Date: Tue, 1 Apr 2014 09:47:25 -0700
From: Greg KH <gregkh@linuxfoundation.org>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: stuart.yoder@freescale.com, kvm@vger.kernel.org, jan.kiszka@siemens.com,
        will.deacon@arm.com, linux-kernel@vger.kernel.org, mhocko@suse.cz,
        bhelgaas@google.com, Varun.Sethi@freescale.com,
        kvmarm@lists.cs.columbia.edu, rafael.j.wysocki@intel.com,
        agraf@suse.de, linux-pci@vger.kernel.org, linux@roeck-us.net,
        konrad.wilk@oracle.com, d.kasatkin@samsung.com, tj@kernel.org,
        scottwood@freescale.com, a.motakis@virtualopensystems.com,
        tech@virtualopensystems.com, Bharat.Bhushan@freescale.com,
        toshi.kani@hp.com, kim.phillips@linaro.org,
        a.rigo@virtualopensystems.com, iommu@lists.linux-foundation.org,
        joe@perches.com, christoffer.dall@linaro.org
Subject: Re: [RFC PATCH] PCI: Introduce new device binding path using
 pci_dev.driver_override
Message-ID: <20140401164725.GA4649@kroah.com>
References: <20140401161851.18815.31108.stgit@bling.home>
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On Tue, Apr 01, 2014 at 10:28:54AM -0600, Alex Williamson wrote:
> The driver_override field allows us to specify the driver for a device
> rather than relying on the driver to provide a positive match of the
> device.  This shortcuts the existing process of looking up the vendor
> and device ID, adding them to the driver new_id, binding the device,
> then removing the ID, but it also provides a couple advantages.
> 
> First, the above process allows the driver to bind to any device
> matching the new_id for the window where it's enabled.  This is often
> not desired, such as the case of trying to bind a single device to a
> meta driver like pci-stub or vfio-pci.  Using driver_override we can
> do this deterministically using:
> 
> echo pci-stub > /sys/bus/pci/devices/0000:03:00.0/driver_override
> echo 0000:03:00.0 > /sys/bus/pci/devices/0000:03:00.0/driver/unbind
> echo 0000:03:00.0 > /sys/bus/pci/drivers_probe
> 
> Previously we could not invoke drivers_probe after adding a device
> to new_id for a driver as we get non-deterministic behavior whether
> the driver we intend or the standard driver will claim the device.
> Now it becomes a deterministic process, only the driver matching
> driver_override will probe the device.
> 
> To return the device to the standard driver, we simply clear the
> driver_override and reprobe the device, ex:
> 
> echo > /sys/bus/pci/devices/0000:03:00.0/preferred_driver
> echo 0000:03:00.0 > /sys/bus/pci/devices/0000:03:00.0/driver/unbind
> echo 0000:03:00.0 > /sys/bus/pci/drivers_probe
> 
> Another advantage to this approach is that we can specify a driver
> override to force a specific binding or prevent any binding.  For
> instance when an IOMMU group is exposed to userspace through VFIO
> we require that all devices within that group are owned by VFIO.
> However, devices can be hot-added into an IOMMU group, in which case
> we want to prevent the device from binding to any driver (preferred
> driver = "none") or perhaps have it automatically bind to vfio-pci.
> With driver_override it's a simple matter for this field to be set
> internally when the device is first discovered to prevent driver
> matches.
> 
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
> 
> Apologies for the exceptionally long cc list, this is a follow-up to
> Stuart's "Subject: mechanism to allow a driver to bind to any device"
> thread.  This is effectively a v2 of the proof-of-concept patch I
> posted in that thread.  This version changes to use a dummy id struct
> to return on an "override" match, which removes the collateral damage
> and greatly simplifies the patch.  This feels fairly well baked for
> PCI and I would expect that platform drivers could do a similar
> implementation.  From there perhaps we can discuss whether there's
> any advantage to placing driver_override on struct device.  The logic
> for incorporating it into the match still needs to happen per bus
> driver, so it might only contribute to consistency of the show/store
> sysfs attributes to move it up to struct device.  Please comment.
> Thanks,
> 
> Alex
> 
>  drivers/pci/pci-driver.c |   25 ++++++++++++++++++++++---
>  drivers/pci/pci-sysfs.c  |   40 ++++++++++++++++++++++++++++++++++++++++
>  include/linux/pci.h      |    1 +
>  3 files changed, 63 insertions(+), 3 deletions(-)

No Documentation/ABI/ update to reflect the ABI you are creating?

thanks,

greg k-h
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