Re: [PATCH 1/2] fpga: dfl: add driver_override support

[gregkh@xxxxxxxxxxxxxxxxxxx]

On Mon, Oct 19, 2020 at 03:50:32PM +0800, Xu Yilun wrote:
> On Mon, Oct 19, 2020 at 03:46:23PM +0800, Wu, Hao wrote:
> > > On Fri, Oct 16, 2020 at 09:21:50AM -0700, Tom Rix wrote:
> > > >
> > > > On 10/15/20 11:02 PM, Xu Yilun wrote:
> > > > > Add support for overriding the default matching of a dfl device to a dfl
> > > > > driver. It follows the same way that can be used for PCI and platform
> > > > > devices. This patch adds the 'driver_override' sysfs file.
> > > > >
> > > > > Signed-off-by: Xu Yilun <yilun.xu@xxxxxxxxx>
> > > > > ---
> > > > >  Documentation/ABI/testing/sysfs-bus-dfl | 28 ++++++++++++++---
> > > > >  drivers/fpga/dfl.c                      | 54
> > > ++++++++++++++++++++++++++++++++-
> > > > >  include/linux/dfl.h                     |  2 ++
> > > > >  3 files changed, 79 insertions(+), 5 deletions(-)
> > > > >
> > > > > diff --git a/Documentation/ABI/testing/sysfs-bus-dfl
> > > b/Documentation/ABI/testing/sysfs-bus-dfl
> > > > > index 23543be..db7e8d3 100644
> > > > > --- a/Documentation/ABI/testing/sysfs-bus-dfl
> > > > > +++ b/Documentation/ABI/testing/sysfs-bus-dfl
> > > > > @@ -1,15 +1,35 @@
> > > > >  What:/sys/bus/dfl/devices/dfl_dev.X/type
> > > > > -Date:Aug 2020
> > > > > -KernelVersion:5.10
> > > > > +Date:Oct 2020
> > > > > +KernelVersion:5.11
> > > > >  Contact:Xu Yilun <yilun.xu@xxxxxxxxx>
> > > > >  Description:Read-only. It returns type of DFL FIU of the device.
> > > Now DFL
> > > > >  supports 2 FIU types, 0 for FME, 1 for PORT.
> > > > >  Format: 0x%x
> > > > >
> > > > >  What:/sys/bus/dfl/devices/dfl_dev.X/feature_id
> > > > > -Date:Aug 2020
> > > > > -KernelVersion:5.10
> > > > > +Date:Oct 2020
> > > > > +KernelVersion:5.11
> > > > >  Contact:Xu Yilun <yilun.xu@xxxxxxxxx>
> > > > >  Description:Read-only. It returns feature identifier local to its DFL
> > > FIU
> > > > >  type.
> > > > >  Format: 0x%x
> > > >
> > > > These updates, do not match the comment.
> > > >
> > > > Consider splitting this out.
> > >
> > > I'm sorry it's a typo. The above code should not be changed.
> > >
> > > >
> > > > > +
> > > > > +What:           /sys/bus/dfl/devices/.../driver_override
> > > > > +Date:           Oct 2020
> > > > > +KernelVersion:  5.11
> > > > > +Contact:        Xu Yilun <yilun.xu@xxxxxxxxx>
> > > > I am looking at description and trying to make it consistent with sysfs-bus-
> > > pci
> > > > > +Description:    This file allows the driver for a device to be specified.
> > > >
> > > > 'to be specified which will override the standard dfl bus feature id to driver
> > > mapping.'
> > >
> > > Yes, it could be improved.
> > >
> > > Actually now it is the "type" and "feature id" matching, the 2 fields
> > > are defined for dfl_driver.id_table. In future for dfl v1, it may be
> > > GUID matching, which will be added to id_table. So how about we make it
> > > more generic:
> > >
> > > 'to be specified which will override the standard ID table matching.'
> > >
> > > >
> > > >
> > > > >  When
> > > > > +                specified, only a driver with a name matching the value written
> > > > > +                to driver_override will have an opportunity to bind to the
> > > > > +                device. The override is specified by writing a string to the
> > > > > +                driver_override file (echo dfl-uio-pdev > driver_override) and
> > > > > +                may be cleared with an empty string (echo > driver_override).
> > > > > +                This returns the device to standard matching rules binding.
> > > > > +                Writing to driver_override does not automatically unbind the
> > > > > +                device from its current driver or make any attempt to
> > > > > +                automatically load the specified driver.  If no driver with a
> > > > > +                matching name is currently loaded in the kernel, the device
> > > > > +                will not bind to any driver.  This also allows devices to
> > > > > +                opt-out of driver binding using a driver_override name such as
> > > > > +                "none".  Only a single driver may be specified in the override,
> > > > > +                there is no support for parsing delimiters.
> > > > > diff --git a/drivers/fpga/dfl.c b/drivers/fpga/dfl.c
> > > > > index 511b20f..bc35750 100644
> > > > > --- a/drivers/fpga/dfl.c
> > > > > +++ b/drivers/fpga/dfl.c
> > > > > @@ -262,6 +262,10 @@ static int dfl_bus_match(struct device *dev,
> > > struct device_driver *drv)
> > > > >  struct dfl_driver *ddrv = to_dfl_drv(drv);
> > > > >  const struct dfl_device_id *id_entry;
> > > > >
> > > > > +/* When driver_override is set, only bind to the matching driver */
> > > > > +if (ddev->driver_override)
> > > > > +return !strcmp(ddev->driver_override, drv->name);
> > > > > +
> > > > >  id_entry = ddrv->id_table;
> > > > >  if (id_entry) {
> > > > >  while (id_entry->feature_id) {
> > > > > @@ -303,6 +307,53 @@ static int dfl_bus_uevent(struct device *dev,
> > > struct kobj_uevent_env *env)
> > > > >        ddev->type, ddev->feature_id);
> > > > >  }
> > > > >
> > > >
> > > > I am looking at other implementations of driver_override* and looking for
> > > consistency.
> > > >
> > > > > +static ssize_t driver_override_show(struct device *dev,
> > > > > +    struct device_attribute *attr, char *buf)
> > > > > +{
> > > > > +struct dfl_device *ddev = to_dfl_dev(dev);
> > > > > +ssize_t len;
> > > > > +
> > > > > +device_lock(dev);
> > > > > +len = sprintf(buf, "%s\n", ddev->driver_override);
> > > > len = snprintf(buf, PAGE_SIZE ...
> > >
> > > It is good to me.
> > >
> > > Some bus drivers use snprintf, some use sprintf.
> > >
> > > I think it is reasonable snprintf is used here, unlike %d, %u ... it is
> > > uncertain for the output size of %s.

No, no one should care at all about this.

If you are caring, then you are using sysfs wrong.

Just use sprintf(), you never care about the size of the buffer for a
show/store function.

Or better yet, use the new sysfs_emit() call, that is what everything
will be converted over to anyway.

> > you limited the size < a page in store function for driver_override?
> 
> Yes. So normally the sprintf should be OK. But I think it may be safer
> if the driver_override pointer is corrupted in some unexpected cases.

How can that happen?

greg k-h