linux-kernel - Re: RFC: (re-)binding the VFIO platform driver to a platform device

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Message-ID: <1381348191.7979.348.camel@snotra.buserror.net>
Date:	Wed, 9 Oct 2013 14:49:51 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
CC:	Yoder Stuart-B08248 <B08248@...escale.com>,
	Wood Scott-B07421 <B07421@...escale.com>,
	Kim Phillips <kim.phillips@...aro.org>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"a.motakis@...tualopensystems.com" <a.motakis@...tualopensystems.com>,
	"agraf@...e.de" <agraf@...e.de>,
	Sethi Varun-B16395 <B16395@...escale.com>,
	Bhushan Bharat-R65777 <R65777@...escale.com>,
	"peter.maydell@...aro.org" <peter.maydell@...aro.org>,
	"santosh.shukla@...aro.org" <santosh.shukla@...aro.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: RFC: (re-)binding the VFIO platform driver to a platform device

On Wed, 2013-10-09 at 12:16 -0700, gregkh@...uxfoundation.org wrote:
> On Wed, Oct 09, 2013 at 07:02:25PM +0000, Yoder Stuart-B08248 wrote:
> > Have been thinking about this issue some more.  As Scott mentioned,
> > 'wildcard' matching for a driver can be fairly done in the platform
> > bus driver.  We could add a new flag to the platform driver struct:
> > 
> > diff --git a/drivers/base/platform.c b/drivers/base/platform.c
> > index 4f8bef3..4d6cf14 100644
> > --- a/drivers/base/platform.c
> > +++ b/drivers/base/platform.c
> > @@ -727,6 +727,10 @@ static int platform_match(struct device *dev, struct device_driver *drv)
> >         struct platform_device *pdev = to_platform_device(dev);
> >         struct platform_driver *pdrv = to_platform_driver(drv);
> > 
> > +       /* the driver matches any device */
> > +       if (pdrv->match_any)
> > +               return 1;
> > +
> >         /* Attempt an OF style match first */
> >         if (of_driver_match_device(dev, drv))
> >                 return 1;
> > 
> > However, the more problematic issue is that a bus driver has no way to 
> > differentiate from an explicit bind request via sysfs and a bind that
> > happened through bus probing.
> 
> That was by design, nice to see I implemented it properly :)
> 
> > I think something like the new flag in the snippet below would enable the platform
> > bus to support platform drivers that only bind on explicit request:
> > 
> > diff --git a/drivers/base/bus.c b/drivers/base/bus.c
> > index 4c289ab..daf6d24 100644
> > --- a/drivers/base/bus.c
> > +++ b/drivers/base/bus.c
> > @@ -201,7 +201,7 @@ static ssize_t bind_store(struct device_driver *drv, const char *buf,
> >         int err = -ENODEV;
> > 
> >         dev = bus_find_device_by_name(bus, NULL, buf);
> > -       if (dev && dev->driver == NULL && driver_match_device(drv, dev)) {
> > +       if (dev && dev->driver == NULL && driver_match_device(drv, dev, 1)) {
> 
> Magic flags are the spawn of your favorite anti-$DIETY.  I'm never going
> to accept that, sorry.
> 
> If you really want to do something "special" for the platform bus, then
> do it only for the platform bus.  But even then, you'll find me arguing
> that you really don't want to do it at all, sorry.

It's not (or shouldn't be) special for the platform bus.  The "don't
bind by default" flag (note that I am *not* referring to the above code
snippet) would be useful for VFIO PCI as well, as it would replace the
hacky and racy usage of new_id.

> I'm still yet to be convinced this is even an issue at all, but maybe
> that's just the jetlag talking...

If this is because you think we should use new_id, we just had a
discussion in this thread about why that's no good.

-Scott



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