Re: [Xen-devel] [PATCH v2 RESEND] xen-netback: prefer xenbus_scanf() over xenbus_gather()

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 24 October 2016 16:08
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; Wei Liu <wei.liu2@xxxxxxxxxx>
> Cc: David Vrabel <david.vrabel@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> boris.ostrovsky@xxxxxxxxxx; Juergen Gross <JGross@xxxxxxxx>;
> netdev@xxxxxxxxxxxxxxx
> 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@xxxxxxxx>
> ---
> 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)
> 
> 

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