netdev - [PATCH v2 26/35] mISDN: optimize get_free

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Message-Id: <20231203193307.542794-25-yury.norov@gmail.com>
Date: Sun,  3 Dec 2023 11:32:58 -0800
From: Yury Norov <yury.norov@...il.com>
To: linux-kernel@...r.kernel.org,
	Karsten Keil <isdn@...ux-pingi.de>,
	netdev@...r.kernel.org
Cc: Yury Norov <yury.norov@...il.com>,
	Jan Kara <jack@...e.cz>,
	Mirsad Todorovac <mirsad.todorovac@....unizg.hr>,
	Matthew Wilcox <willy@...radead.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>,
	Alexey Klimov <klimov.linux@...il.com>,
	Bart Van Assche <bvanassche@....org>,
	Sergey Shtylyov <s.shtylyov@....ru>
Subject: [PATCH v2 26/35] mISDN: optimize get_free_devid()

get_free_devid() traverses each bit in device_ids in an open-coded loop.
We can do it faster by using dedicated find_and_set_bit().

It makes the whole function a nice one-liner, and because MAX_DEVICE_ID
is a small constant-time value (63), on 64-bit platforms find_and_set_bit()
call will be optimized to:

	ffs();
	test_and_set_bit().

Signed-off-by: Yury Norov <yury.norov@...il.com>
---
 drivers/isdn/mISDN/core.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/drivers/isdn/mISDN/core.c b/drivers/isdn/mISDN/core.c
index ab8513a7acd5..3f97db006cf3 100644
--- a/drivers/isdn/mISDN/core.c
+++ b/drivers/isdn/mISDN/core.c
@@ -197,14 +197,9 @@ get_mdevice_count(void)
 static int
 get_free_devid(void)
 {
-	u_int	i;
+	u_int i = find_and_set_bit((u_long *)&device_ids, MAX_DEVICE_ID + 1);
 
-	for (i = 0; i <= MAX_DEVICE_ID; i++)
-		if (!test_and_set_bit(i, (u_long *)&device_ids))
-			break;
-	if (i > MAX_DEVICE_ID)
-		return -EBUSY;
-	return i;
+	return i <= MAX_DEVICE_ID ? i : -EBUSY;
 }
 
 int
-- 
2.40.1