linux-kernel - Re: [RFC PATCH] PCI: Introduce new device binding path using pci_dev.driver

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Date:	Tue, 1 Apr 2014 09:47:25 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	stuart.yoder@...escale.com, kvm@...r.kernel.org,
	jan.kiszka@...mens.com, will.deacon@....com,
	linux-kernel@...r.kernel.org, mhocko@...e.cz, bhelgaas@...gle.com,
	Varun.Sethi@...escale.com, kvmarm@...ts.cs.columbia.edu,
	rafael.j.wysocki@...el.com, agraf@...e.de,
	linux-pci@...r.kernel.org, linux@...ck-us.net,
	konrad.wilk@...cle.com, d.kasatkin@...sung.com, tj@...nel.org,
	scottwood@...escale.com, a.motakis@...tualopensystems.com,
	tech@...tualopensystems.com, Bharat.Bhushan@...escale.com,
	toshi.kani@...com, kim.phillips@...aro.org,
	a.rigo@...tualopensystems.com, iommu@...ts.linux-foundation.org,
	joe@...ches.com, christoffer.dall@...aro.org
Subject: Re: [RFC PATCH] PCI: Introduce new device binding path using
 pci_dev.driver_override

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@...hat.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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