Re: [PATCH 2/5] vmbus: add driver_override support

["gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>]

On Mon, Aug 13, 2018 at 07:30:50PM +0000, Michael Kelley (EOSG) wrote:
> From: kys@linuxonhyperv.com <kys@linuxonhyperv.com>  Sent: Friday, August 10, 2018 4:06 PM
> 
> > From: Stephen Hemminger <stephen@networkplumber.org>
> > 
> > Add support for overriding the default driver for a VMBus device
> > in the same way that it can be done for PCI devices. This patch
> > adds the /sys/bus/vmbus/devices/.../driver_override file
> > and the logic for matching.
> > 
> > This is used by driverctl tool to do driver override.
> > https://gitlab.com/driverctl/driverctl
> > 
> > Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
> > Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
> > ---
> > diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> > index b1b548a21f91..e6d8fdac6d8b 100644
> > --- a/drivers/hv/vmbus_drv.c
> > +++ b/drivers/hv/vmbus_drv.c
> > @@ -498,6 +498,54 @@ static ssize_t device_show(struct device *dev,
> >  }
> >  static DEVICE_ATTR_RO(device);
> > 
> > +static ssize_t driver_override_store(struct device *dev,
> > +				     struct device_attribute *attr,
> > +				     const char *buf, size_t count)
> > +{
> > +	struct hv_device *hv_dev = device_to_hv_device(dev);
> > +	char *driver_override, *old, *cp;
> > +
> > +	/* We need to keep extra room for a newline */
> > +	if (count >= (PAGE_SIZE - 1))
> > +		return -EINVAL;
> 
> Does 'count' actually have a relationship to PAGE_SIZE, or
> is PAGE_SIZE just used as an arbitrary size limit?  I'm
> wondering what happens on ARM64 with a 64K page size,
> for example.  If it's just arbitrary, coding such a constant
> would be better.

sysfs buffers are PAGE_SIZE big.