Re: [PATCH v2 2/4] driver core: handle -EPROBE_DEFER from bus_type.match()

[Marek Szyprowski]

Hello,

On 2015-11-30 14:36, Ulf Hansson wrote:
> On 26 November 2015 at 13:49, Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> wrote:
> > From: Tomeu Vizoso <tomeu.vizoso@xxxxxxxxxxxxx>
> >
> > Lets implementations of the match() callback in struct bus_type to
> /s/Lets/Allow
>
> > return errors and if it's -EPROBE_DEFER then queue the device for
> > deferred probing.
> >
> > This is useful to buses such as AMBA in which devices are registered
> > before their matching information can be retrieved from the HW
> > (typically because a clock driver hasn't probed yet).
> >
> > Signed-off-by: Tomeu Vizoso <tomeu.vizoso@xxxxxxxxxxxxx>
> > Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> > ---
> >   drivers/base/dd.c      | 24 ++++++++++++++++++++++--
> >   include/linux/device.h |  2 +-
> >   2 files changed, 23 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> > index a641cf3..a20c119 100644
> > --- a/drivers/base/dd.c
> > +++ b/drivers/base/dd.c
> > @@ -490,6 +490,7 @@ static int __device_attach_driver(struct device_driver *drv, void *_data)
> >          struct device_attach_data *data = _data;
> >          struct device *dev = data->dev;
> >          bool async_allowed;
> > +       int ret;
> >
> >          /*
> >           * Check if device has already been claimed. This may
> > @@ -500,8 +501,17 @@ static int __device_attach_driver(struct device_driver *drv, void *_data)
> >          if (dev->driver)
> >                  return -EBUSY;
> >
> > -       if (!driver_match_device(drv, dev))
> > +       ret = driver_match_device(drv, dev);
> > +       if (!ret)
> >                  return 0;
> > +       else if (ret < 0) {
> Depending on what happens with the added dev_warn() below, perhaps a
> switch statement can make it a bit clearer, instead of these messy if
> clauses?

Frankly, I have no idea how to convert this to switch statement and make
the code easier to read. Please note that we have the following 4 cases:

ret > 0: positive match
ret == 0: negative match
ret == -EPROBE_DEFER: deferred probe
ret < 0: other, unknown error

Another way to encode this logic is the following code:

if (ret == 0) {
        /* no match */
        return 0;
} else if (ret == -EPROBE_DEFER) {
        dev_dbg(dev, "Device match requests probe deferral\n");
        driver_deferred_probe_add(dev);
        return ret;
} else if (ret < 0) {
        dev_dbg(dev, "Bus failed to match device: %d", ret);
        return ret;
} /* ret > 0 means positive match */


> > +               if (ret == -EPROBE_DEFER) {
> > +                       dev_dbg(dev, "Device match requests probe deferral\n");
> > +                       driver_deferred_probe_add(dev);
> > +               } else
> > +                       dev_warn(dev, "Bus failed to match device: %d", ret);
> Greg commented on this before, as it may introduce some noise [1].
>
> I started browsing various busâs implementation of the ->match()
> callback. A quick search tells me that most implementations are
> following the Documentation/driver-model/porting.txt, which means
> returning 0 or 1. Actually I couldn't find anyone returning any other
> value, though it was a quick search.
>
> On the other hand, include/linux/device.h states a "non-zero" value is
> allowed to be return, so there's a tiny conflict between the code and
> the documentation. I guess we should fix that!?

Okay, I will update the documentation as well.

> No matter what, I realize that it could be useful to print a message
> when receiving a negative error code, maybe dev_dbg() could be
> sufficient?
>
> > +               return ret;
> > +       }
> >
> >          async_allowed = driver_allows_async_probing(drv);
> >
> > @@ -621,6 +631,7 @@ void device_initial_probe(struct device *dev)
> >   static int __driver_attach(struct device *dev, void *data)
> >   {
> >          struct device_driver *drv = data;
> > +       int ret;
> >
> >          /*
> >           * Lock device and try to bind to it. We drop the error
> > @@ -632,8 +643,17 @@ static int __driver_attach(struct device *dev, void *data)
> >           * is an error.
> >           */
> >
> > -       if (!driver_match_device(drv, dev))
> > +       ret = driver_match_device(drv, dev);
> > +       if (!ret)
> > +               return 0;
> > +       else if (ret < 0) {
> > +               if (ret == -EPROBE_DEFER) {
> > +                       dev_dbg(dev, "Device match requests probe deferral\n");
> > +                       driver_deferred_probe_add(dev);
> > +               } else
> > +                       dev_warn(dev, "Bus failed to match device: %d", ret);
> >                  return 0;
> > +       }
> >
> >          if (dev->parent)        /* Needed for USB */
> >                  device_lock(dev->parent);
> > diff --git a/include/linux/device.h b/include/linux/device.h
> > index b8f411b..d4e7d1f 100644
> > --- a/include/linux/device.h
> > +++ b/include/linux/device.h
> > @@ -70,7 +70,7 @@ extern void bus_remove_file(struct bus_type *, struct bus_attribute *);
> >    * @dev_groups:        Default attributes of the devices on the bus.
> >    * @drv_groups: Default attributes of the device drivers on the bus.
> >    * @match:     Called, perhaps multiple times, whenever a new device or driver
> > - *             is added for this bus. It should return a nonzero value if the
> > + *             is added for this bus. It should return a positive value if the
> >    *             given device can be handled by the given driver.
> >    * @uevent:    Called when a device is added, removed, or a few other things
> >    *             that generate uevents to add the environment variables.
> > --
> > 1.9.2

Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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