Re: [RFC][PATCH] ALSA: firewire-lib: support Linux tracing to dump a part of packet data

[Takashi Iwai <tiwai@suse.de>]

On Sun, 27 Mar 2016 14:18:06 +0200,
Takashi Sakamoto wrote:
> 
> When audio and music units have some quirks in their sequence of packet,
> it's really hard for non-owners to identify the quirks. To handle the
> quirks, developers need a dump for sequence of packets at least, however
> it's difficult to users who have no knowledge and equipment for this
> purpose.
> 
> This commit adds tracepoints for this situation. When users encounter
> the issue, they can dump a part of packet data via Linux tracing framework
> as long as using drivers in ALSA firewire stack.
> 
> This commit adds 'snd-firewire-lib' subsystem with 'in_packet' and
> 'out_packet' events. In the events, the content of CIP headers, the number
> of included quadlets and the index of packet managed by this module are
> recorded per packet.
> 
> This is a sample:
> 
> $ trace-cmd record -e snd_firewire_lib:out_packet -e snd_firewire_lib:in_packet
> /sys/kernel/tracing/events/snd_firewire_lib/out_packet/filter
> /sys/kernel/tracing/events/snd_firewire_lib/in_packet/filter
> Hit Ctrl^C to stop recording
> ^C
> $ trace-cmd report trace.dat
> ...
> <idle>-0  [002] 40398.221702: out_packet: 00070008 9001427a: 058: 31
> <idle>-0  [002] 40398.221703: in_packet:  01020010 9001ffff: 002: 15
> <idle>-0  [002] 40398.221703: out_packet: 00070010 9001ffff: 002: 32
> <idle>-0  [002] 40398.223679: in_packet:  010b0010 900157e4: 090: 16
> <idle>-0  [002] 40398.223681: out_packet: 00070010 900157e4: 058: 33
> <idle>-0  [002] 40398.223681: in_packet:  010b0018 9001714f: 090: 17
> ...
> 
> One line represent one packet. The legend for the last four fields is:
>  - The first quadlet of CIP header
>  - The second quadlet of CIP header
>  - The number of included quadlets
>  - The index of packet inner buffer maintained by this module
> 
> Currently, when detecting packet discontinuity, this module stops packet
> streaming. This is reasonable to packet streaming implementation. However,
> to identify the quirks, packet streaming should continue to dump enough
> sequence of packet. This commit adds a condition statement and a marker
> for this purpose.

The packet tracing is nice, and I don't see any reason against it.
But changing the driver behavior depending on the trace state is
unusual, rather a thing to be avoided in general.


thanks,

Takashi