On Thu, Nov 11, 2021 at 03:02:04PM -0700, Jim Cromie wrote:

Sean Paul proposed, in:
https://patchwork.freedesktop.org/series/78133/
drm/trace: Mirror DRM debug logs to tracefs

His patchset's objective is to be able to independently steer some of
the drm.debug stream to an alternate tracing destination, by splitting
drm_debug_enabled() into syslog & trace flavors, and enabling them
separately.  2 advantages were identified:

1- syslog is heavyweight, tracefs is much lighter
2- separate selection of enabled categories means less traffic

Dynamic-Debug can do 2nd exceedingly well:

A- all work is behind jump-label's NOOP, zero off cost.
B- exact site selectivity, precisely the useful traffic.
   can tailor enabled set interactively, at shell.

Since the tracefs interface is effective for drm (the threads suggest
so), adding that interface to dynamic-debug has real potential for
everyone including drm.

if CONFIG_TRACING:

Grab Sean's trace_init/cleanup code, use it to provide tracefs
available by default to all pr_debugs.  This will likely need some
further per-module treatment; perhaps something reflecting hierarchy
of module,file,function,line, maybe with a tuned flattening.

endif CONFIG_TRACING

Add a new +T flag to enable tracing, independent of +p, and add and
use 3 macros: dyndbg_site_is_enabled/logging/tracing(), to encapsulate
the flag checks.  Existing code treats T like other flags.

I posted a patchset a while ago to do something very similar, but that
got stalled for some reason and I unfortunately didn't follow it up:

 https://lore.kernel.org/lkml/20200825153338.17061-1-vincent.whitchurch@axis.com/ (local)

A key difference between that patchset and this patch (besides that
small fact that I used +x instead of +T) was that my patchset allowed
the dyndbg trace to be emitted to the main buffer and did not force them
to be in an instance-specific buffer.

That feature is quite important at least for my use case since I often
use dyndbg combined with function tracing, and the latter doesn't work
on non-main instances according to Documentation/trace/ftrace.rst.

For example, here's a random example of a bootargs from one of my recent
debugging sessions:

 trace_event=printk:* ftrace_filter=_mmc*,mmc*,sd*,dw_mci*,mci*
 ftrace=function trace_buf_size=20M dyndbg="file drivers/mmc/* +x"


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