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

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Date:   Fri, 24 Jul 2020 21:00:20 +0000
From:   "Eads, Gage" <gage.eads@...el.com>
To:     Greg KH <gregkh@...uxfoundation.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "arnd@...db.de" <arnd@...db.de>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        "Topel, Bjorn" <bjorn.topel@...el.com>
Subject: RE: [PATCH 01/20] dlb2: add skeleton for DLB 2.0 driver



> -----Original Message-----
> From: Greg KH <gregkh@...uxfoundation.org>
> Sent: Monday, July 20, 2020 2:19 PM
> To: Eads, Gage <gage.eads@...el.com>
> Cc: linux-kernel@...r.kernel.org; arnd@...db.de; Karlsson, Magnus
> <magnus.karlsson@...el.com>; Topel, Bjorn <bjorn.topel@...el.com>
> Subject: Re: [PATCH 01/20] dlb2: add skeleton for DLB 2.0 driver
> 
> On Mon, Jul 20, 2020 at 07:02:05PM +0000, Eads, Gage wrote:
> >
> >
> > > -----Original Message-----
> > > From: Greg KH <gregkh@...uxfoundation.org>
> > > Sent: Saturday, July 18, 2020 1:47 AM
> > > To: Eads, Gage <gage.eads@...el.com>
> > > Cc: linux-kernel@...r.kernel.org; arnd@...db.de; Karlsson, Magnus
> > > <magnus.karlsson@...el.com>; Topel, Bjorn <bjorn.topel@...el.com>
> > > 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@...uxfoundation.org>
> > > > > Sent: Sunday, July 12, 2020 10:58 AM
> > > > > To: Eads, Gage <gage.eads@...el.com>
> > > > > Cc: linux-kernel@...r.kernel.org; arnd@...db.de; Karlsson, Magnus
> > > > > <magnus.karlsson@...el.com>; Topel, Bjorn <bjorn.topel@...el.com>
> > > > > 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.
> 
> Stop going through crazy gyrations for something that your own legal
> team has told you not to do anymore in the first place.
> 
> No "hardware access library" files please, that's not how Linux drivers
> are written.
> 
> you all know better...
> 
> > But if tracepoints are the preferred method and/or you think the driver
> would
> > benefit, I'll make the change.
> 
> I don't think you need any of that stuff, now that the code works
> properly, right?

There are no known issues, correct. The logging (whether it's
dev_dbg/tracepoints/etc.) would be for user-space developers -- visibility into
the driver could help them debug issues in their own code.

It's hardly a critical feature; I'm happy to change or remove it if necessary.
But it could be helpful, isn't a maintenance burden or performance hindrance,
and (AFAICT) shouldn't pose any security risks.

Thanks,
Gage

> 
> greg k-h