netdev - Re: [PATCH 1/2] net: dsa: mv88e6xxx: sleep in _mv88e6xxx_stats

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Date:	Fri, 10 Jul 2015 10:10:27 -0700
From:	Guenter <linux@...ck-us.net>
To:	Vivien Didelot <vivien.didelot@...oirfairelinux.com>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Andrew Lunn <andrew@...n.ch>, linux-kernel@...r.kernel.org,
	kernel@...oirfairelinux.com
Subject: Re: [PATCH 1/2] net: dsa: mv88e6xxx: sleep in _mv88e6xxx_stats_wait

On Fri, Jul 10, 2015 at 12:57:28PM -0400, Vivien Didelot wrote:
> The current _mv88e6xxx_stats_wait function does not sleep while testing
> the stats busy bit. Fix this by using the generic _mv88e6xxx_wait
> function.
> 
> Note that it requires to move _mv88e6xxx_wait on top of
> _mv88e6xxx_stats_wait to avoid undefined reference compilation error.
> 
> Signed-off-by: Vivien Didelot <vivien.didelot@...oirfairelinux.com>
> ---
>  drivers/net/dsa/mv88e6xxx.c | 52 +++++++++++++++++++--------------------------
>  1 file changed, 22 insertions(+), 30 deletions(-)
> 
> diff --git a/drivers/net/dsa/mv88e6xxx.c b/drivers/net/dsa/mv88e6xxx.c
> index f6c7409..7753db1 100644
> --- a/drivers/net/dsa/mv88e6xxx.c
> +++ b/drivers/net/dsa/mv88e6xxx.c
> @@ -557,19 +557,31 @@ static bool mv88e6xxx_6352_family(struct dsa_switch *ds)
>  	return false;
>  }
>  
> +/* Must be called with SMI lock held */
> +static int _mv88e6xxx_wait(struct dsa_switch *ds, int reg, int offset,
> +			   u16 mask)
> +{
> +	unsigned long timeout = jiffies + HZ / 10;
> +
> +	while (time_before(jiffies, timeout)) {
> +		int ret;
> +
> +		ret = _mv88e6xxx_reg_read(ds, reg, offset);
> +		if (ret < 0)
> +			return ret;
> +		if (!(ret & mask))
> +			return 0;
> +
> +		usleep_range(1000, 2000);
> +	}
> +	return -ETIMEDOUT;
> +}
> +
>  /* Must be called with SMI mutex held */
>  static int _mv88e6xxx_stats_wait(struct dsa_switch *ds)
>  {
> -	int ret;
> -	int i;
> -
> -	for (i = 0; i < 10; i++) {
> -		ret = _mv88e6xxx_reg_read(ds, REG_GLOBAL, GLOBAL_STATS_OP);
> -		if ((ret & GLOBAL_STATS_OP_BUSY) == 0)
> -			return 0;
> -	}

Hi Vivien,

is this really beneficial and/or needed ? It adds at least 1ms delay
to a loop which did not have any delay at all unless the register
read itself was sleeping. Is the original function seen to return
a timeout error under some circumstances ?

Thanks,
Guenter
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