usb: gadget: hid: allow dynamic interval configuration via configfs

This patch enhances the HID gadget driver to support dynamic configuration
of the interrupt polling interval (bInterval) via configfs.  A new
‘interval’ attribute is exposed under each HID function’s configfs
directory, and any write to it will adjust the poll rate for all endpoints
without requiring a rebuild.

When the attribute has never been written, legacy defaults are preserved:
  • Full-Speed (FS) endpoints (IN & OUT) poll every 10 ms
  • High-Speed (HS) endpoints (IN & OUT) poll every 4 micro-frames
    (~1 ms)

To implement this cleanly:
  • Add two new fields to f_hid_opts and f_hidg:
      – unsigned char interval
      – bool           interval_user_set
  • Introduce dedicated f_hid_opts_interval_show/store functions.
    The store routine parses into an unsigned int, bounds‐checks,
    assigns to opts->interval, and sets opts->interval_user_set = true.
  • Initialize opts->interval = 4 and opts->interval_user_set = false in
    hidg_alloc_inst(), then copy both into the live f_hidg instance in
    hidg_alloc().
  • In hidg_bind(), set each endpoint’s bInterval based on whether the
  user has written the attribute:
      – If interval_user_set == false, use FS=10 / HS=4
      – If interval_user_set == true, use the user’s value for both FS
        & HS

Signed-off-by: Ben Hoff <hoff.benjamin.k@gmail.com>
Link: https://lore.kernel.org/r/20250429182809.811786-1-hoff.benjamin.k@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/f_hid.c

@@ -62,6 +62,9 @@ struct f_hidg {
 	unsigned short			report_desc_length;
 	char				*report_desc;
 	unsigned short			report_length;
+	unsigned char			interval;
+	bool				interval_user_set;
+
 	/*
 	 * use_out_ep - if true, the OUT Endpoint (interrupt out method)
 	 *              will be used to receive reports from the host
@@ -157,10 +160,7 @@ static struct usb_endpoint_descriptor hidg_ss_in_ep_desc = {
 	.bEndpointAddress	= USB_DIR_IN,
 	.bmAttributes		= USB_ENDPOINT_XFER_INT,
 	/*.wMaxPacketSize	= DYNAMIC */
-	.bInterval		= 4, /* FIXME: Add this field in the
-				      * HID gadget configuration?
-				      * (struct hidg_func_descriptor)
-				      */
+	/*.bInterval		= DYNAMIC */
 };
 
 static struct usb_ss_ep_comp_descriptor hidg_ss_in_comp_desc = {
@@ -178,10 +178,7 @@ static struct usb_endpoint_descriptor hidg_ss_out_ep_desc = {
 	.bEndpointAddress	= USB_DIR_OUT,
 	.bmAttributes		= USB_ENDPOINT_XFER_INT,
 	/*.wMaxPacketSize	= DYNAMIC */
-	.bInterval		= 4, /* FIXME: Add this field in the
-				      * HID gadget configuration?
-				      * (struct hidg_func_descriptor)
-				      */
+	/*.bInterval		= DYNAMIC */
 };
 
 static struct usb_ss_ep_comp_descriptor hidg_ss_out_comp_desc = {
@@ -219,10 +216,7 @@ static struct usb_endpoint_descriptor hidg_hs_in_ep_desc = {
 	.bEndpointAddress	= USB_DIR_IN,
 	.bmAttributes		= USB_ENDPOINT_XFER_INT,
 	/*.wMaxPacketSize	= DYNAMIC */
-	.bInterval		= 4, /* FIXME: Add this field in the
-				      * HID gadget configuration?
-				      * (struct hidg_func_descriptor)
-				      */
+	/* .bInterval		= DYNAMIC */
 };
 
