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

From: Antonios Motakis
Date: Wed Oct 08 2014 - 08:19:20 EST


On Fri, Sep 26, 2014 at 5:37 PM, Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxx>
>
> 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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