linux-kernel - Re: [REPOST][PATCH 1/2] driver core: Add new device_driver flag to allow binding via sysfs only

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Date:	Thu, 19 Dec 2013 14:22:11 -0600
From:	Scott Wood <scottwood@...escale.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	Kim Phillips <kim.phillips@...aro.org>,
	<linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
	<R65777@...escale.com>, <B08248@...escale.com>,
	<christoffer.dall@...aro.org>, <alex.williamson@...hat.com>,
	<a.motakis@...tualopensystems.com>, <agraf@...e.de>,
	<B16395@...escale.com>
Subject: Re: [REPOST][PATCH 1/2] driver core: Add new device_driver flag to
 allow binding via sysfs only

On Wed, 2013-12-18 at 17:07 -0800, Greg Kroah-Hartman wrote:
> On Tue, Dec 03, 2013 at 12:34:46PM +0000, Kim Phillips wrote:
> > VFIO supports pass-through of devices to user space - for sake
> > of illustration, say a PCI e1000 device:
> > 
> > - the e1000 is first unbound from the PCI e1000 driver via sysfs
> > - the vfio-pci driver is told via new_id that it now handles e1000 devices
> > - the e1000 is explicitly bound to vfio-pci through sysfs
> > 
> > However, now we have two drivers in the system that both handle e1000
> > devices.  A hotplug event could then occur and it is ambiguous as to which
> > driver will claim the device.  The desired semantics is that vfio-pci is
> > only bound to devices by explicit request in sysfs.  This patch makes this
> > possible by introducing a sysfs_bind_only flag in struct device_driver.
> 
> Why deal with this at all and not just deal with the "bind" sysfs file
> instead?  That way no driver core logic needs to be changed at all, and
> your userspace tools know _exactly_ which device is being bound to the
> new device.
> 
> Don't mess with the "new_id" file for stuff like this, as you point out,
> it's "tricky"...

As discussed before, "bind" does not bypass the ID checks, and thus it
does not work without either "new_id" or a wildcard match.

Or are you proposing changing "bind" so that it does bypass the ID
checks?  Or perhaps a new "force_bind" file that does?

-Scott


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