 static struct usb_endpoint_descriptor hidg_hs_out_ep_desc = {
@@ -231,10 +225,7 @@ static struct usb_endpoint_descriptor hidg_hs_out_ep_desc = {
 	.bEndpointAddress	= USB_DIR_OUT,
 	.bmAttributes		= USB_ENDPOINT_XFER_INT,
 	/*.wMaxPacketSize	= DYNAMIC */
-	.bInterval		= 4, /* FIXME: Add this field in the
-				      * HID gadget configuration?
-				      * (struct hidg_func_descriptor)
-				      */
+	/*.bInterval		= DYNAMIC */
 };
 
 static struct usb_descriptor_header *hidg_hs_descriptors_intout[] = {
@@ -260,10 +251,7 @@ static struct usb_endpoint_descriptor hidg_fs_in_ep_desc = {
 	.bEndpointAddress	= USB_DIR_IN,
 	.bmAttributes		= USB_ENDPOINT_XFER_INT,
 	/*.wMaxPacketSize	= DYNAMIC */
-	.bInterval		= 10, /* FIXME: Add this field in the
-				       * HID gadget configuration?
-				       * (struct hidg_func_descriptor)
-				       */
+	/*.bInterval		= DYNAMIC */
 };
 
 static struct usb_endpoint_descriptor hidg_fs_out_ep_desc = {
@@ -272,10 +260,7 @@ static struct usb_endpoint_descriptor hidg_fs_out_ep_desc = {
 	.bEndpointAddress	= USB_DIR_OUT,
 	.bmAttributes		= USB_ENDPOINT_XFER_INT,
 	/*.wMaxPacketSize	= DYNAMIC */
-	.bInterval		= 10, /* FIXME: Add this field in the
-				       * HID gadget configuration?
-				       * (struct hidg_func_descriptor)
-				       */
+	/*.bInterval		= DYNAMIC */
 };
 
 static struct usb_descriptor_header *hidg_fs_descriptors_intout[] = {
@@ -1217,6 +1202,16 @@ static int hidg_bind(struct usb_configuration *c, struct usb_function *f)
 	hidg_hs_in_ep_desc.wMaxPacketSize = cpu_to_le16(hidg->report_length);
 	hidg_fs_in_ep_desc.wMaxPacketSize = cpu_to_le16(hidg->report_length);
 	hidg_ss_out_ep_desc.wMaxPacketSize = cpu_to_le16(hidg->report_length);
+
+	/* IN endpoints: FS default=10ms, HS default=4µ-frame; user override if set */
+	if (!hidg->interval_user_set) {
+		hidg_fs_in_ep_desc.bInterval = 10;
+		hidg_hs_in_ep_desc.bInterval = 4;
+	} else {
+		hidg_fs_in_ep_desc.bInterval = hidg->interval;
+		hidg_hs_in_ep_desc.bInterval = hidg->interval;
+	}
+
 	hidg_ss_out_comp_desc.wBytesPerInterval =
 				cpu_to_le16(hidg->report_length);
 	hidg_hs_out_ep_desc.wMaxPacketSize = cpu_to_le16(hidg->report_length);
@@ -1239,19 +1234,27 @@ static int hidg_bind(struct usb_configuration *c, struct usb_function *f)
 	hidg_ss_out_ep_desc.bEndpointAddress =
 		hidg_fs_out_ep_desc.bEndpointAddress;
 
-	if (hidg->use_out_ep)
+	if (hidg->use_out_ep) {
+		/* OUT endpoints: same defaults (FS=10, HS=4) unless user set */
+		if (!hidg->interval_user_set) {
+			hidg_fs_out_ep_desc.bInterval = 10;
+			hidg_hs_out_ep_desc.bInterval = 4;
+		} else {
+			hidg_fs_out_ep_desc.bInterval = hidg->interval;
+			hidg_hs_out_ep_desc.bInterval = hidg->interval;
+		}
 		status = usb_assign_descriptors(f,
-			hidg_fs_descriptors_intout,
-			hidg_hs_descriptors_intout,
-			hidg_ss_descriptors_intout,
-			hidg_ss_descriptors_intout);
-	else
+			    hidg_fs_descriptors_intout,
+			    hidg_hs_descriptors_intout,
+			    hidg_ss_descriptors_intout,
+			    hidg_ss_descriptors_intout);
+	} else {
 		status = usb_assign_descriptors(f,
 			hidg_fs_descriptors_ssreport,
 			hidg_hs_descriptors_ssreport,
 			hidg_ss_descriptors_ssreport,
 			hidg_ss_descriptors_ssreport);
-
+	}
 	if (status)
 		goto fail;
 
