[PATCH v2] char: misc: Increase the maximum number of dynamic misc devices to 1048448

[D Scott Phillips <scott@os.amperecomputing.com>]

On AmpereOne, 128 dynamic misc devices is not enough for the per-cpu
coresight_tmc devices.  Switch the dynamic minors allocator to an ida and
add logic to allocate in the ranges [0..127] and [256..1048575], leaving
[128..255] for static misc devices.  Dynamic allocations start from 127
growing downwards and then increasing from 256, so device numbering for the
first 128 devices remain the same as before.

Signed-off-by: D Scott Phillips <scott@os.amperecomputing.com>
---
Changes since v1 (https://lore.kernel.org/lkml/20220906195213.2370775-1-scott@os.amperecomputing.com/)
 - Update the commit message to better express the intent.

 drivers/char/misc.c | 41 ++++++++++++++++++++++++++++-------------
 1 file changed, 28 insertions(+), 13 deletions(-)

diff --git a/drivers/char/misc.c b/drivers/char/misc.c
index cba19bfdc44d..05727f0daa6b 100644
--- a/drivers/char/misc.c
+++ b/drivers/char/misc.c
@@ -61,7 +61,29 @@ static DEFINE_MUTEX(misc_mtx);
  * Assigned numbers, used for dynamic minors
  */
 #define DYNAMIC_MINORS 128 /* like dynamic majors */
-static DECLARE_BITMAP(misc_minors, DYNAMIC_MINORS);
+static DEFINE_IDA(misc_minors_ida);
+
+static int misc_minor_alloc(void)
+{
+	int ret;
+
+	ret = ida_alloc_max(&misc_minors_ida, DYNAMIC_MINORS - 1, GFP_KERNEL);
+	if (ret >= 0) {
+		ret = DYNAMIC_MINORS - ret - 1;
+	} else {
+		ret = ida_alloc_range(&misc_minors_ida, MISC_DYNAMIC_MINOR + 1,
+				      MINORMASK, GFP_KERNEL);
+	}
+	return ret;
+}
+
+static void misc_minor_free(int minor)
+{
+	if (minor < DYNAMIC_MINORS)
+		ida_free(&misc_minors_ida, DYNAMIC_MINORS - minor - 1);
+	else if (minor > MISC_DYNAMIC_MINOR)
+		ida_free(&misc_minors_ida, minor);
+}
 
 #ifdef CONFIG_PROC_FS
 static void *misc_seq_start(struct seq_file *seq, loff_t *pos)
@@ -183,14 +205,13 @@ int misc_register(struct miscdevice *misc)
 	mutex_lock(&misc_mtx);
 
 	if (is_dynamic) {
-		int i = find_first_zero_bit(misc_minors, DYNAMIC_MINORS);
+		int i = misc_minor_alloc();
 
-		if (i >= DYNAMIC_MINORS) {
+		if (i < 0) {
 			err = -EBUSY;
 			goto out;
 		}
-		misc->minor = DYNAMIC_MINORS - i - 1;
-		set_bit(i, misc_minors);
+		misc->minor = i;
 	} else {
 		struct miscdevice *c;
 
@@ -209,10 +230,7 @@ int misc_register(struct miscdevice *misc)
 					  misc, misc->groups, "%s", misc->name);
 	if (IS_ERR(misc->this_device)) {
 		if (is_dynamic) {
-			int i = DYNAMIC_MINORS - misc->minor - 1;
-
-			if (i < DYNAMIC_MINORS && i >= 0)
-				clear_bit(i, misc_minors);
+			misc_minor_free(misc->minor);
 			misc->minor = MISC_DYNAMIC_MINOR;
 		}
 		err = PTR_ERR(misc->this_device);
@@ -240,16 +258,13 @@ EXPORT_SYMBOL(misc_register);
 
 void misc_deregister(struct miscdevice *misc)
 {
-	int i = DYNAMIC_MINORS - misc->minor - 1;
-
 	if (WARN_ON(list_empty(&misc->list)))
 		return;
 
 	mutex_lock(&misc_mtx);
 	list_del(&misc->list);
 	device_destroy(misc_class, MKDEV(MISC_MAJOR, misc->minor));
-	if (i < DYNAMIC_MINORS && i >= 0)
-		clear_bit(i, misc_minors);
+	misc_minor_free(misc->minor);
 	mutex_unlock(&misc_mtx);
 }
 EXPORT_SYMBOL(misc_deregister);
-- 
2.37.3