Re: [RFC] Allowing modules to control tracing subsystem

[Steven Rostedt]

On Tue, 02 Apr 2019 08:44:31 +0300
Felipe Balbi <felipe.balbi@xxxxxxxxxxxxxxx> wrote:

> Hi,
> 
> Felipe Balbi <felipe.balbi@xxxxxxxxxxxxxxx> writes:

Hi Felipe,

I somehow missed both your original email and this gentle reminder.

> > Hi guys,
> >
> > I've been thinking about turning my f-trace.c USB function [1] into an
> > implementation of the USB Debug Class [2]. In order to support all
> > requests, I would need means to control parts of the tracing subsystem,
> > for example enabling and disabling tracing, flushing the trace buffer,
> > changing trace buffer size, choose which trace events to enable, etc.

I believe a lot of these are already available.

Also note, that Divya made it possible to have your own tracing
instance for a module.

> >
> > Some of those functions (e.g. print_trace_line()) are available for
> > things like trace_kdb.c [3], but they're not available for modules;
> > i.e. they're not exported symbols.
> >
> > This will involve a rather large change to tracing subsystem however we
> > could rely on such infrastructure for remote debugging of e.g. IVIs,
> > phones, etc.
> >
> > The USB Debug Class already predicts the presence of JTAG access and
> > Hardware tracing modules to be exported over USB. It also allows for
> > GDB-type access. Adding our linux kernel tracing would make it into a
> > complete tracing solution.
> >
> > In order to fully support this we would need, not only to expose some
> > internal trace functions for modules, but I suppose the trace format
> > would have to be slightly better defined so that it be decoded remotely
> > give access to vmlinux.
> >
> > What do you guys think about this? Would this be something that folks
> > would be interested in? Currently we can only export ftrace data, not
> > trace events or anything else. Ideally we would export raw trace buffer
> > entries to be decoded at a later moment.
> >

Let me know what changes are needed and we can work something out.

Sorry for the delay, it appears I missed quite a bit of emails in this period :-/


-- Steve

> > cheers
> >
> > [1] https://marc.info/?i=20190321094748.7031-1-felipe.balbi@xxxxxxxxxxxxxxx
> > [2] https://www.usb.org/sites/default/files/documents/usb_debug_class_rev_1_0_final_0.pdf
> > [3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/trace/trace_kdb.c#n20  
> 
> A gentle reminder here.
>