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

From: D Scott Phillips
Date: Mon Oct 10 2022 - 22:30:49 EST


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@xxxxxxxxxxxxxxxxxxxxxx>
---
Changes since v1 (https://lore.kernel.org/lkml/20220906195213.2370775-1-scott@xxxxxxxxxxxxxxxxxxxxxx/)
- 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