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

[Sylwester Nawrocki <sylvester.nawrocki@gmail.com>]

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.

--
Regards,
Sylwester