Re: [PATCH] v4l2-dev: Add tracepoints for QBUF and DQBUF

[Mauro Carvalho Chehab <m.chehab@samsung.com>]

Em Sat, 23 Nov 2013 17:54:48 +0100
Hans Verkuil <hverkuil@xs4all.nl> escreveu:

> On 11/23/2013 05:30 PM, Sylwester Nawrocki wrote:
> > Hi,
> > 
> > On 11/23/2013 12:25 PM, Hans Verkuil wrote:
> >> Hi Wade,
> >>
> >> On 11/22/2013 08:48 PM, Wade Farnsworth wrote:
> >>> Add tracepoints to the QBUF and DQBUF ioctls to enable rudimentary
> >>> performance measurements using standard kernel tracers.
> >>>
> >>> Signed-off-by: Wade Farnsworth<wade_farnsworth@mentor.com>
> >>> ---
> >>>
> >>> This is the update to the RFC patch I posted a few weeks back.  I've added
> >>> several bits of metadata to the tracepoint output per Mauro's suggestion.
> >>
> >> I don't like this. All v4l2 ioctls can already be traced by doing e.g.
> >> echo 1 (or echo 2)>/sys/class/video4linux/video0/debug.
> >>
> >> So this code basically duplicates that functionality. It would be nice to be able
> >> to tie in the existing tracing code (v4l2-ioctl.c) into tracepoints.
> > 
> > I think it would be really nice to have this kind of support for standard
> > traces at the v4l2 subsystem. Presumably it could even gradually replace
> > the v4l2 custom debug infrastructure.
> > 
> > If I understand things correctly, the current tracing/profiling 
> > infrastructure
> > is much less invasive than inserting printks all over, which may cause 
> > changes
> > in control flow. I doubt the system could be reliably profiled by 
> > enabling all
> > those debug prints.
> > 
> > So my vote would be to add support for standard tracers, like in other
> > subsystems in the kernel.
> 
> The reason for the current system is to trace which ioctls are called in
> what order by a misbehaving application. It's very useful for that,
> especially when trying to debug user problems.
> 
> I don't mind switching to tracepoints as long as this functionality is
> kept one way or another.

I agree with Sylwester: we should move to tracepoints, and this is a good
start.

Regards,
Mauro