rpmsg: Add driver_override device attribute for rpmsg_device

This patch adds "driver_override" device attribute for rpmsg_device which
will allow users to explicitly specify the rpmsg_driver to be used via
sysfs entry.

The "driver_override" device attribute implemented here is very similar
to "driver_override" implemented for platform, pci, and amba bus types.

One important use-case of "driver_override" device attribute is to force
use of rpmsg_chrdev driver for certain rpmsg_device instances.
Signed-off-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

Documentation/ABI/testing/sysfs-bus-rpmsg

@@ -73,3 +73,23 @@ Description:
 		This sysfs entry tells us whether the channel is a local
 		server channel that is announced (values are either
 		true or false).
+
+What:		/sys/bus/rpmsg/devices/.../driver_override
+Date:		April 2018
+KernelVersion:	4.18
+Contact:	Bjorn Andersson <bjorn.andersson@linaro.org>
+Description:
+		Every rpmsg device is a communication channel with a remote
+		processor. Channels are identified by a textual name (see
+		/sys/bus/rpmsg/devices/.../name above) and have a local
+		("source") rpmsg address, and remote ("destination") rpmsg
+		address.
+
+		The listening entity (or client) which communicates with a
+		remote processor is referred as rpmsg driver. The rpmsg device
+		and rpmsg driver are matched based on rpmsg device name and
+		rpmsg driver ID table.
+
+		This sysfs entry allows the rpmsg driver for a rpmsg device
+		to be specified which will override standard OF, ID table
+		and name matching.

drivers/rpmsg/rpmsg_core.c

@@ -333,11 +333,49 @@ field##_show(struct device *dev,					\
 }									\
 static DEVICE_ATTR_RO(field);
 
+#define rpmsg_string_attr(field, member)				\
+static ssize_t								\
+field##_store(struct device *dev, struct device_attribute *attr,	\
+	      const char *buf, size_t sz)				\
+{									\
+	struct rpmsg_device *rpdev = to_rpmsg_device(dev);		\
+	char *new, *old;						\
+									\
+	new = kstrndup(buf, sz, GFP_KERNEL);				\
+	if (!new)							\
+		return -ENOMEM;						\
+	new[strcspn(new, "\n")] = '\0';					\
+									\
+	device_lock(dev);						\
+	old = rpdev->member;						\
+	if (strlen(new)) {						\
+		rpdev->member = new;					\
+	} else {							\
+		kfree(new);						\
+		rpdev->member = NULL;					\
+	}								\
+	device_unlock(dev);						\
+									\
+	kfree(old);							\
+									\
+	return sz;							\
+}									\
+static ssize_t								\
+field##_show(struct device *dev,					\
+	     struct device_attribute *attr, char *buf)			\
+{									\
+	struct rpmsg_device *rpdev = to_rpmsg_device(dev);		\
+									\
+	return sprintf(buf, "%s\n", rpdev->member);			\
+}									\
+static DEVICE_ATTR_RW(field)
+
 /* for more info, see Documentation/ABI/testing/sysfs-bus-rpmsg */
 rpmsg_show_attr(name, id.name, "%s\n");
 rpmsg_show_attr(src, src, "0x%x\n");
 rpmsg_show_attr(dst, dst, "0x%x\n");
 rpmsg_show_attr(announce, announce ? "true" : "false", "%s\n");
+rpmsg_string_attr(driver_override, driver_override);
 
 static ssize_t modalias_show(struct device *dev,
 			     struct device_attribute *attr, char *buf)
@@ -359,6 +397,7 @@ static struct attribute *rpmsg_dev_attrs[] = {
 	&dev_attr_dst.attr,
 	&dev_attr_src.attr,
 	&dev_attr_announce.attr,
+	&dev_attr_driver_override.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(rpmsg_dev);