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

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Message-ID: <20191110194601.0d6ed1a0@cakuba>
Date:   Sun, 10 Nov 2019 19:46:01 -0800
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Parav Pandit <parav@...lanox.com>,
        Jiri Pirko <jiri@...nulli.us>,
        David M <david.m.ertman@...el.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Saeed Mahameed <saeedm@...lanox.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "leon@...nel.org" <leon@...nel.org>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        Jiri Pirko <jiri@...lanox.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        Or Gerlitz <gerlitz.or@...il.com>
Subject: Re: [PATCH net-next 00/19] Mellanox, mlx5 sub function support

On Sun, 10 Nov 2019 10:18:55 +0100, gregkh@...uxfoundation.org wrote:
> > What I'm missing is why is it so bad to have a driver register to
> > multiple subsystems.  
> 
> Because these PCI devices seem to do "different" things all in one PCI
> resource set.  Blame the hardware designers :)

See below, I don't think you can blame the HW designers in this
particular case :)

> > For the nfp I think the _real_ reason to have a bus was that it
> > was expected to have some out-of-tree modules bind to it. Something 
> > I would not encourage :)  
> 
> That's not ok, and I agree with you.
> 
> But there seems to be some more complex PCI devices that do lots of
> different things all at once.  Kind of like a PCI device that wants to
> be both a keyboard and a storage device at the same time (i.e. a button
> on a disk drive...)

The keyboard which is also a storage device may be a clear cut case
where multiple devices were integrated into one bus endpoint.

The case with these advanced networking adapters is a little different
in that they are one HW device which has oodles of FW implementing
clients or acceleration for various networking protocols.

The nice thing about having a fake bus is you can load out-of-tree
drivers to operate extra protocols quite cleanly.

I'm not saying that's what the code in question is doing, I'm saying 
I'd personally like to understand the motivation more clearly before
every networking driver out there starts spawning buses. The only
argument I've heard so far for the separate devices is reloading subset
of the drivers, which I'd rate as moderately convincing.