linux-kernel - [PATCH] hwmon: applesmc: avoid overlong udelay()

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Date:   Wed, 27 May 2020 15:51:57 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Henrik Rydberg <rydberg@...math.org>,
        Jean Delvare <jdelvare@...e.com>,
        Guenter Roeck <linux@...ck-us.net>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Richard Fontana <rfontana@...hat.com>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: [PATCH] hwmon: applesmc: avoid overlong udelay()

Building this driver with "clang -O3" produces a link error
after the compiler partially unrolls the loop and 256ms
becomes a compile-time constant that triggers the check
in udelay():

ld.lld: error: undefined symbol: __bad_udelay
>>> referenced by applesmc.c
>>>               hwmon/applesmc.o:(read_smc) in archive drivers/built-in.a

I can see no reason against using a sleeping function here,
as no part of the driver runs in atomic context, so instead use
usleep_range() with a wide range and use jiffies for the
end condition.

Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 drivers/hwmon/applesmc.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/hwmon/applesmc.c b/drivers/hwmon/applesmc.c
index ec93b8d673f5..316618409315 100644
--- a/drivers/hwmon/applesmc.c
+++ b/drivers/hwmon/applesmc.c
@@ -156,14 +156,19 @@ static struct workqueue_struct *applesmc_led_wq;
  */
 static int wait_read(void)
 {
+	unsigned long end = jiffies + (APPLESMC_MAX_WAIT * HZ) / USEC_PER_SEC;
 	u8 status;
 	int us;
+
 	for (us = APPLESMC_MIN_WAIT; us < APPLESMC_MAX_WAIT; us <<= 1) {
-		udelay(us);
+		usleep_range(us, us * 16);
 		status = inb(APPLESMC_CMD_PORT);
 		/* read: wait for smc to settle */
 		if (status & 0x01)
 			return 0;
+		/* timeout: give up */
+		if (time_after(jiffies, end))
+			break;
 	}
 
 	pr_warn("wait_read() fail: 0x%02x\n", status);
@@ -178,10 +183,11 @@ static int send_byte(u8 cmd, u16 port)
 {
 	u8 status;
 	int us;
+	unsigned long end = jiffies + (APPLESMC_MAX_WAIT * HZ) / USEC_PER_SEC;
 
 	outb(cmd, port);
 	for (us = APPLESMC_MIN_WAIT; us < APPLESMC_MAX_WAIT; us <<= 1) {
-		udelay(us);
+		usleep_range(us, us * 16);
 		status = inb(APPLESMC_CMD_PORT);
 		/* write: wait for smc to settle */
 		if (status & 0x02)
@@ -190,7 +196,7 @@ static int send_byte(u8 cmd, u16 port)
 		if (status & 0x04)
 			return 0;
 		/* timeout: give up */
-		if (us << 1 == APPLESMC_MAX_WAIT)
+		if (time_after(jiffies, end))
 			break;
 		/* busy: long wait and resend */
 		udelay(APPLESMC_RETRY_WAIT);
-- 
2.26.2