Date:   Mon, 19 Oct 2020 15:50:32 +0800
From:   Xu Yilun <yilun.xu@...el.com>
To:     "Wu, Hao" <hao.wu@...el.com>
Cc:     Tom Rix <trix@...hat.com>, "mdf@...nel.org" <mdf@...nel.org>,
        "linux-fpga@...r.kernel.org" <linux-fpga@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "lgoncalv@...hat.com" <lgoncalv@...hat.com>, yilun.xu@...el.com
Subject: Re: [PATCH 1/2] fpga: dfl: add driver_override support

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@...el.com>
> > > > ---
> > > >  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@...el.com>
> > > >  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@...el.com>
> > > >  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@...el.com>
> > > 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.
> 
> 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.

Thanks,
Yilun

> 
> Hao
> 
> >
> > > > +device_unlock(dev);
> > > > +return len;
> > > > +}
> > > > +
> > > > +static ssize_t driver_override_store(struct device *dev,
> > > > +     struct device_attribute *attr,
> > > > +     const char *buf, size_t count)
> > > > +{
> > > > +struct dfl_device *ddev = to_dfl_dev(dev);
> > > > +char *driver_override, *old, *cp;
> > > > +
> > > > +/* We need to keep extra room for a newline */
> > > > +if (count >= (PAGE_SIZE - 1))
> > > > +return -EINVAL;
> > > > +
> > > > +driver_override = kstrndup(buf, count, GFP_KERNEL);
> > > > +if (!driver_override)
> > > > +return -ENOMEM;
> > > > +
> > > > +cp = strchr(driver_override, '\n');
> > > > +if (cp)
> > > > +*cp = '\0';
> > > > +
> > > > +device_lock(dev);
> > > > +old = ddev->driver_override;
> > > > +if (strlen(driver_override)) {
> > > > +ddev->driver_override = driver_override;
> > > > +} else {
> > > > +kfree(driver_override);
> > > > +ddev->driver_override = NULL;
> > > > +}
> > > > +device_unlock(dev);
> > > > +
> > > > +kfree(old);
> > > > +
> > > > +return count;
> > > > +}
> > > > +static DEVICE_ATTR_RW(driver_override);
> > > > +
> > > >  static ssize_t
> > > >  type_show(struct device *dev, struct device_attribute *attr, char *buf)
> > > >  {
> > > > @@ -324,6 +375,7 @@ static DEVICE_ATTR_RO(feature_id);
> > > >  static struct attribute *dfl_dev_attrs[] = {
> > > >  &dev_attr_type.attr,
> > > >  &dev_attr_feature_id.attr,
> > > > +&dev_attr_driver_override.attr,
> > > >  NULL,
> > > >  };
> > > >  ATTRIBUTE_GROUPS(dfl_dev);
> > > > @@ -469,7 +521,7 @@ static int dfl_devs_add(struct
> > dfl_feature_platform_data *pdata)
> > > >
> > > >  int __dfl_driver_register(struct dfl_driver *dfl_drv, struct module *owner)
> > > >  {
> > > > -if (!dfl_drv || !dfl_drv->probe || !dfl_drv->id_table)
> > > > +if (!dfl_drv || !dfl_drv->probe)
> > >
> > > id_table is still needed for the normal case.
> > >
> > > Instead of removing this check, could you add something like
> > >
> > > || (!dfl_drv->is_override && !dfl_drv->id_table)
> >
> > I don't think it is needed. Seems is_override and !id_table are duplicated
> > conditions for this implementation. And it may make confusing, e.g. could
> > a driver been force matched when is_override is not set?
> >
> > I think we could make it simple, if the dfl driver didn't provide the
> > id_table, normally it could not match any device. I think it could be
> > easily understood by dfl driver developers.
> >
> >
> > Thanks,
> > Yilun
> >
> > >
> > > Tom
> > >
> > > >  return -EINVAL;
> > > >
> > > >  dfl_drv->drv.owner = owner;
> > > > diff --git a/include/linux/dfl.h b/include/linux/dfl.h
> > > > index 7affba2f..e1b2471 100644
> > > > --- a/include/linux/dfl.h
> > > > +++ b/include/linux/dfl.h
> > > > @@ -32,6 +32,7 @@ enum dfl_id_type {
> > > >   * @num_irqs: number of IRQs supported by this dfl device.
> > > >   * @cdev: pointer to DFL FPGA container device this dfl device belongs to.
> > > >   * @id_entry: matched id entry in dfl driver's id table.
> > > > + * @driver_override: driver name to force a match
> > > >   */
> > > >  struct dfl_device {
> > > >  struct device dev;
> > > > @@ -43,6 +44,7 @@ struct dfl_device {
> > > >  unsigned int num_irqs;
> > > >  struct dfl_fpga_cdev *cdev;
> > > >  const struct dfl_device_id *id_entry;
> > > > +char *driver_override;
> > > >  };
> > > >
> > > >  /**

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