RE: [PATCH v2 0/2] Simplify mtty driver and mdev core

[Parav Pandit]

> -----Original Message-----
> From: Christophe de Dinechin <christophe.de.dinechin@xxxxxxxxx>
> Subject: Re: [PATCH v2 0/2] Simplify mtty driver and mdev core
> 
> 
> Parav Pandit writes:
> 
> > + Dave.
> >
> > Hi Jiri, Dave, Alex, Kirti, Cornelia,
> >
> > Please provide your feedback on it, how shall we proceed?
> >
> > Hence, I would like to discuss below options.
> >
> > Option-1: mdev index
> > Introduce an optional mdev index/handle as u32 during mdev create time.
> > User passes mdev index/handle as input.
> >
> > phys_port_name=mIndex=m%u
> > mdev_index will be available in sysfs as mdev attribute for udev to name the
> mdev's netdev.
> >
> > example mdev create command:
> > UUID=$(uuidgen)
> > echo $UUID index=10 >
> > /sys/class/net/ens2f0/mdev_supported_types/mlx5_core_mdev/create
> > example netdevs:
> > repnetdev=ens2f0_m10	/*ens2f0 is parent PF's netdevice */
> > mdev_netdev=enm10
> >
> > Pros:
> > 1. mdevctl and any other existing tools are unaffected.
> > 2. netdev stack, ovs and other switching platforms are unaffected.
> > 3. achieves unique phys_port_name for representor netdev 4. achieves
> > unique mdev eth netdev name for the mdev using udev/systemd extension.
> > 5. Aligns well with mdev and netdev subsystem and similar to existing sriov
> bdf's.
> >
> > Option-2: shorter mdev name
> > Extend mdev to have shorter mdev device name in addition to UUID.
> > such as 'foo', 'bar'.
> > Mdev will continue to have UUID.
> > phys_port_name=mdev_name
> >
> > Pros:
> > 1. All same as option-1, except mdevctl needs upgrade for newer usage.
> > It is common practice to upgrade iproute2 package along with the kernel.
> > Similar practice to be done with mdevctl.
> > 2. Newer users of mdevctl who wants to work with non_UUID names, will use
> newer mdevctl/tools.
> > Cons:
> > 1. Dual naming scheme of mdev might affect some of the existing tools.
> > It's unclear how/if it actually affects.
> > mdevctl [2] is very recently developed and can be enhanced for dual naming
> scheme.
> >
> > Option-3: mdev uuid alias
> > Instead of shorter mdev name or mdev index, have alpha-numeric name
> alias.
> > Alias is an optional mdev sysfs attribute such as 'foo', 'bar'.
> > example mdev create command:
> > UUID=$(uuidgen)
> > echo $UUID alias=foo >
> > /sys/class/net/ens2f0/mdev_supported_types/mlx5_core_mdev/create
> > example netdevs:
> > examle netdevs:
> > repnetdev = ens2f0_mfoo
> > mdev_netdev=enmfoo
> >
> > Pros:
> > 1. All same as option-1.
> > 2. Doesn't affect existing mdev naming scheme.
> > Cons:
> > 1. Index scheme of option-1 is better which can number large number of
> mdevs with fewer characters, simplifying the management tool.
> 
> I believe that Alex pointed out another "Cons" to all three options, which is that
> it forces user-space to resolve potential race conditions when creating an index
> or short name or alias.
> 
This race condition exists for at least two subsystems that I know of, i.e. netdev and rdma.
If a device with a given name exists, subsystem returns error.
When user space gets error code EEXIST, and it can picks up different identifier(s).

> Also, what happens if `index=10` is not provided on the command-line?
> Does that make the device unusable for your purpose?
Yes, it is unusable to an extent.
Currently we have DEVLINK_PORT_FLAVOUR_PCI_VF in include/uapi/linux/devlink.h
Similar to it, we need to have DEVLINK_PORT_FLAVOUR_MDEV for mdev eswitch ports.
This port flavour needs to generate phys_port_name(). This should be user parameter driven.
Because representor netdevice name is generated based on this parameter.