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

[Francesco Valla]

Hello Greg,

thank you for the quick feedback.

On Tue, Jun 30, 2026 at 12:21:39PM +0200, Greg Kroah-Hartman wrote:
> On Mon, Jun 29, 2026 at 11:51:18PM +0200, Francesco Valla wrote:
> > 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);
> 
> So you now get 2 lines per driver probe attempt?
> 
> What exactly is this going to help out with?  You aleady get the probe
> result line, how is doing 2 going to change anything except explode your
> kernel log?  Why not just modify the one existing line instead?
> 

I was being overzealous with the "don't break the userspace ABI" rule
and included the existing print into this kind of ABI. Given your
response, though, this might not be the case. I'll wait a couple of days
to see if anyone has something to add and then send a V2 that adds the
driver name to the existent print (solution that I technically prefer).

> And the comment "see below" should mean that you move the comment up
> here so that it doesn't need to be repeated.
> 
> thanks,
> 
> greg k-h

Regards,
Francesco