linux-kernel - Re: [RFC PATCH] tty: serial: kgdboc: Fix 8250

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Message-ID: <CAOf5uwmTg_T+vHsZwMtkPbxqQdQx4VucMni+f71KtGZY_XdgcA@mail.gmail.com>
Date: Sat, 16 Dec 2023 14:45:55 +0100
From: Michael Nazzareno Trimarchi <michael@...rulasolutions.com>
To: Doug Anderson <dianders@...omium.org>
Cc: Jason Wessel <jason.wessel@...driver.com>, 
	Daniel Thompson <daniel.thompson@...aro.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby <jirislaby@...nel.org>, 
	kgdb-bugreport@...ts.sourceforge.net, linux-kernel@...r.kernel.org, 
	linux-serial@...r.kernel.org
Subject: Re: [RFC PATCH] tty: serial: kgdboc: Fix 8250_* kgd over serial

Hi Doug

On Tue, Dec 12, 2023 at 9:54 AM Michael Nazzareno Trimarchi
<michael@...rulasolutions.com> wrote:
>
> Hi Doug
>
> On Mon, Dec 11, 2023 at 11:00 PM Doug Anderson <dianders@...omium.org> wrote:
> >
> > Hi,
> >
> > On Mon, Dec 11, 2023 at 1:42 PM Michael Nazzareno Trimarchi
> > <michael@...rulasolutions.com> wrote:
> > >
> > > > 1. init_kgdboc() runs and registers the singleton kgdb "platform driver".
> > > >
> > > > 2. The platform driver's probe function, kgdboc_probe(), runs and
> > > > checks to see if the console is ready by looking at the return value
> > > > of configure_kgdboc(). If it's ready then we're good to go. If it's
> > > > not ready then we defer.
> > > >
> > > > So I think the bug here is that somehow the console looks "ready"
> > > > (because tty_find_polling_driver() can find it) but it isn't actually
> > > > ready yet (because it crashes). That's what you need to fix.
> > > >
> > >
> > > The polling driver look for uart and uart8250_core is registered and 4 fake uart
> > > are there but there are not still replaced by platform driver that can
> > > come later.
> > > The try_polling find it but it's the isa-8250 driver. It means that
> > > add_uart 8250 is
> > > not still happen
> >
> > The 8250 driver is always a maze, so you might need to do a bunch of
> > digging. ...but it sure sounds like the console shouldn't be
> > registered until the correct ops are in place. That either means
> > getting the ops put in place earlier or deferring when the console is
> > registered...
> >
>
> Your point is pretty clear and my initial idea was to find a real fix.
> This come to avoid
> breaking existing setup but anyway I will dig in it more
>
> Michael

What about this?

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -385,6 +385,7 @@ struct tty_driver *tty_find_polling_driver(char
*name, int *line)
        int tty_line = 0;
        int len;
        char *str, *stp;
+       int index;

        for (str = name; *str; str++)
                if ((*str >= '0' && *str <= '9') || *str == ',')
@@ -406,7 +407,7 @@ struct tty_driver *tty_find_polling_driver(char
*name, int *line)
                if (*stp == '\0')
                        stp = NULL;

-               if (tty_line >= 0 && tty_line < p->num && p->ops &&
+               if (tty_line >= 0 && tty_line < p->num && p->ops &&
console_device(&index) == p &&
                    p->ops->poll_init && !p->ops->poll_init(p, tty_line, stp)) {
                        res = tty_driver_kref_get(p);
                        *line = tty_line;

I will send proper patch

[   18.885348] printk: legacy console [ttyS2] disabled
[   18.890821] 2800000.serial: ttyS2 at MMIO 0x2800000 (irq = 283,
base_baud = 3000000) is a 8250
[   18.899727] printk: legacy console [ttyS2] enabled
[   18.909440] printk: legacy bootconsole [ns16550a0] disabled
[   18.923263] omap8250_probe: register uart 2800000.serial

Michael
>
> > -Doug
>
>
>
> --
> Michael Nazzareno Trimarchi
> Co-Founder & Chief Executive Officer
> M. +39 347 913 2170
> michael@...rulasolutions.com
> __________________________________
>
> Amarula Solutions BV
> Joop Geesinkweg 125, 1114 AB, Amsterdam, NL
> T. +31 (0)85 111 9172
> info@...rulasolutions.com
> www.amarulasolutions.com



-- 
Michael Nazzareno Trimarchi
Co-Founder & Chief Executive Officer
M. +39 347 913 2170
michael@...rulasolutions.com
__________________________________

Amarula Solutions BV
Joop Geesinkweg 125, 1114 AB, Amsterdam, NL
T. +31 (0)85 111 9172
info@...rulasolutions.com
www.amarulasolutions.com