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

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Date:   Wed, 27 Feb 2019 13:42:56 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Harini Katakam <harinik@...inx.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Nicolas Ferre <nicolas.ferre@...el.com>,
        David Miller <davem@...emloft.net>,
        Michal Simek <michal.simek@...inx.com>,
        Harini Katakam <harini.katakam@...inx.com>,
        Harini Katakam <harinikatakamlinux@...il.com>
Subject: Re: Request for suggestion on net device re-naming/re-ordering
 based on DT alias

On Wed, 27 Feb 2019 10:45:44 -0800
Florian Fainelli <f.fainelli@...il.com> wrote:

> On 2/27/19 10:40 AM, Stephen Hemminger wrote:
> > On Wed, 27 Feb 2019 17:24:03 +0530
> > Harini Katakam <harinik@...inx.com> 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= <mac1’s interrupt id>, <mac1’s base address>, eth0
> >> netdev=<mac0’s interrupt id>, <mac0’s 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.   
> 
> And Device Tree does already, if you look at the uevent attributes that
> the kernel sends, there should be ample information to uniquely
> determine which physical network device the network device maps to, e
> for instance:
> 
> cat /sys/class/net/eth*/device/uevent
> DRIVER=brcm-systemport
> OF_NAME=ethernet
> OF_FULLNAME=/rdb/ethernet@...0000
> OF_TYPE=network
> OF_COMPATIBLE_0=brcm,systemportlite-v1.00
> OF_COMPATIBLE_1=brcm,systemport
> OF_COMPATIBLE_N=2
> OF_ALIAS_0=eth0
> MODALIAS=of:NethernetTnetworkCbrcm,systemportlite-v1.00Cbrcm,systemport

Then you need to work with udev developers to handle device tree better.

By design ethN is not used for persistent naming, only other names like ensX.
This allows for safer renaming and provides way to handle devices that may
not match any rules.

Most of udev naming is based of properties in sysfs like pci-slot, port etc.
If you had that available on these devices, it would just work now.