Re: Request for suggestion on net device re-naming/re-ordering based on DT alias

[Stephen Hemminger]

On Wed, 27 Feb 2019 17:24:03 +0530
Harini Katakam <harinik@xxxxxxxxxx> wrote:

> Hi,
> 
> We've had some users requesting control over net device name order
> when multiple ethernet devices are present on a system. I've tried a
> few solutions to this and looked it up on forums. But I apologize if
> I have missed something.
> 
> I know that the current system allocates eth<n> as per probe order
> but that is obviously not stably controlled by user (tried DT
> re-ordering and defer probe). One solution is to use DT alias names
> to write to (net_device)->name as follows:
> Devicetree:
> aliases {
>    ethernet0 = &mac1;
>    ethernet1 = &mac0;
> };
> Driver probe:
> +       /* Read ethernet DT alias id and assign to right device name*/
> +       id = of_alias_get_id(np, "ethernet");
> +       if (id < 0) {
> +               dev_warn(&pdev->dev, "failed to get alias id (%u)\n", id);
> +               return id;
> +       }
> +       snprintf(dev->name, sizeof(dev->name), "eth%d", id);
> +
> 
> These three drivers seem to have something similar for mdio/phy bus IDs:
> drivers/net/ethernet/broadcom/genet/bcmmii.c:409: id =
> of_alias_get_id(dn, "eth");
> drivers/net/ethernet/samsung/sxgbe/sxgbe_platform.c:43: plat->bus_id =
> of_alias_get_id(np, "ethernet");
> drivers/net/ethernet/stmicro/stmmac/stmmac_platform.c:404:
> plat->bus_id = of_alias_get_id(np, "ethernet");
> 
> Drawback: This approach will break if alias is not provided for one
> of the interfaces on board. Not to mention, there could be systems
> with multiple ethernet makes (for ex. Cadence macb and Xilinx axienet)
> If one of the drivers does not have an alias read mechanism, it is
> possible to have clashing ID assignments. Is there any way this
> solution can be changed to be stable/acceptable?
> 
> One other alternative I've tried is netdev kernel bootargs but this
> device name was not being picked by the kernel:
> https://www.kernel.org/doc/html/v4.19/admin-guide/kernel-parameters.html
> netdev= <mac1âs interrupt id>, <mac1âs base address>, eth0
> netdev=<mac0âs interrupt id>, <mac0âs base address>, eth1
> 
> Could you please suggest any alternatives?
> Thanks!
> 
> Regards,
> Harini

Device naming is a hard problem, and there is no perfect solution.

Device tree should be providing hints to userspace policy for naming, not
trying to do it in the kernel. 
See: https://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames/