RE: [PATCH v4] PCI: add kernel parameter to override devid<->driver mapping.

[Stuart Yoder]

> -----Original Message-----
> From: Marcel Apfelbaum [mailto:marcel.a@xxxxxxxxxx]
> Sent: Thursday, October 23, 2014 7:32 AM
> To: Alex Williamson
> Cc: linux-pci@xxxxxxxxxxxxxxx; bhelgaas@xxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; marcel@xxxxxxxxxx;
> mst@xxxxxxxxxx; Yoder Stuart-B08248
> Subject: Re: [PATCH v4] PCI: add kernel parameter to override devid<->driver mapping.
> 
> On Wed, 2014-10-22 at 12:32 -0600, Alex Williamson wrote:
> > [cc+ stuart]
> >
> > On Mon, 2014-10-20 at 17:04 +0300, Marcel Apfelbaum wrote:
> > > Scanning a lot of devices during boot requires a lot of time.
> > > On other scenarios there is a need to bind a driver to a specific slot.
> > >
> > > Binding devices to pci-stub driver does not work,
> > > as it will not differentiate between devices of the
> > > same type. Using some start scripts is error prone.
> > >
> > > The solution leverages driver_override functionality introduced by
> > >
> > > 	commit: 782a985d7af26db39e86070d28f987cad21313c0
> > > 	Author: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > > 	Date:   Tue May 20 08:53:21 2014 -0600
> > >
> > >     	PCI: Introduce new device binding path using pci_dev.driver_override
> > >
> > > In order to bind PCI slots to specific drivers use:
> > > 	pci=driver[xxxx:xx:xx.x]=foo,driver[xxxx:xx:xx.x]=bar,...
> > >
> > > Signed-off-by: Marcel Apfelbaum <marcel.a@xxxxxxxxxx>
> > > ---
> > > v3 -> v4:
> > >  - Addressed Alex Williamson's comments:
> > >    - Modified the type of driver_override_entry's fields
> > >    - Used PCI_DEVFN when appropriated
> > >    - Removed redundant checks
> > >    - Replaced BUG_ON with pr_err messages
> > >    - Simpler command line parsing
> > >  - Addressed Michael S. Tsirkin comments
> > >    - removed DRIVER_OVERRIDE_NAME_LENGTH limitation
> > > v2 -> v3:
> > >  - Corrected subject line
> > > v1 -> v2:
> > >  - Addressed Michael S. Tsirkin comments
> > >    - Removed 32 slots limitation
> > >    - Better handling of memory allocation failures
> > >      (preferred BUG_ON over error messages)
> > >  - Addressed Alex Williamson's comments:
> > >    - Modified commit message to show parameter usage more clear.
> > >  - I preferred to re-use parse_args instead of manually using
> > >    strstr in order to better comply with command line parsing
> > >    rules.
> > >  - I didn't use any locking when parsing the command line args
> > >    (see parse_done usage) assuming that first call will be
> > >    early in system boot and no race can occur. Please correct
> > >    me if I am wrong.
> > >
> > > Notes:
> > >  - I have further ideas on top of this patch based on your reviews.
> > >    I thought of:
> > >    - Use wildcards to specify entire buses/devices, something like:
> > >      	driver[0001:02:*.*]=pci-stub
> > >    - Use comma to separate several devices:
> > >      	driver[0001:02:03.4,0001:02:04.0,...]=pci-stub
> > >    - Make domain optional:
> > >    	driver[00:03.0]=pci-stub
> > >
> > > Comments will be appreciated,
> > > Thanks,
> > > Marcel
> > >  Documentation/kernel-parameters.txt |   4 ++
> > >  drivers/pci/bus.c                   | 111 ++++++++++++++++++++++++++++++++++++
> > >  drivers/pci/pci.c                   |   2 +
> > >  3 files changed, 117 insertions(+)
> >
> > The driver_override feature that we're making use of here is also going
> > to be supported by platform devices and potentially more bustypes in the
> > future, so I'm concerned that making a pci specific kernel parameter is
> > too shortsighted.  Instead we could hook on to BUS_NOTIFY_ADD_DEVICE for
> > bustypes that support driver_override so we can have a common interface.
> The real question here if those bus types/devices would benefit from this
> feature, and I also must confess that I have no knowledge of the other buses.
> Can anyone confirm that it does make sense for them?

We don't have vfio-platform in use yet, so not much actual real world user
experience yet.  But, I think it makes sense.   Especially, given that we are
inventing a kernel parameter I think we should design the syntax so that it
can work buses can implement support for this.  The driver_override mechanism
is not bus specific, so let's not make the kernel parameter bus specific.

> > Perhaps:
> >
> > driver_override=pci,0000:02:00.0=pci-stub;platform,fakename=vfio-platform
> >
> > Finding delimiters that don't conflict may be challenging.  Also, can we
> > assume that bus-name:dev-name is unique for every bustype?  It is for
> > pci, platform?
> For PCI, sure the domain:bus:dev.func is unique, for platform I have no idea,
> can anyone that knows "platform" confirm or deny?

Yes, dev-name will be unique.  All platform devices are under a single
platform bus.

Stuart