netdev - Re: [net-next 1/3] ice: Initialize and register platform device to provide RDMA

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Date:   Thu, 4 Jul 2019 12:48:29 +0000
From:   Jason Gunthorpe <jgg@...lanox.com>
To:     Greg KH <gregkh@...uxfoundation.org>
CC:     Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "dledford@...hat.com" <dledford@...hat.com>,
        Tony Nguyen <anthony.l.nguyen@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "nhorman@...hat.com" <nhorman@...hat.com>,
        "sassmann@...hat.com" <sassmann@...hat.com>,
        "poswald@...e.com" <poswald@...e.com>,
        "mustafa.ismail@...el.com" <mustafa.ismail@...el.com>,
        "shiraz.saleem@...el.com" <shiraz.saleem@...el.com>,
        Dave Ertman <david.m.ertman@...el.com>,
        Andrew Bowers <andrewx.bowers@...el.com>
Subject: Re: [net-next 1/3] ice: Initialize and register platform device to
 provide RDMA

On Thu, Jul 04, 2019 at 02:42:47PM +0200, Greg KH wrote:
> On Thu, Jul 04, 2019 at 12:37:33PM +0000, Jason Gunthorpe wrote:
> > On Thu, Jul 04, 2019 at 02:29:50PM +0200, Greg KH wrote:
> > > On Thu, Jul 04, 2019 at 12:16:41PM +0000, Jason Gunthorpe wrote:
> > > > On Wed, Jul 03, 2019 at 07:12:50PM -0700, Jeff Kirsher wrote:
> > > > > From: Tony Nguyen <anthony.l.nguyen@...el.com>
> > > > > 
> > > > > The RDMA block does not advertise on the PCI bus or any other bus.
> > > > > Thus the ice driver needs to provide access to the RDMA hardware block
> > > > > via a virtual bus; utilize the platform bus to provide this access.
> > > > > 
> > > > > This patch initializes the driver to support RDMA as well as creates
> > > > > and registers a platform device for the RDMA driver to register to. At
> > > > > this point the driver is fully initialized to register a platform
> > > > > driver, however, can not yet register as the ops have not been
> > > > > implemented.
> > > > 
> > > > I think you need Greg's ack on all this driver stuff - particularly
> > > > that a platform_device is OK.
> > > 
> > > A platform_device is almost NEVER ok.
> > > 
> > > Don't abuse it, make a real device on a real bus.  If you don't have a
> > > real bus and just need to create a device to hang other things off of,
> > > then use the virtual one, that's what it is there for.
> > 
> > Ideally I'd like to see all the RDMA drivers that connect to ethernet
> > drivers use some similar scheme.
> 
> Why?  They should be attached to a "real" device, why make any up?

? A "real" device, like struct pci_device, can only bind to one
driver. How can we bind it concurrently to net, rdma, scsi, etc?

> > This is for a PCI device that plugs into multiple subsystems in the
> > kernel, ie it has net driver functionality, rdma functionality, some
> > even have SCSI functionality
> 
> Sounds like a MFD device, why aren't you using that functionality
> instead?

This was also my advice, but in another email Jeff says:

  MFD architecture was also considered, and we selected the simpler
  platform model. Supporting a MFD architecture would require an
  additional MFD core driver, individual platform netdev, RDMA function
  drivers, and stripping a large portion of the netdev drivers into
  MFD core. The sub-devices registered by MFD core for function
  drivers are indeed platform devices.  

Thanks,
Jason