@@ -1423,6 +1426,53 @@ static ssize_t f_hid_opts_report_desc_store(struct config_item *item,
 
 CONFIGFS_ATTR(f_hid_opts_, report_desc);
 
+static ssize_t f_hid_opts_interval_show(struct config_item *item, char *page)
+{
+	struct f_hid_opts *opts = to_f_hid_opts(item);
+	int result;
+
+	mutex_lock(&opts->lock);
+	result = sprintf(page, "%d\n", opts->interval);
+	mutex_unlock(&opts->lock);
+
+	return result;
+}
+
+static ssize_t f_hid_opts_interval_store(struct config_item *item,
+		const char *page, size_t len)
+{
+	struct f_hid_opts *opts = to_f_hid_opts(item);
+	int ret;
+	unsigned int tmp;
+
+	mutex_lock(&opts->lock);
+	if (opts->refcnt) {
+		ret = -EBUSY;
+		goto end;
+	}
+
+	/* parse into a wider type first */
+	ret = kstrtouint(page, 0, &tmp);
+	if (ret)
+		goto end;
+
+	/* range-check against unsigned char max */
+	if (tmp > 255) {
+		ret = -EINVAL;
+		goto end;
+	}
+
+	opts->interval = (unsigned char)tmp;
+	opts->interval_user_set = true;
+	ret = len;
+
+end:
+	mutex_unlock(&opts->lock);
+	return ret;
+}
+
+CONFIGFS_ATTR(f_hid_opts_, interval);
+
 static ssize_t f_hid_opts_dev_show(struct config_item *item, char *page)
 {
 	struct f_hid_opts *opts = to_f_hid_opts(item);
@@ -1437,6 +1487,7 @@ static struct configfs_attribute *hid_attrs[] = {
 	&f_hid_opts_attr_protocol,
 	&f_hid_opts_attr_no_out_endpoint,
 	&f_hid_opts_attr_report_length,
+	&f_hid_opts_attr_interval,
 	&f_hid_opts_attr_report_desc,
 	&f_hid_opts_attr_dev,
 	NULL,
@@ -1483,6 +1534,10 @@ static struct usb_function_instance *hidg_alloc_inst(void)
 	if (!opts)
 		return ERR_PTR(-ENOMEM);
 	mutex_init(&opts->lock);
+
+	opts->interval = 4;
+	opts->interval_user_set = false;
+
 	opts->func_inst.free_func_inst = hidg_free_inst;
 	ret = &opts->func_inst;
 
@@ -1561,6 +1616,8 @@ static struct usb_function *hidg_alloc(struct usb_function_instance *fi)
 	hidg->bInterfaceProtocol = opts->protocol;
 	hidg->report_length = opts->report_length;
 	hidg->report_desc_length = opts->report_desc_length;
+	hidg->interval = opts->interval;
+	hidg->interval_user_set = opts->interval_user_set;
 	if (opts->report_desc) {
 		hidg->report_desc = kmemdup(opts->report_desc,
 					    opts->report_desc_length,

drivers/usb/gadget/function/u_hid.h

@@ -25,6 +25,8 @@ struct f_hid_opts {
 	unsigned short			report_desc_length;
 	unsigned char			*report_desc;
 	bool				report_desc_alloc;
+	unsigned char			interval;
+	bool				interval_user_set;
 
 	/*
 	 * Protect the data form concurrent access by read/write