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

On 2015-11-30 14:36, Ulf Hansson wrote:
> On 26 November 2015 at 13:49, Marek Szyprowski <m.szyprowski@samsung.com> wrote:
>> From: Tomeu Vizoso <tomeu.vizoso@collabora.com>
>>
>> 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@collabora.com>
>> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
>> ---
>> 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 bus’s 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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