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

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Date:   Fri, 8 Nov 2019 16:32:09 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Parav Pandit <parav@...lanox.com>
Cc:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jiri Pirko <jiri@...nulli.us>,
        David M <david.m.ertman@...el.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "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 Fri, Nov 08, 2019 at 08:20:43PM +0000, Parav Pandit wrote:
> 
> 
> > From: Jason Gunthorpe <jgg@...pe.ca>
> > On Fri, Nov 08, 2019 at 11:12:38AM -0800, Jakub Kicinski wrote:
> > > On Fri, 8 Nov 2019 15:40:22 +0000, Parav Pandit wrote:
> > > > > The new intel driver has been having a very similar discussion
> > > > > about how to model their 'multi function device' ie to bind RDMA
> > > > > and other drivers to a shared PCI function, and I think that discussion
> > settled on adding a new bus?
> > > > >
> > > > > Really these things are all very similar, it would be nice to have
> > > > > a clear methodology on how to use the device core if a single PCI
> > > > > device is split by software into multiple different functional
> > > > > units and attached to different driver instances.
> > > > >
> > > > > Currently there is alot of hacking in this area.. And a consistent
> > > > > scheme might resolve the ugliness with the dma_ops wrappers.
> > > > >
> > > > > We already have the 'mfd' stuff to support splitting platform
> > > > > devices, maybe we need to create a 'pci-mfd' to support splitting PCI
> > devices?
> > > > >
> > > > > I'm not really clear how mfd and mdev relate, I always thought
> > > > > mdev was strongly linked to vfio.
> > > > >
> > > >
> > > > Mdev at beginning was strongly linked to vfio, but as I mentioned
> > > > above it is addressing more use case.
> > > >
> > > > I observed that discussion, but was not sure of extending mdev further.
> > > >
> > > > One way to do for Intel drivers to do is after series [9].
> > > > Where PCI driver says, MDEV_CLASS_ID_I40_FOO RDMA driver
> > > > mdev_register_driver(), matches on it and does the probe().
> > >
> > > Yup, FWIW to me the benefit of reusing mdevs for the Intel case vs
> > > muddying the purpose of mdevs is not a clear trade off.
> > 
> > IMHO, mdev has amdev_parent_ops structure clearly intended to link it to vfio,
> > so using a mdev for something not related to vfio seems like a poor choice.
> > 
> Splitting mdev_parent_ops{} is already in works for larger use case in series [1] for virtio.
> 
> [1] https://patchwork.kernel.org/patch/11233127/

Weird. So what is mdev actually providing and what does it represent
if the entire driver facing API surface is under a union?

This smells a lot like it is re-implementing a bus.. AFAIK bus is
supposed to represent the in-kernel API the struct device presents to
drivers.

Jason