Re: mechanism to allow a driver to bind to any device

[Antonios Motakis <a.motakis-lrHrjnjw1UfHK3s98zE1ajGjJy/sRE9J@public.gmane.org>]

(Resend because of html)

On Wed, Mar 26, 2014 at 2:40 AM, Stuart Yoder
<stuart.yoder-KZfg59tc24xl57MIdRCFDg@public.gmane.org> wrote:
> Hi Greg,
>
> We (Linaro, Freescale, Virtual Open Systems) are trying get an issue
> closed that has been perculating for a while around creating a mechanism
> that will allow kernel drivers like vfio can bind to devices of any type.
>
> This thread with you:
> http://www.spinics.net/lists/kvm-arm/msg08370.html
> ...seems to have died out, so am trying to get your response
> and will summarize again.  Vfio drivers in the kernel (regardless of
> bus type) need to bind to devices of any type.  The driver's function
> is to simply export hardware resources of any type to user space.
>
> There are several approaches that have been proposed:
>
>    1.  new_id -- (current approach) the user explicitly registers
>        each new device type with the vfio driver using the new_id
>        mechanism.
>
>        Problem: multiple drivers will be resident that handle the
>        same device type...and there is nothing user space hotplug
>        infrastructure can do to help.
>

Of note is that new_id doesn't work particularly well for platform
devices. Before trying any of the other two solutions, a nasty hack
was applied on the device tree used with the system in order to let
vfio-platform match with a specific device, which is certainly not an
acceptable solution.

Implementing wildcard matching quickly reveals the problems mentioned,
which motivates the other two approaches.

>    2.  "any id" -- the vfio driver could specify a wildcard match
>        of some kind in its ID match table which would allow it to
>        match and bind to any possible device id.  However,
>        we don't want the vfio driver grabbing _all_ devices...just the ones we
>        explicitly want to pass to user space.
>
>        The proposed patch to support this was to create a new flag
>        "sysfs_bind_only" in struct device_driver.  When this flag
>        is set, the driver can only bind to devices via the sysfs
>        bind file.  This would allow the wildcard match to work.
>
>        Patch is here:
>        https://lkml.org/lkml/2013/12/3/253
>
>    3.  "Driver initiated explicit bind" -- with this approach the
>        vfio driver would create a private 'bind' sysfs object
>        and the user would echo the requested device into it:
>
>        echo 0001:03:00.0 > /sys/bus/pci/drivers/vfio-pci/vfio_bind
>
>        In order to make that work, the driver would need to call
>        driver_probe_device() and thus we need this patch:
>        https://lkml.org/lkml/2014/2/8/175
>
>
> Would like your comment on these options-- option #3 is preferred
> and is literally a 2 line patch.

I would definitely agree with approach #3, for which Kim has already
provided a patch. Not having this would make using VFIO with platform
devices really inelegant and strange.

>
> Thanks,
> Stuart



-- 
Antonios Motakis
Virtual Open Systems