diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index 5c8f58114677..877b637e7f7b 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -46,6 +46,7 @@
 #include <linux/usb/cdc.h>
 #include <asm/byteorder.h>
 #include <asm/unaligned.h>
+#include <linux/idr.h>
 #include <linux/list.h>
 
 #include "cdc-acm.h"
@@ -56,27 +57,27 @@
 
 static struct usb_driver acm_driver;
 static struct tty_driver *acm_tty_driver;
-static struct acm *acm_table[ACM_TTY_MINORS];
 
-static DEFINE_MUTEX(acm_table_lock);
+static DEFINE_IDR(acm_minors);
+static DEFINE_MUTEX(acm_minors_lock);
 
 static void acm_tty_set_termios(struct tty_struct *tty,
 				struct ktermios *termios_old);
 
 /*
- * acm_table accessors
+ * acm_minors accessors
  */
 
 /*
- * Look up an ACM structure by index. If found and not disconnected, increment
+ * Look up an ACM structure by minor. If found and not disconnected, increment
  * its refcount and return it with its mutex held.
  */
-static struct acm *acm_get_by_index(unsigned index)
+static struct acm *acm_get_by_minor(unsigned int minor)
 {
 	struct acm *acm;
 
-	mutex_lock(&acm_table_lock);
-	acm = acm_table[index];
+	mutex_lock(&acm_minors_lock);
+	acm = idr_find(&acm_minors, minor);
 	if (acm) {
 		mutex_lock(&acm->mutex);
 		if (acm->disconnected) {
@@ -87,7 +88,7 @@ static struct acm *acm_get_by_index(unsigned index)
 			mutex_unlock(&acm->mutex);
 		}
 	}
-	mutex_unlock(&acm_table_lock);
+	mutex_unlock(&acm_minors_lock);
 	return acm;
 }
 
@@ -98,14 +99,9 @@ static int acm_alloc_minor(struct acm *acm)
 {
 	int minor;
 
-	mutex_lock(&acm_table_lock);
-	for (minor = 0; minor < ACM_TTY_MINORS; minor++) {
-		if (!acm_table[minor]) {
-			acm_table[minor] = acm;
-			break;
-		}
-	}
-	mutex_unlock(&acm_table_lock);
+	mutex_lock(&acm_minors_lock);
+	minor = idr_alloc(&acm_minors, acm, 0, ACM_TTY_MINORS, GFP_KERNEL);
+	mutex_unlock(&acm_minors_lock);
 
 	return minor;
 }
@@ -113,9 +109,9 @@ static int acm_alloc_minor(struct acm *acm)
 /* Release the minor number associated with 'acm'.  */
 static void acm_release_minor(struct acm *acm)
 {
-	mutex_lock(&acm_table_lock);
-	acm_table[acm->minor] = NULL;
-	mutex_unlock(&acm_table_lock);
+	mutex_lock(&acm_minors_lock);
+	idr_remove(&acm_minors, acm->minor);
+	mutex_unlock(&acm_minors_lock);
 }
 
 /*
@@ -497,7 +493,7 @@ static int acm_tty_install(struct tty_driver *driver, struct tty_struct *tty)
 
 	dev_dbg(tty->dev, "%s\n", __func__);
 
-	acm = acm_get_by_index(tty->index);
+	acm = acm_get_by_minor(tty->index);
 	if (!acm)
 		return -ENODEV;
 
@@ -1316,7 +1312,7 @@ made_compressed_probe:
 		goto alloc_fail;
 
 	minor = acm_alloc_minor(acm);
-	if (minor == ACM_TTY_MINORS) {
+	if (minor < 0) {
 		dev_err(&intf->dev, "no more free acm devices\n");
 		kfree(acm);
 		return -ENODEV;