linux-kernel - Re: [PATCHv7 07/26] driver core: amba: add device binding path 'driver

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Date:	Wed, 8 Oct 2014 14:18:54 +0200
From:	Antonios Motakis <a.motakis@...tualopensystems.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Alex Williamson <alex.williamson@...hat.com>,
	kvm-arm <kvmarm@...ts.cs.columbia.edu>,
	Linux IOMMU <iommu@...ts.linux-foundation.org>,
	VirtualOpenSystems Technical Team <tech@...tualopensystems.com>,
	KVM devel mailing list <kvm@...r.kernel.org>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	Will Deacon <will.deacon@....com>,
	Kim Phillips <kim.phillips@...escale.com>,
	Eric Auger <eric.auger@...aro.org>,
	Marc Zyngier <marc.zyngier@....com>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv7 07/26] driver core: amba: add device binding path 'driver_override'

On Fri, Sep 26, 2014 at 5:37 PM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
>
> On Tue, Sep 23, 2014 at 04:46:06PM +0200, Antonios Motakis wrote:
> > As already demonstrated with PCI [1] and the platform bus [2], a
> > driver_override property in sysfs can be used to bypass the id matching
> > of a device to a AMBA driver. This can be used by VFIO to bind to any AMBA
> > device requested by the user.
> >
> > [1] http://lists-archives.com/linux-kernel/28030441-pci-introduce-new-device-binding-path-using-pci_dev-driver_override.html
> > [2] https://www.redhat.com/archives/libvir-list/2014-April/msg00382.html
> >
> > Signed-off-by: Antonios Motakis <a.motakis@...tualopensystems.com>
>
> I have to ask why this is even needed in the first place.  To take the
> example in [2], what's wrong with:
>
> echo fff51000.ethernet > /sys/bus/platform/devices/fff51000.ethernet/driver/unbind
> echo fff51000.ethernet > /sys/bus/platform/drivers/vfio-platform/bind
>
> and similar for AMBA.
>
> All we would need to do is to introduce a way of having a driver accept
> explicit bind requests.
>
> In any case:
>
> > +static ssize_t driver_override_store(struct device *_dev,
> > +                                  struct device_attribute *attr,
> > +                                  const char *buf, size_t count)
> > +{
> > +     struct amba_device *dev = to_amba_device(_dev);
> > +     char *driver_override, *old = dev->driver_override, *cp;
> > +
> > +     if (count > PATH_MAX)
> > +             return -EINVAL;
> > +
> > +     driver_override = kstrndup(buf, count, GFP_KERNEL);
> > +     if (!driver_override)
> > +             return -ENOMEM;
> > +
> > +     cp = strchr(driver_override, '\n');
> > +     if (cp)
> > +             *cp = '\0';
>
> I hope that is not replicated everywhere.  This allows up to a page to be
> allocated, even when the first byte may be a newline.  This is wasteful.
>
> How about:
>
>         if (count > PATH_MAX)
>                 return -EINVAL;
>
>         cp = strnchr(buf, count, '\n');
>         if (cp)
>                 count = cp - buf - 1;
>
>         if (count) {
>                 driver_override = kstrndup(buf, count, GFP_KERNEL);
>                 if (!driver_override)
>                         return -ENOMEM;
>         } else {
>                 driver_override = NULL;
>         }
>
>         kfree(dev->driver_override);
>         dev->driver_override = driver_override;

I implemented something along this lines and tested it a bit. The
kernel already splits the input by newlines, and the function is being
called for each one separately; so this change doesn't save any
memory.

>
> Also:
>
> > +static ssize_t driver_override_show(struct device *_dev,
> > +                                 struct device_attribute *attr, char *buf)
> > +{
> > +     struct amba_device *dev = to_amba_device(_dev);
> > +
> > +     return sprintf(buf, "%s\n", dev->driver_override);
> > +}
>
> Do we really want to do a NULL pointer dereference here?
>
> --
> FTTC broadband for 0.8mile line: currently at 9.5Mbps down 400kbps up
> according to speedtest.net.




-- 
Antonios Motakis
Virtual Open Systems
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