[PATCH v4] staging: greybus: Convert uart.c from IDR to XArray

["Fabio M. De Francesco" <fmdefrancesco@gmail.com>]

Convert greybus/uart.c from IDR to XArray. The abstract data type XArray
is more memory-efficient, parallelisable, and cache friendly. It takes
advantage of RCU to perform lookups without locking. Furthermore, IDR is
deprecated because XArray has a better (cleaner and more consistent) API.

Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
---

v3->v4: 
	Remove mutex_lock/unlock around xa_load(). These locks seem to
	be unnecessary because there is a 1:1 correspondence between
	a specific minor and its gb_tty and there is no reference
	counting. I think that the RCU locks used inside xa_load()
	are sufficient to protect this API from returning an invalid
	gb_tty in case of concurrent access. Some more considerations 
	on this topic are in the following message to linux-kernel list:
	https://lore.kernel.org/lkml/3554184.2JXonMZcNW@localhost.localdomain/
v2->v3:
        Fix some issues according to a review by Alex Elder <elder@ieee.org>
v1->v2:
        Fix an issue found by the kernel test robot. It is due to
        passing to xa_*lock() the same old mutex that IDR used with
        the previous version of the code.

 drivers/staging/greybus/uart.c | 32 ++++++++++++++------------------
 1 file changed, 14 insertions(+), 18 deletions(-)

diff --git a/drivers/staging/greybus/uart.c b/drivers/staging/greybus/uart.c
index 73f01ed1e5b7..f66983adb51b 100644
--- a/drivers/staging/greybus/uart.c
+++ b/drivers/staging/greybus/uart.c
@@ -22,7 +22,7 @@
 #include <linux/serial.h>
 #include <linux/tty_driver.h>
 #include <linux/tty_flip.h>
-#include <linux/idr.h>
+#include <linux/xarray.h>
 #include <linux/fs.h>
 #include <linux/kdev_t.h>
 #include <linux/kfifo.h>
@@ -32,8 +32,9 @@
 
 #include "gbphy.h"
 
-#define GB_NUM_MINORS	16	/* 16 is more than enough */
-#define GB_NAME		"ttyGB"
+#define GB_NUM_MINORS		16	/* 16 is more than enough */
+#define GB_RANGE_MINORS		XA_LIMIT(0, GB_NUM_MINORS)
+#define GB_NAME			"ttyGB"
 
 #define GB_UART_WRITE_FIFO_SIZE		PAGE_SIZE
 #define GB_UART_WRITE_ROOM_MARGIN	1	/* leave some space in fifo */
@@ -67,8 +68,7 @@ struct gb_tty {
 };
 
 static struct tty_driver *gb_tty_driver;
-static DEFINE_IDR(tty_minors);
-static DEFINE_MUTEX(table_lock);
+static DEFINE_XARRAY(tty_minors);
 
 static int gb_uart_receive_data_handler(struct gb_operation *op)
 {
@@ -341,8 +341,7 @@ static struct gb_tty *get_gb_by_minor(unsigned int minor)
 {
 	struct gb_tty *gb_tty;
 
-	mutex_lock(&table_lock);
-	gb_tty = idr_find(&tty_minors, minor);
+	gb_tty = xa_load(&tty_minors, minor);
 	if (gb_tty) {
 		mutex_lock(&gb_tty->mutex);
 		if (gb_tty->disconnected) {
@@ -353,19 +352,18 @@ static struct gb_tty *get_gb_by_minor(unsigned int minor)
 			mutex_unlock(&gb_tty->mutex);
 		}
 	}
-	mutex_unlock(&table_lock);
 	return gb_tty;
 }
 
 static int alloc_minor(struct gb_tty *gb_tty)
 {
 	int minor;
+	int ret;
 
-	mutex_lock(&table_lock);
-	minor = idr_alloc(&tty_minors, gb_tty, 0, GB_NUM_MINORS, GFP_KERNEL);
-	mutex_unlock(&table_lock);
-	if (minor >= 0)
-		gb_tty->minor = minor;
+	ret = xa_alloc(&tty_minors, &minor, gb_tty, GB_RANGE_MINORS, GFP_KERNEL);
+	if (ret)
+		return ret;
+	gb_tty->minor = minor;
 	return minor;
 }
 
@@ -374,9 +372,7 @@ static void release_minor(struct gb_tty *gb_tty)
 	int minor = gb_tty->minor;
 
 	gb_tty->minor = 0;	/* Maybe should use an invalid value instead */
-	mutex_lock(&table_lock);
-	idr_remove(&tty_minors, minor);
-	mutex_unlock(&table_lock);
+	xa_erase(&tty_minors, minor);
 }
 
 static int gb_tty_install(struct tty_driver *driver, struct tty_struct *tty)
@@ -837,7 +833,7 @@ static int gb_uart_probe(struct gbphy_device *gbphy_dev,
 
 	minor = alloc_minor(gb_tty);
 	if (minor < 0) {
-		if (minor == -ENOSPC) {
+		if (minor == -EBUSY) {
 			dev_err(&gbphy_dev->dev,
 				"no more free minor numbers\n");
 			retval = -ENODEV;
@@ -982,7 +978,7 @@ static void gb_tty_exit(void)
 {
 	tty_unregister_driver(gb_tty_driver);
 	put_tty_driver(gb_tty_driver);
-	idr_destroy(&tty_minors);
+	xa_destroy(&tty_minors);
 }
 
 static const struct gbphy_device_id gb_uart_id_table[] = {
-- 
2.32.0