netdev - Re: [PATCH net-next 00/19] Mellanox, mlx5 sub function support

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Date:   Fri, 8 Nov 2019 11:06:40 -0800
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     Parav Pandit <parav@...lanox.com>, alex.williamson@...hat.com,
        davem@...emloft.net, kvm@...r.kernel.org, netdev@...r.kernel.org,
        saeedm@...lanox.com, kwankhede@...dia.com, leon@...nel.org,
        cohuck@...hat.com, jiri@...lanox.com, linux-rdma@...r.kernel.org,
        Or Gerlitz <gerlitz.or@...il.com>
Subject: Re: [PATCH net-next 00/19] Mellanox, mlx5 sub function support

On Fri, 8 Nov 2019 13:12:33 +0100, Jiri Pirko wrote:
> Thu, Nov 07, 2019 at 09:32:34PM CET, jakub.kicinski@...ronome.com wrote:
> >On Thu,  7 Nov 2019 10:04:48 -0600, Parav Pandit wrote:  
> >> Mellanox sub function capability allows users to create several hundreds
> >> of networking and/or rdma devices without depending on PCI SR-IOV support.  
> >
> >You call the new port type "sub function" but the devlink port flavour
> >is mdev.
> >
> >As I'm sure you remember you nacked my patches exposing NFP's PCI 
> >sub functions which are just regions of the BAR without any mdev
> >capability. Am I in the clear to repost those now? Jiri?  
> 
> Well question is, if it makes sense to have SFs without having them as
> mdev? I mean, we discussed the modelling thoroughtly and eventually we
> realized that in order to model this correctly, we need SFs on "a bus".
> Originally we were thinking about custom bus, but mdev is already there
> to handle this.

But the "main/real" port is not a mdev in your case. NFP is like mlx4. 
It has one PCI PF for multiple ports.

> Our SFs are also just regions of the BAR, same thing as you have.
> 
> Can't you do the same for nfp SFs?
> Then the "mdev" flavour is enough for all.

Absolutely not. 

Why not make the main device of mlx5 a mdev, too, if that's acceptable.
There's (a) long precedence for multiple ports on one PCI PF in
networking devices, (b) plenty deployed software 
which depend on the main devices hanging off the PCI PF directly.

The point of mdevs is being able to sign them to VFs or run DPDK on
them (map to user space).

For normal devices existing sysfs hierarchy were one device has
multiple children of a certain class, without a bus and a separate
driver is perfectly fine. Do you think we should also slice all serial
chips into mdevs if they have multiple lines.

Exactly as I predicted much confusion about what's being achieved here,
heh :)