing the .read()
callback function. If a region does not support such direct reads, a
suitable extended error message is reported.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/uapi/linux/devlink.h

@@ -610,6 +610,8 @@ enum devlink_attr {
 	DEVLINK_ATTR_RATE_TX_PRIORITY,		/* u32 */
 	DEVLINK_ATTR_RATE_TX_WEIGHT,		/* u32 */
 
+	DEVLINK_ATTR_REGION_DIRECT,		/* flag */
+
 	/* add new attributes above here, update the policy in devlink.c */
 
 	__DEVLINK_ATTR_MAX,