remoteproc: introduce deny_sysfs_ops flag

The remoteproc framework provides sysfs interfaces for changing
the firmware name and for starting/stopping a remote processor
through the sysfs files 'state' and 'firmware'. These interfaces
are currently allowed irrespective of how the remoteprocs were
booted (like remoteproc self auto-boot, remoteproc client-driven
boot etc). These interfaces can adversely affect a remoteproc
and its clients especially when a remoteproc is being controlled
by a remoteproc client driver(s). Also, not all remoteproc
drivers may want to support the sysfs interfaces by default.

Add support to deny the sysfs state/firmware change by introducing
a state flag 'deny_sysfs_ops' that the individual remoteproc drivers
can set based on their usage needs. The default behavior is to
allow the sysfs operations as before.

Signed-off-by: Suman Anna <s-anna@ti.com>

drivers/remoteproc/remoteproc_sysfs.c

@@ -35,6 +35,10 @@ static ssize_t firmware_store(struct device *dev,
 	char *p;
 	int err, len = count;
 
+	/* restrict sysfs operations if not allowed by remoteproc drivers */
+	if (rproc->deny_sysfs_ops)
+		return -EPERM;
+
 	err = mutex_lock_interruptible(&rproc->lock);
 	if (err) {
 		dev_err(dev, "can't lock rproc %s: %d\n", rproc->name, err);
@@ -101,6 +105,10 @@ static ssize_t state_store(struct device *dev,
 	struct rproc *rproc = to_rproc(dev);
 	int ret = 0;
 
+	/* restrict sysfs operations if not allowed by remoteproc drivers */
+	if (rproc->deny_sysfs_ops)
+		return -EPERM;
+
 	if (sysfs_streq(buf, "start")) {
 		if (rproc->state == RPROC_RUNNING)
 			return -EBUSY;

include/linux/remoteproc.h

@@ -441,6 +441,7 @@ struct rproc_dump_segment {
  * @cached_table: copy of the resource table
  * @table_sz: size of @cached_table
  * @has_iommu: flag to indicate if remote processor is behind an MMU
+ * @deny_sysfs_ops: flag to not permit sysfs operations on state and firmware
  * @dump_segments: list of segments in the firmware
  */
 struct rproc {
@@ -473,6 +474,7 @@ struct rproc {
 	size_t table_sz;
 	bool has_iommu;
 	bool auto_boot;
+	unsigned int deny_sysfs_ops		: 1;
 	struct list_head dump_segments;
 };