linux-kernel - Re: [PATCH] net: dsa: use NET_NAME_PREDICTABLE for user ports with name given in DT

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Date:   Sun, 13 Nov 2022 21:03:52 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: dsa: use NET_NAME_PREDICTABLE for user ports with
 name given in DT

On 11/11/2022 18.10, Andrew Lunn wrote:
>> diff --git a/net/dsa/slave.c b/net/dsa/slave.c
>> index a9fde48cffd4..dfefcc4a9ccf 100644
>> --- a/net/dsa/slave.c
>> +++ b/net/dsa/slave.c
>> @@ -2374,16 +2374,25 @@ int dsa_slave_create(struct dsa_port *port)
>>  {
>>  	struct net_device *master = dsa_port_to_master(port);
>>  	struct dsa_switch *ds = port->ds;
>> -	const char *name = port->name;
>>  	struct net_device *slave_dev;
>>  	struct dsa_slave_priv *p;
>> +	const char *name;
>> +	int assign_type;
>>  	int ret;
>>  
>>  	if (!ds->num_tx_queues)
>>  		ds->num_tx_queues = 1;
>>  
>> +	if (port->name) {
>> +		name = port->name;
>> +		assign_type = NET_NAME_PREDICTABLE;
>> +	} else {
>> +		name = "eth%d";
>> +		assign_type = NET_NAME_UNKNOWN;
>> +	}
> 
> I know it is a change in behaviour, but it seems like NET_NAME_ENUM
> should be used, not NET_NAME_UNKNOWN. alloc_etherdev_mqs() uses
> NET_NAME_ENUM.

I don't really have any strong opinion on the case where we fall back to
eth%d, as its not relevant to any board I've worked on.

> https://elixir.bootlin.com/linux/latest/source/include/uapi/linux/netdevice.h#L42
> says that NET_NAME_UNKNOWN does not get passed to user space, but i
> assume NET_NAME_ENUM does. So maybe changing it would be an ABI
> change?

Well, the name_assign_type ABI is kind of silly. I mean, userspace knows
that when one gets EINVAL trying to read the value, that really means
that the value is NET_NAME_UNKNOWN. But I won't propose changing that.

However, what I do propose here is obviously already an ABI change; I
_want_ to expose more proper information in the case where the port has
a label, and just kept the NET_NAME_UNKNOWN for the eth%d case to make
the minimal change. But if people want to change that to NET_NAME_ENUM
while we're here, I can certainly do that. I can't think of any real
scenario where NET_NAME_ENUM would be treated differently than
NET_NAME_UNKNOWN - in both cases, userspace don't know that the name can
be trusted to be predictable.

Rasmus