netdev - Re: [PATCH 4/4] asix: Add a new driver for the AX88172A

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Date:	Wed, 11 Jul 2012 17:10:14 +0200
From:	Christian Riesch <christian.riesch@...cron.at>
To:	michael@...sch.at
Cc:	Ben Hutchings <bhutchings@...arflare.com>, netdev@...r.kernel.org,
	Oliver Neukum <oneukum@...e.de>,
	Eric Dumazet <edumazet@...gle.com>,
	Allan Chou <allan@...x.com.tw>,
	Mark Lord <kernel@...savvy.com>,
	Grant Grundler <grundler@...omium.org>,
	Ming Lei <tom.leiming@...il.com>
Subject: Re: [PATCH 4/4] asix: Add a new driver for the AX88172A

Hi again,

On Wed, Jul 11, 2012 at 10:27 AM, Christian Riesch
<christian.riesch@...cron.at> wrote:
> Hi Ben and Michael,
>
> On Mon, Jul 9, 2012 at 12:30 PM, Christian Riesch
> <christian.riesch@...cron.at> wrote:
>> Hi Ben and Michael,
>>
>> On Sun, Jul 8, 2012 at 5:39 PM, Michael Riesch <michael@...sch.at> wrote:
>>> On Fri, 2012-07-06 at 18:37 +0100, Ben Hutchings wrote:
>>>> > +   priv->mdio->priv = (void *)dev;
>>>> > +   priv->mdio->read = &asix_mdio_bus_read;
>>>> > +   priv->mdio->write = &asix_mdio_bus_write;
>>>> > +   priv->mdio->name = "Asix MDIO Bus";
>>>> > +   snprintf(priv->mdio->id, MII_BUS_ID_SIZE, "asix-%s",
>>>> > +            dev_name(dev->net->dev.parent));
>>>> [...]
>>>>
>>>> I think you need to ensure that the bus identifier is unique throughout
>>>> its lifetime, but net devices can be renamed and that could lead to a
>>>> collision.  Perhaps you could use the ifindex or the USB device path
>>>
>>> Ben,
>>>
>>> the dev_name function in the code above returns the sysfs filename of
>>> the USB device (e.g. 1-0:1.0).
>>>
>>>> (though that might be too long).
>>>
>>> This may be a problem. The bus identifier may be 17 characters long, so
>>> if we leave the endpoint/configuration part (:1.0) and the prefix away
>>> it should be fine in any "normal" system. However, on a system with a
>>> more-than-9-root-hubs 5-tier 127-devices-each USB infrastructure it
>>> results in collisions. So is this approach acceptable?
>>>
>>> Using the ifindex sounds good to me,
>>>
>>> snprintf(priv->mdio->id, MII_BUS_ID_SIZE, "asix-%d",
>>>         dev->net->ifindex);
>>>
>>> works on any system with less than 10^12 network interfaces.
>>
>> Ok, I'll change that to use ifindex.
>
> No, I won't.
> At the time the mdio bus is registered, ifindex is not yet set, so the
> snprintf would always result in "asix-0".

What do you think about
snprintf(priv->mdio->id, MII_BUS_ID_SIZE, "usb-%03d:%03d",
dev->udev->bus->busnum, dev->udev->devnum);
??

This would use the busnum/devnum identifier as reported by lsusb and
would be short enough for an mdio bus name.

Thanks, Christian
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