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

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Message-ID: <20200527162203.GA149714@roeck-us.net>
Date:   Wed, 27 May 2020 09:22:03 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Henrik Rydberg <rydberg@...math.org>,
        Jean Delvare <jdelvare@...e.com>,
        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: Re: [PATCH] hwmon: applesmc: avoid overlong udelay()

On Wed, May 27, 2020 at 03:51:57PM +0200, Arnd Bergmann wrote:
> 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.
> 
Me not either.

> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Applied.

Thanks,
Guenter

> ---
>  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);