On Fri, 15 Mar 2024 17:49:00 +0100
Luca Ceresoli <luca.ceres...@bootlin.com> wrote:
> Hello Linux tracing maintainers,
Hi Luca!
>
> I've come across an unexpected behaviour in the kernel tracing
> infrastructure that looks like a bug, or maybe two.
>
> Cc-ing ASoC maintainers for as it appeared using ASoC traces, but it
> does not look ASoC-specific.
>
> It all started when using this trace-cmd sequence on an ARM64 board
> running a mainline 6.8.0-rc7 kernel:
>
> trace-cmd record -e snd_soc_dapm_path ./my-play
> trace-cmd report
>
> While this produces perfectly valid traces for other asoc events,
> the snd_soc_dapm_path produces:
>
> snd_soc_dapm_path: >c<* MIC1_EN <- (direct) <-
>
> instead of the expected:
>
> snd_soc_dapm_path: *MIC1 <- (direct) <- MIC1_EN
>
> The originating macro is:
>
> TP_printk("%c%s %s %s %s %s",
> (int) __entry->path_node &&
> (int) __entry->path_connect ? '*' : ' ',
> __get_str(wname), DAPM_ARROW(__entry->path_dir),
> __get_str(pname), DAPM_ARROW(__entry->path_dir),
> __get_str(pnname))
>
> It appears as if the %c placeholder always produces the three ">c<"
> characters, the '*' or ' ' char is printed as the first %s, all the
> other strings are shifted right by one position and the last string is
> never printed.
>
> On my x86_64 laptop running the default Ubuntu kernel (6.5) I'm able to
> trace a few events having a '%c' in their TP_printk() macros and the
> result is:
>
> intel_pipe_update_start: dev 0000:00:02.0, pipe >c<, frame=1,
> scanline=107856, min=2208, max=2154
>
What does /sys/kernel/tracing/trace show?
If that's fine, then the bug is in libtraceevent and not the kernel.
I'm testing it out now, and I see %c not being processed properly by
libtraceevent. I'll take a deeper look.
Thanks for the report.
-- Steve
> originating from:
>
> TP_printk("dev %s, pipe %c, frame=%u, scanline=%u, min=%u, max=%u",
>
> Here it looks like the %c produced ">c<" again, but apparently without
> any shifting.
>
> Back on the ARM64 board I found a couple interesting clues.
>
> First, using the <debugfs>/tracing/ interface instead of trace-cmd, I'm
> getting correctly formatted strings:
>
> trace-cmd: snd_soc_dapm_path: >c<* HPOUT_L -> (direct) ->
> debugfs: snd_soc_dapm_path: *HPOUT_L <- (direct) <- HPOUT_POP_SOUND_L
>
> Notice the arrows pointing to the opposite direction though. The correct
> arrow is the one in the debugfs run.
>
> Second, I tried a simple test:
>
> TP_printk("(%c,%c,%c,%c) [%s,%s,%s,%s]",
>
>
