linux-kernel - Re: Spurious timeouts in mvmdio

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Date:	Tue, 03 Dec 2013 23:38:23 +0000
From:	Leigh Brown <leigh@...inno.co.uk>
To:	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Nicolas Schichan <nschichan@...ebox.fr>,
	Jason Cooper <jason@...edaemon.net>, netdev@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Florian Fainelli <florian@...nwrt.org>,
	"David S. Miller" <davem@...emloft.net>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Spurious timeouts in mvmdio

On 2013-12-03 23:17, Sebastian Hesselbarth wrote:
> On 12/04/2013 12:20 AM, Leigh Brown wrote:
>> On 2013-12-03 22:45, Sebastian Hesselbarth wrote:
>>> On 12/03/2013 09:57 PM, Leigh Brown wrote:
>> [...]
>>>> Nicolas' patch should fix the issue, but I prefer the following as 
>>>> it is
>>>> more
>>>> correct, as it only adjusts the timeout when calling
>>>> wait_event_timeout().  As
>>>> I said above,I believe the polling code is correct.
>>>> 
>>>> diff --git a/drivers/net/ethernet/marvell/mvmdio.c
>>>> b/drivers/net/ethernet/marvell/mvmdio.c
>>>> index 7354960..b187c08 100644
>>>> --- a/drivers/net/ethernet/marvell/mvmdio.c
>>>> +++ b/drivers/net/ethernet/marvell/mvmdio.c
>>>> @@ -92,6 +92,14 @@ static int orion_mdio_wait_ready(struct mii_bus 
>>>> *bus)
>>>>               if (time_is_before_jiffies(end))
>>>>                   ++timedout;
>>>>               } else {
>>>> +            /*
>>>> +             * wait_event_timeout does not guarantee a delay of at
>>>> +             * least one whole jiffie, so timeout must be no less
>>>> +             * than two.
>>>> +             */
>>>> +            if (timeout < 2)
>>>> +                timeout = 2;
>>> 
>>> If you always want to wait at least two jiffies, why not just 
>>> increase
>>> TIMEOUT makro to 20ms instead of messing here with it again?
>>> As said on IRC log above, originally timeout was 100ms.
>> 
>> You could do that, but would you not feel bad leaving a latent bug in
>> the code?
>> I know it's unlikely that someone would set HZ to 50, but if they did,
>> the same
>> bug would appear again.
> 
> If you want to ensure timeout > 2, why not then just use:
> 
> - 	unsigned long timeout = usecs_to_jiffies(MVMDIO_SMI_TIMEOUT);
> + 	unsigned long timeout = 1 + usecs_to_jiffies(MVMDIO_SMI_TIMEOUT);

This will make it correct when using interrupts but it will make the 
loop wait one
jiffie longer than it should when polling.

I think I might be being a little to anal about this, but I guess what 
you could do
is make the above change, then replace the call to 
time_is_before_jiffies() with
time_is_before_eq_jiffies() so that the behaviour when polling and when 
using
interrupts is the same.

diff --git a/drivers/net/ethernet/marvell/mvmdio.c 
b/drivers/net/ethernet/marvell/mvmdio.c
index 7354960..48b8ded 100644
--- a/drivers/net/ethernet/marvell/mvmdio.c
+++ b/drivers/net/ethernet/marvell/mvmdio.c
@@ -75,7 +75,7 @@ static int orion_mdio_smi_is_done(struct 
orion_mdio_dev *dev)
  static int orion_mdio_wait_ready(struct mii_bus *bus)
  {
  	struct orion_mdio_dev *dev = bus->priv;
-	unsigned long timeout = usecs_to_jiffies(MVMDIO_SMI_TIMEOUT);
+	unsigned long timeout = 1 + usecs_to_jiffies(MVMDIO_SMI_TIMEOUT);
  	unsigned long end = jiffies + timeout;
  	int timedout = 0;

@@ -89,7 +89,7 @@ static int orion_mdio_wait_ready(struct mii_bus *bus)
  			usleep_range(MVMDIO_SMI_POLL_INTERVAL_MIN,
  				     MVMDIO_SMI_POLL_INTERVAL_MAX);

-			if (time_is_before_jiffies(end))
+			if (time_is_before_eq_jiffies(end))
  				++timedout;
  	        } else {
  			wait_event_timeout(dev->smi_busy_wait,

Regards,

Leigh.
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