RE: [PATCH 01/20] dlb2: add skeleton for DLB 2.0 driver

[Eads, Gage]

> -----Original Message-----
> From: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
> Sent: Saturday, July 18, 2020 1:47 AM
> To: Eads, Gage <gage.eads@xxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx; arnd@xxxxxxxx; Karlsson, Magnus
> <magnus.karlsson@xxxxxxxxx>; Topel, Bjorn <bjorn.topel@xxxxxxxxx>
> Subject: Re: [PATCH 01/20] dlb2: add skeleton for DLB 2.0 driver
> 
> On Fri, Jul 17, 2020 at 06:18:46PM +0000, Eads, Gage wrote:
> >
> >
> > > -----Original Message-----
> > > From: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > Sent: Sunday, July 12, 2020 10:58 AM
> > > To: Eads, Gage <gage.eads@xxxxxxxxx>
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx; arnd@xxxxxxxx; Karlsson, Magnus
> > > <magnus.karlsson@xxxxxxxxx>; Topel, Bjorn <bjorn.topel@xxxxxxxxx>
> > > Subject: Re: [PATCH 01/20] dlb2: add skeleton for DLB 2.0 driver
> > >
> > > On Sun, Jul 12, 2020 at 08:43:12AM -0500, Gage Eads wrote:
> > > > +static int dlb2_probe(struct pci_dev *pdev,
> > > > +		      const struct pci_device_id *pdev_id) {
> > > > +	struct dlb2_dev *dlb2_dev;
> > > > +	int ret;
> > > > +
> > > > +	dev_dbg(&pdev->dev, "probe\n");
> > >
> > > ftrace is your friend.  Remove all of your debugging code now, you don't
> need
> > > it anymore, especially for stuff like this where you didn't even need it in
> the
> > > first place :(
> >
> > I'll remove this and other similar dev_dbg() calls. This was an oversight on
> my part.
> >
> > I have other instances that a kprobe can't easily replace, such as printing
> structure contents, that are useful for tracing the usage of the driver. It looks
> like other misc drivers use dev_dbg() similarly -- do you consider this an
> acceptable use of a debug print?
> 
> Why can't a kernel tracepoint print a structure?

I meant the command-line installed kprobes[1], but instrumenting the driver is
certainly an option. We don't require the much lower overhead of a tracepoint,
so I didn't choose it. This driver handles the (performance-insensitive)
device configuration, while the fast-path operations take place in user-space.

Another reason is the "hardware access library" files use only non-GPL external
symbols, and some tracepoint functions are exported GPL. Though it's probably
feasible to lift that tracing code up into a (GPLv2-only) caller function.

But if tracepoints are the preferred method and/or you think the driver would
benefit, I'll make the change.

Thanks,
Gage

[1] https://www.kernel.org/doc/html/latest/trace/kprobetrace.html