[PATCH] driver core: add driver name to probe debug print

[Francesco Valla]

The initcall_debug command line option is a useful tool while debugging
and optimizing the initialization of a new system, mainly because it
allows to see probe failures and deferrals without recompiling the
kernel (e.g., with CONFIG_DEBUG_DRIVER). However, matching a device
with the driver it is being probed with can become difficult, since
some devices use names that are not explicit, at least at a first sight
(e.g.: '1-0:1.0' or '1-0060').

Add an additional debug print to inform the user which driver is being
used for a device, allowing for a quick match. The print is inserted in
the same really_probe_debug() wrapper that is already used to report
the result of the probe, and is thus not affecting executions not using
the initcall_debug option.

Suggested-by: Tim Bird <tim.bird@xxxxxxxx>
Signed-off-by: Francesco Valla <francesco@xxxxxxxx>
---
Hello,

this very small patch comes from a discussion started at the end of
2024 after a Boot Time SIG meeting [1]; I decided to reduce the patch
proposed there by Tim to the bare minimum, as this should already be
enough information for a developer to work with.

I was unsure on whether to add information to the existing print or
introduce a new one; while IMO technically worse, I opted for this
second solution, since the existing print *might* be viewed as
userspace-facing ABI. I'll be happy to do otherwise if there is
consensus.

Thank you!

Regards,
Francesco

[1] https://lore.kernel.org/linux-embedded/MW5PR13MB563277AF5972FD2B56026CF9FD3C2@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
---
 drivers/base/dd.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/base/dd.c b/drivers/base/dd.c
index 60c005223844..3c0930020050 100644
--- a/drivers/base/dd.c
+++ b/drivers/base/dd.c
@@ -782,6 +782,9 @@ static int really_probe_debug(struct device *dev, const struct device_driver *dr
 	ktime_t calltime, rettime;
 	int ret;
 
+	/* Don't change this to pr_debug() - see comment below. */
+	printk(KERN_DEBUG "probing %s with driver %s\n", dev_name(dev), drv->name);
+
 	calltime = ktime_get();
 	ret = really_probe(dev, drv);
 	rettime = ktime_get();

---
base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
change-id: 20260628-probe_driver-3f14e94573f6

Best regards,
--  
Francesco Valla <francesco@xxxxxxxx>