linux-kernel - Re: [PATCH] firewire: core: option to log bus reset initiation

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Date: Tue, 26 Mar 2024 21:38:52 +0900
From: Takashi Sakamoto <o-takashi@...amocchi.jp>
To: Adam Goldman <adamg@...ox.com>
Cc: linux1394-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] firewire: core: option to log bus reset initiation

Hi,

On Tue, Mar 26, 2024 at 05:18:32AM -0700, Adam Goldman wrote:
> Hi Takashi,
> 
> On Mon, Mar 25, 2024 at 09:41:34AM +0900, Takashi Sakamoto wrote:
> > Now we have two debug parameters per module for the slightly-similar
> > purpose. In my opinion, it is a pretty cumbersome to enable them when
> > checking bus-reset behaviour. I think it is time to investigate the other
> > way.
> > 
> > Linux Kernel Tracepoints[2] is one of options. Roughly describing, the
> > tracepoints mechanism allows users to deliver structured data from kernel
> > space to user space via ring-buffer when enabling it by either sysfs or
> > kernel command-line parameters. Linux kernel also has a command-line
> > parameter to redirect the human-readable formatted data to kernel log[3].
> > I think it is suitable in the case.
> > 
> > It requires many work to replace the existent debug parameter of
> > firewire-ohci, while it is a good start to work just for bus-reset debug.
> > The data structure layout should be pre-defined in each subsystem, thus we
> > need to decide it. In my opinion, it would be like:
> > 
> > ```
> > struct bus_reset_event {
> >     enum reason {
> >         Initiate,
> > 	Schedule,
> > 	Postpone,
> > 	Detect,
> >     },
> >     // We can put any other data if prefering.
> > }
> > ```
> 
> Maybe these should be four separate trace events?
> 
> > Would I ask your opinion about my idea?
> 
> It seems that tracepoints are the modern way to make debugging logs, so 
> if we want to modernize the FireWire driver, we should replace the 
> existent logging with tracepoints.

Thanks for your positive comment.

I pushed my work-in-progress patches to the following specific topic
branch::
https://github.com/takaswie/linux-firewire-dkms/tree/topic/backport-to-v6.8/tracepoints

You can see some patches onto your commits:

* 145da78e firewire: ohci: obsolete OHCI_PARAM_DEBUG_BUSRESETS from debug parameter with tracepoints event
* 3bdad35d firewire: core: obsolete debug parameter with tracepoints event
* 30f489af firewire: ohci: support bus_reset tracepoints event
* 4937d9c8 firewire: core: support bus_reset tracepoints event
* 0da26087 firewire: core: add support for Linux kernel tracepoints
* 961cba18 firewire: core: option to log bus reset initiation
* b3124560 firewire: ohci: mask bus reset interrupts between ISR and bottom half

In the above, I added 'bus_reset' events in 'firewire' tracepoints
subsystem. The structure is something like:

```
struct bus_reset {
    enum fw_trace_bus_reset_issue issue;
    bool short_reset;
};
```

The issue enumerations are in 'drivers/firewire/core.h':

```
enum fw_trace_bus_reset_issue {
	FW_TRACE_BUS_RESET_ISSUE_INITIATE = 0,
	FW_TRACE_BUS_RESET_ISSUE_SCHEDULE,
	FW_TRACE_BUS_RESET_ISSUE_POSTPONE,
	FW_TRACE_BUS_RESET_ISSUE_DETECT,
};
```

You can see the above event is trigerred by two kernel modules:

* firewire-core
* firewire-ohci

When merging the above changes and build/load the kernel modules, we can
see 'firewire:bus_reset' event in Linux Kernel tracepoints system, like:

```
$ ls /sys/kernel/debug/tracing/events/firewire/bus_reset
```

I currently consider about a pair of events for OHCI interrupts and PHY
operation, instead of the above event. I'm happy if receiving your
opinion about it or the other ideas.


Regards

Takashi Sakamoto