linux-kernel - Re: [PATCH v3] net: ocelot: fix wrong time

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Date:   Wed, 6 Jul 2022 13:13:01 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Pavel Skripkin <paskripkin@...il.com>
CC:     Claudiu Manoil <claudiu.manoil@....com>,
        "alexandre.belloni@...tlin.com" <alexandre.belloni@...tlin.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "clement.leger@...tlin.com" <clement.leger@...tlin.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Lunn <andrew@...n.ch>
Subject: Re: [PATCH v3] net: ocelot: fix wrong time_after usage

Hi Pavel,

On Wed, Jul 06, 2022 at 01:50:44PM +0300, Pavel Skripkin wrote:
> Accidentally noticed, that this driver is the only user of
> while (time_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.
> 
> There is one possible problem with this poll loop: 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.
> 
> Fix it by using sane poll API that avoids all problems described above
> 
> Fixes: 753a026cfec1 ("net: ocelot: add FDMA support")
> Suggested-by: Andrew Lunn <andrew@...n.ch>
> Signed-off-by: Pavel Skripkin <paskripkin@...il.com>
> ---
> 
> Changes since v2:
> 	- Use _atomic variant of readx_poll_timeout
> 
> Changes since v1:
> 	- Fixed typos in title and commit message
> 	- Remove while loop and use readx_poll_timeout as suggested by
> 	  Andrew
> 
> ---
>  drivers/net/ethernet/mscc/ocelot_fdma.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/net/ethernet/mscc/ocelot_fdma.c b/drivers/net/ethernet/mscc/ocelot_fdma.c
> index 083fddd263ec..c93fba0a2a7d 100644
> --- a/drivers/net/ethernet/mscc/ocelot_fdma.c
> +++ b/drivers/net/ethernet/mscc/ocelot_fdma.c
> @@ -94,19 +94,18 @@ static void ocelot_fdma_activate_chan(struct ocelot *ocelot, dma_addr_t dma,
>  	ocelot_fdma_writel(ocelot, MSCC_FDMA_CH_ACTIVATE, BIT(chan));
>  }
>  
> +static u32 ocelot_fdma_read_ch_safe(struct ocelot *ocelot)
> +{
> +	return ocelot_fdma_readl(ocelot, MSCC_FDMA_CH_SAFE);
> +}
> +
>  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;
> +	return readx_poll_timeout_atomic(ocelot_fdma_read_ch_safe, ocelot, safe,
> +				  safe & BIT(chan), 0,
> +				  OCELOT_FDMA_CH_SAFE_TIMEOUT_US);

Can you please indent the arguments to the open bracket?

>  }
>  
>  static void ocelot_fdma_dcb_set_data(struct ocelot_fdma_dcb *dcb,
> -- 
> 2.35.1
>