netdev - RE: [PATCH v2 RESEND] xen-netback: prefer xenbus_scanf() over xenbus

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Date:   Tue, 25 Oct 2016 07:52:39 +0000
From:   Paul Durrant <Paul.Durrant@...rix.com>
To:     Jan Beulich <JBeulich@...e.com>, Wei Liu <wei.liu2@...rix.com>
CC:     David Vrabel <david.vrabel@...rix.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        Juergen Gross <JGross@...e.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH v2 RESEND] xen-netback: prefer xenbus_scanf() over
 xenbus_gather()

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@...e.com]
> Sent: 24 October 2016 16:08
> To: Paul Durrant <Paul.Durrant@...rix.com>; Wei Liu <wei.liu2@...rix.com>
> Cc: David Vrabel <david.vrabel@...rix.com>; xen-devel@...ts.xenproject.org;
> boris.ostrovsky@...cle.com; Juergen Gross <JGross@...e.com>;
> netdev@...r.kernel.org
> Subject: [PATCH v2 RESEND] xen-netback: prefer xenbus_scanf() over
> xenbus_gather()
> 
> For single items being collected this should be preferred as being more
> typesafe (as the compiler can check format string and to-be-written-to
> variable match) and more efficient (requiring one less parameter to be
> passed).
> 
> Signed-off-by: Jan Beulich <jbeulich@...e.com>
> ---
> v2: Avoid commit message to continue from subject.
> ---
>  drivers/net/xen-netback/xenbus.c |   14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> --- 4.9-rc2/drivers/net/xen-netback/xenbus.c
> +++ 4.9-rc2-xen-netback-prefer-xenbus_scanf/drivers/net/xen-
> netback/xenbus.c
> @@ -889,16 +889,16 @@ static int connect_ctrl_ring(struct back
>  	unsigned int evtchn;
>  	int err;
> 
> -	err = xenbus_gather(XBT_NIL, dev->otherend,
> -			    "ctrl-ring-ref", "%u", &val, NULL);
> -	if (err)
> +	err = xenbus_scanf(XBT_NIL, dev->otherend,
> +			   "ctrl-ring-ref", "%u", &val);
> +	if (err <= 0)

Looking at other uses of xenbus_scanf() in the same code I think the check here should be if (err < 0). It's a nit, since xenbus_scanf() cannot return 0, but it would be better for consistency I think.

>  		goto done; /* The frontend does not have a control ring */
> 
>  	ring_ref = val;
> 
> -	err = xenbus_gather(XBT_NIL, dev->otherend,
> -			    "event-channel-ctrl", "%u", &val, NULL);
> -	if (err) {
> +	err = xenbus_scanf(XBT_NIL, dev->otherend,
> +			   "event-channel-ctrl", "%u", &val);
> +	if (err <= 0) {
>  		xenbus_dev_fatal(dev, err,
>  				 "reading %s/event-channel-ctrl",
>  				 dev->otherend);
> @@ -919,7 +919,7 @@ done:
>  	return 0;
> 
>  fail:
> -	return err;
> +	return err ?: -ENODATA;

I don't think you need this.

  Paul

>  }
> 
>  static void connect(struct backend_info *be)
> 
>