linux-kernel - Re: [PATCH] applesmc: Bump max wait and rearrange udelay

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Date:	Sun, 16 Sep 2012 15:00:54 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Parag Warudkar <parag.lkml@...il.com>
Cc:	Henrik Rydberg <rydberg@...omail.se>, lm-sensors@...sensors.org,
	linux-kernel@...r.kernel.org, khali@...ux-fr.org
Subject: Re: [PATCH] applesmc: Bump max wait and rearrange udelay

On Sun, Sep 16, 2012 at 05:22:21PM -0400, Parag Warudkar wrote:
> 
> 
> On Sun, 16 Sep 2012, Henrik Rydberg wrote:
> 
> > On Sat, Sep 15, 2012 at 09:31:16PM -0700, Guenter Roeck wrote:
> > > That looks terribly complicated. Better keep the loop, and just replace
> > > 	udelay(us);
> > > with something like
> > > 	usleep_range(us, us << 1);
> > > 
> > > Alternatively, just use a constant such as
> > > 	usleep_range(us, us + APPLESMC_MIN_WAIT);
> > 
> Well I don't think there is anything terribly complicated there - but I 
> tried to make it simpler. Below patch seems to work better for me for my 
> normal workload - I got no failures or other oddities with the default 
> 32ms timeout. I haven't tried very hard to get to the corner cases which 
> earlier required a higher timeout. 
> 
> I would like to get this in for now if there aren't any further 
> suggestions, as it fixes the failures by making it use the 
> 32ms maximum it was originally supposed to and for bonus it's also 
> converted to use usleep_range which is better from PM standpoint.
> 
> > It would be worthwhile to check if there are other bits in status that
> > encodes a busy state, similar to what we now have in send_byte(). This
> > is what has finally made almost all machines error-free. Increasing
> > the max wait is possible, of course, but it only kills the symptoms.
> > 
> > So, Parag, would it be possible for you to print the status field as
> > you go through one of those long waits? If you find a bit that seems
> > to change occasionally, you could try to use it as a busy indicator,
> > and use the APPLESMC_RETRY_WAIT for that case.
> > 
> Henrik - if the below patch still results in failures we can 
> revisit the long  wait cases. For now I am satisfied that there aren't any 
> normal case failures like before.
> 
> Thanks,
> Parag
> 
> Signed-off-by: Parag Warudkar <parag.lkml@...il.com>
> 
> diff --git a/drivers/hwmon/applesmc.c b/drivers/hwmon/applesmc.c
> index 2827088..569aa8d 100644
> --- a/drivers/hwmon/applesmc.c
> +++ b/drivers/hwmon/applesmc.c
> @@ -168,14 +168,14 @@ static struct workqueue_struct *applesmc_led_wq;
>  static int wait_read(void)
>  {
>  	u8 status;
> -	int us;
> -	for (us = APPLESMC_MIN_WAIT; us < APPLESMC_MAX_WAIT; us <<= 1) {
> -		udelay(us);
> +	int us = APPLESMC_MIN_WAIT;
> +	do {
>  		status = inb(APPLESMC_CMD_PORT);
>  		/* read: wait for smc to settle */
>  		if (status & 0x01)
>  			return 0;
> -	}
> +		usleep_range(us, us << 1);
> +	} while ((us <<= 1) <= APPLESMC_MAX_WAIT);
>  
No difference to the original for loop, so might as well keep it.

Also, the problem with usleep_range() after the status read is that you end up
waiting just to abort which doesn't really make sense.

>  	pr_warn("wait_read() fail: 0x%02x\n", status);
>  	return -EIO;
> @@ -192,19 +192,22 @@ static int send_byte(u8 cmd, u16 port)
>  
>  	outb(cmd, port);
>  	for (us = APPLESMC_MIN_WAIT; us < APPLESMC_MAX_WAIT; us <<= 1) {
> -		udelay(us);
>  		status = inb(APPLESMC_CMD_PORT);
>  		/* write: wait for smc to settle */
> -		if (status & 0x02)
> +		if (status & 0x02) {
> +			usleep_range(us, us << 1);

For the last loop iteration, this sleep doesn't provide any value
and just delays the inevitable error return.

>  			continue;
> +		}
>  		/* ready: cmd accepted, return */
>  		if (status & 0x04)
>  			return 0;
>  		/* timeout: give up */
> -		if (us << 1 == APPLESMC_MAX_WAIT)
> +		if (us << 1 > APPLESMC_MAX_WAIT) {

With this change, you never get here: us is at most APPLESMC_MAX_WAIT/2 and
us << 1 is thus never larger than APPLESMC_MAX_WAIT. Did you want to change
"us < APPLESMC_MAX_WAIT" to "us <= APPLESMC_MAX_WAIT" above ?

> +			pr_warn("Timeout with us: %d\n", us);
>  			break;
> +		}
>  		/* busy: long wait and resend */
> -		udelay(APPLESMC_RETRY_WAIT);
> +		usleep_range(APPLESMC_RETRY_WAIT, APPLESMC_RETRY_WAIT << 1);
>  		outb(cmd, port);
>  	}
>  
>  
> 
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