[PATCH v3 10/11] usb: gadget: uvc: Allow linking function to string descs

[Daniel Scally <dan.scally@ideasonboard.com>]

Currently the string descriptors for the IAD and VideoStreaming
Interfaces are hardcoded into f_uvc. Now that we can create arbitrary
string descriptors, add a mechanism to define string descriptors for
the IAD, VC and VS interfaces by linking to the appropriate directory
at function level.

Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
---
Changes in v3:

	- Changed target of link to be within <gadget root>/strings
	- Dropped the VideoControl description since there's an attribute for
	that now

Changes in v2:

	- New patch

 drivers/usb/gadget/function/u_uvc.h        |  8 +++
 drivers/usb/gadget/function/uvc_configfs.c | 60 ++++++++++++++++++++++
 2 files changed, 68 insertions(+)

diff --git a/drivers/usb/gadget/function/u_uvc.h b/drivers/usb/gadget/function/u_uvc.h
index 5119cfe5ee4e..565c14fb3d73 100644
--- a/drivers/usb/gadget/function/u_uvc.h
+++ b/drivers/usb/gadget/function/u_uvc.h
@@ -81,6 +81,14 @@ struct f_uvc_opts {
 	struct uvc_descriptor_header			**uvc_hs_streaming_cls;
 	struct uvc_descriptor_header			**uvc_ss_streaming_cls;
 
+	/*
+	 * Indexes into the function's string descriptors allowing users to set
+	 * custom descriptions rather than the hard-coded defaults.
+	 */
+	u8						iad_index;
+	u8						vs0_index;
+	u8						vs1_index;
+
 	/*
 	 * Read/write access to configfs attributes is handled by configfs.
 	 *
diff --git a/drivers/usb/gadget/function/uvc_configfs.c b/drivers/usb/gadget/function/uvc_configfs.c
index 9da3b784f954..ffa338643cc8 100644
--- a/drivers/usb/gadget/function/uvc_configfs.c
+++ b/drivers/usb/gadget/function/uvc_configfs.c
@@ -2901,8 +2901,68 @@ static void uvc_func_item_release(struct config_item *item)
 	usb_put_function_instance(&opts->func_inst);
 }
 
+static int uvc_func_allow_link(struct config_item *src, struct config_item *tgt)
+{
+	struct mutex *su_mutex = &src->ci_group->cg_subsys->su_mutex;
+	struct gadget_string *string;
+	struct config_item *strings;
+	struct f_uvc_opts *opts;
+	int ret = 0;
+
+	mutex_lock(su_mutex); /* for navigating configfs hierarchy */
+
+	/* Validate that the target is an entry in strings/<langid> */
+	strings = config_group_find_item(to_config_group(src->ci_parent->ci_parent),
+					 "strings");
+	if (!strings || tgt->ci_parent->ci_parent != strings) {
+		ret = -EINVAL;
+		goto put_strings;
+	}
+
+	string = to_gadget_string(tgt);
+
+	opts = to_f_uvc_opts(src);
+	mutex_lock(&opts->lock);
+
+	if (!strcmp(tgt->ci_name, "iad_desc"))
+		opts->iad_index = string->usb_string.id;
+	else if (!strcmp(tgt->ci_name, "vs0_desc"))
+		opts->vs0_index = string->usb_string.id;
+	else if (!strcmp(tgt->ci_name, "vs1_desc"))
+		opts->vs1_index = string->usb_string.id;
+	else
+		ret = -EINVAL;
+
+	mutex_unlock(&opts->lock);
+
+put_strings:
+	config_item_put(strings);
+	mutex_unlock(su_mutex);
+
+	return ret;
+}
+
+static void uvc_func_drop_link(struct config_item *src, struct config_item *tgt)
+{
+	struct f_uvc_opts *opts;
+
+	opts = to_f_uvc_opts(src);
+	mutex_lock(&opts->lock);
+
+	if (!strcmp(tgt->ci_name, "iad_desc"))
+		opts->iad_index = 0;
+	else if (!strcmp(tgt->ci_name, "vs0_desc"))
+		opts->vs0_index = 0;
+	else if (!strcmp(tgt->ci_name, "vs1_desc"))
+		opts->vs1_index = 0;
+
+	mutex_unlock(&opts->lock);
+}
+
 static struct configfs_item_operations uvc_func_item_ops = {
 	.release	= uvc_func_item_release,
+	.allow_link	= uvc_func_allow_link,
+	.drop_link	= uvc_func_drop_link,
 };
 
 #define UVCG_OPTS_ATTR(cname, aname, limit)				\
-- 
2.34.1