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

[Ulf Hansson]

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?

> +               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!?

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
>

Kind regards
Uffe

[1]
https://lkml.org/lkml/2015/10/17/24
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