Re: [PATCH v3] tty/serial: create debugfs interface for UART register tracing

[Andy Shevchenko <andriy.shevchenko@intel.com>]

On Thu, Aug 31, 2023 at 12:33:33PM -0600, Dan Raymond wrote:
> On 8/31/2023 12:08 PM, Andy Shevchenko wrote:
> > Warning: This email is from an unusual correspondent.
> > Warning: Make sure this is someone you trust.
> > 
> > On Thu, Aug 24, 2023 at 02:59:01PM -0600, Dan Raymond wrote:
> >> Implement a UART register tracing facility using the debugfs.  This can be
> >> used as a "serial port sniffer" to monitor UART traffic and line settings
> >> with timestamps at microsecond granularity.  This can be useful for general
> >> serial port debugging or to debug the UART driver itself.
> > 
> >>  drivers/tty/serial/8250/8250_debug.c | 530 +++++++++++++++++++++++++++
> > 
> > My gosh. Why trace points and trace events can't be used for that?
> 
> I don't understand your post.  Are you saying there is already a way to
> trace UART register reads/writes without this patch? Can you elaborate?

Besides what Greg an I were telling you (another type of infrastructure),
as a matter of fact you may trace any IO on some architectures (at least x86),
it's called mmiotracer (I have used it like 5 years ago or so to trace UART).

Below is the excerpt from my old shell script

rebind() {
	local drvdir="/sys/bus/$1/drivers/$2"
	local devdir="/sys/bus/$1/devices/$3"
	[ -d "$drvdir" -a -d "$devdir" ] || return
	echo "$3" > "$drvdir/unbind"
	echo "$3" > "$drvdir/bind"
}

mmiotrace() {
	echo mmiotrace > /sys/kernel/tracing/current_tracer
	echo 1 > /sys/kernel/tracing/tracing_on

	rebind platform 'dw-apb-uart'   '80860F0A:00'   # BYT ttyS1 (ACPI)
	rebind platform 'dw-apb-uart'   '8086228A:00'   # BSW ttyS1 (ACPI)
	rebind pci      '8250_mid'      '0000:00:04.2'  # Edison ttyS1
	rebind pci      'intel-lpss'    '0000:00:18.1'  # BXT LH ttyS1
	rebind pci      'intel-lpss'    '0000:00:1e.0'  # SKL ttyS1
	rebind pci      '8250_lpss'     '0000:00:1e.3'  # BSW ttyS1 (PCI)
}


-- 
With Best Regards,
Andy Shevchenko