linux-kernel - Re: [PATCH] net: ocelot: fix wront time

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Date:   Sat, 21 May 2022 00:06:16 +0300
From:   Pavel Skripkin <paskripkin@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     vladimir.oltean@....com, claudiu.manoil@....com,
        alexandre.belloni@...tlin.com, UNGLinuxDriver@...rochip.com,
        davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        dan.carpenter@...cle.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: ocelot: fix wront time_after usage

Hi Andrew,

On 5/20/22 15:40, Andrew Lunn wrote:
> On Thu, May 19, 2022 at 11:40:17PM +0300, Pavel Skripkin wrote:
>> Accidentally noticed, that this driver is the only user of
>> while (timer_after(jiffies...)).
>> 
>> It looks like typo, because likely this while loop will finish after 1st
>> iteration, because time_after() returns true when 1st argument _is after_
>> 2nd one.
>> 
>> Fix it by negating time_after return value inside while loops statement
> 
> A better fix would be to use one of the helpers in linux/iopoll.h.
> 
> There is a second bug in the current code:
> 
> static int ocelot_fdma_wait_chan_safe(struct ocelot *ocelot, int chan)
> {
> 	unsigned long timeout;
> 	u32 safe;
> 
> 	timeout = jiffies + usecs_to_jiffies(OCELOT_FDMA_CH_SAFE_TIMEOUT_US);
> 	do {
> 		safe = ocelot_fdma_readl(ocelot, MSCC_FDMA_CH_SAFE);
> 		if (safe & BIT(chan))
> 			return 0;
> 	} while (time_after(jiffies, timeout));
> 
> 	return -ETIMEDOUT;
> }
> 
> The scheduler could put the thread to sleep, and it does not get woken
> up for OCELOT_FDMA_CH_SAFE_TIMEOUT_US. During that time, the hardware
> has done its thing, but you exit the while loop and return -ETIMEDOUT.
> 
> linux/iopoll.h handles this correctly by testing the state one more
> time after the timeout has expired.
> 

I wasn't aware about these macros. Thanks for pointing out to that header!

Will send v2 soon,



With regards,
Pavel Skripkin

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