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

From: Xu Yilun
Date: Mon Oct 19 2020 - 20:47:33 EST

On Mon, Oct 19, 2020 at 11:03:16AM +0200, Greg KH wrote:
> On Mon, Oct 19, 2020 at 04:52:33PM +0800, Xu Yilun wrote:
> > On Mon, Oct 19, 2020 at 12:06:13PM +0800, Xu Yilun 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.
> >
> > Sorry, I checked the Documentation/filesystems/sysfs.rst again and found
> > I didn't remember it correctly.
> >
> > The snprintf is must not be used, sprintf() or scnprintf() should be
> > used. So I prefer sprintf() here, following other implementations.
> >
> > sprintf() should be safe here, as we already limited the input of
> > driver_override string to less then PAGE_SIZE on store().
>
> As already mentioned, please use sysfs_emit() instead.

Yes, I'll do it.

Thanks,
Yilun

>
> thanks,
>
> greg k-h