linux-kernel - Re: [PATCH tty-next v5 5/6] serial: 8250: Switch to nbcon console

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Message-ID: <csyu222cbxrze5yv3qnwj3gl67x2pwkd6ichpotrwxoj7jru5h@cywpyiqycfre>
Date: Thu, 9 Oct 2025 12:04:13 +0200
From: Thierry Reding <thierry.reding@...il.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Jon Hunter <jonathanh@...dia.com>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Thierry Reding <treding@...dia.com>, 
	Jiri Slaby <jirislaby@...nel.org>, Petr Mladek <pmladek@...e.com>, 
	Sergey Senozhatsky <senozhatsky@...omium.org>, Steven Rostedt <rostedt@...dmis.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Esben Haabendal <esben@...nix.com>, linux-serial@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, 
	Arnd Bergmann <arnd@...db.de>, Tony Lindgren <tony@...mide.com>, 
	Niklas Schnelle <schnelle@...ux.ibm.com>, Serge Semin <fancer.lancer@...il.com>, 
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH tty-next v5 5/6] serial: 8250: Switch to nbcon console

On Wed, Oct 08, 2025 at 08:21:49PM +0100, Jon Hunter wrote:
> Hi John,
> 
> On 08/10/2025 16:56, John Ogness wrote:
> > Hi Jon,
> > 
> > On 2025-01-15, Jon Hunter <jonathanh@...dia.com> wrote:
> > > I have noticed a suspend regression on -next for some of our 32-bit
> > > Tegra (ARM) devices (Tegra20, Tegra30 and Tegra124). Bisect is pointing
> > > to this commit and reverting this on top of -next (along with reverting
> > > "serial: 8250: Revert "drop lockdep annotation from
> > > serial8250_clear_IER()") fixes the issue. So far I have not dug in any
> > > further. Unfortunately, I don't have any logs to see if there is some
> > > crash or something happening but I will see if there is any more info I
> > > can get.
> > 
> > I have been looking into reproducing this using other 8250/ARM boards
> > (BeagleBone Black and Phytec WEGA). Unfortunately it is just showing me
> > all kinds of other brokenness (in mainline) and essentially making it
> > impossible to confirm that I am seeing what you are seeing, since
> > suspend/resume is randomly dying without my 8250-nbcon patch.
> > 
> > Before I start spending weeks investigating/fixing most likely totally
> > unrelated PM or BSP issues, is it possible that I could receive one of
> > the boards you mentioned so that I can reproduce and debug the actual
> > problem you are reporting? If this is possible, feel free to take this
> > conversation offline so that we can discuss delivery details. Thanks!
> 
> These boards are really old now and so I don't really have any that we can
> ship. It would be great to get this change merged as I see that it is needed
> for RT support. I could see if I can resurrect a Tegra124 Jetson TK1 and
> test again on that to see if we can get some logs.
> 
> Thierry, do you have a Tegra124 Jetson TK1 handy to test this change on?

Yes, I do. I reapplied patches 5 and 6 from the series (resolved a tiny
conflict for patch 5) and reran the tests. Same results as back in
January, though. Basically the first suspend doesn't work (it exits back
to userspace after a few seconds) and the second attempt then hangs. No
idea why that would be happpening.

I looked a bit at the code, but nothing jumped out that would explain
this. Not that I'm very familiar with any of this code, or the specifics
needed by nbcon. no_console_suspend doesn't have any noticeable effect,
other than providing a few more messages during suspend, but nothing
that would indicate what's going wrong.

John, I'm happy to test any other patches if you've got any ideas on
what could be wrong.

Thierry

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