linux-kernel - Re: [PATCH 4/7] serial: kgdb_nmi: Add support for interrupt based fallback

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Message-ID: <CAFA6WYNc7hdagBknFGjSF3rXnKGXL4FY+PutkA8iensv9dJHEg@mail.gmail.com>
Date:   Tue, 23 Jun 2020 15:29:15 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     kgdb-bugreport@...ts.sourceforge.net, linux-serial@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        Douglas Anderson <dianders@...omium.org>,
        Jiri Slaby <jslaby@...e.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/7] serial: kgdb_nmi: Add support for interrupt based fallback

On Mon, 22 Jun 2020 at 22:06, Daniel Thompson
<daniel.thompson@...aro.org> wrote:
>
> On Mon, Jun 22, 2020 at 07:56:21PM +0530, Sumit Garg wrote:
> > From: Daniel Thompson <daniel.thompson@...aro.org>
> >
> > Add a generic NMI fallback to support kgdb NMI console feature which can
> > be overridden by arch specific implementation.
>
> arch_kgdb_ops.enable_nmi should probably be killed off. Given we now
> have request_nmi() I'm dubious there are any good reasons to use this
> API.
>

Sounds reasonable. Along with this, kgdb_nmi_poll_knock() seems to be
unused and can be removed as well.

-Sumit

>
> Daniel.
>
>
> > This common fallback mechanism utilizes kgdb IO based interrupt in order
> > to support entry into kgdb if a user types in kgdb_nmi_magic sequence. So
> > during NMI console init, NMI handler is installed corresponding to kgdb
> > IO based NMI which is invoked when a character is pending and that can be
> > cleared by calling @read_char until it returns NO_POLL_CHAR.
> >
> > Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
> > Co-developed-by: Sumit Garg <sumit.garg@...aro.org>
> > Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> > ---
> >  drivers/tty/serial/kgdb_nmi.c | 47 ++++++++++++++++++++++++++++++++++++-------
> >  1 file changed, 40 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/tty/serial/kgdb_nmi.c b/drivers/tty/serial/kgdb_nmi.c
> > index b32c6b1..2580f39 100644
> > --- a/drivers/tty/serial/kgdb_nmi.c
> > +++ b/drivers/tty/serial/kgdb_nmi.c
> > @@ -42,9 +42,46 @@ MODULE_PARM_DESC(magic, "magic sequence to enter NMI debugger (default $3#33)");
> >  static atomic_t kgdb_nmi_num_readers = ATOMIC_INIT(0);
> >  static struct console *orig_dbg_cons;
> >
> > +static int kgdb_nmi_poll_one_knock(void);
> > +
> > +static irqreturn_t kgdb_handle_nmi(int irq, void *dev_id)
> > +{
> > +     int ret;
> > +
> > +     if (kgdb_nmi_knock < 0) {
> > +             kgdb_breakpoint();
> > +             return IRQ_HANDLED;
> > +     }
> > +
> > +     ret = kgdb_nmi_poll_one_knock();
> > +     if (ret == NO_POLL_CHAR)
> > +             return IRQ_NONE;
> > +
> > +     while (ret != 1) {
> > +             ret = kgdb_nmi_poll_one_knock();
> > +             if (ret == NO_POLL_CHAR)
> > +                     return IRQ_HANDLED;
> > +     }
> > +
> > +     kgdb_breakpoint();
> > +     return IRQ_HANDLED;
> > +}
> > +
> >  static int kgdb_nmi_console_setup(struct console *co, char *options)
> >  {
> > -     arch_kgdb_ops.enable_nmi(1);
> > +     int res;
> > +
> > +     if (arch_kgdb_ops.enable_nmi) {
> > +             arch_kgdb_ops.enable_nmi(1);
> > +     } else if (dbg_io_ops->request_nmi) {
> > +             res = dbg_io_ops->request_nmi(kgdb_handle_nmi, co);
> > +             if (res) {
> > +                     pr_err("ttyNMI0: Cannot request nmi/irq\n");
> > +                     return res;
> > +             }
> > +     } else {
> > +             return -ENODEV;
> > +     }
> >
> >       /* The NMI console uses the dbg_io_ops to issue console messages. To
> >        * avoid duplicate messages during kdb sessions we must inform kdb's
> > @@ -328,9 +365,6 @@ int kgdb_register_nmi_console(void)
> >  {
> >       int ret;
> >
> > -     if (!arch_kgdb_ops.enable_nmi)
> > -             return 0;
> > -
> >       kgdb_nmi_tty_driver = alloc_tty_driver(1);
> >       if (!kgdb_nmi_tty_driver) {
> >               pr_err("%s: cannot allocate tty\n", __func__);
> > @@ -380,9 +414,8 @@ int kgdb_unregister_nmi_console(void)
> >  {
> >       int ret;
> >
> > -     if (!arch_kgdb_ops.enable_nmi)
> > -             return 0;
> > -     arch_kgdb_ops.enable_nmi(0);
> > +     if (arch_kgdb_ops.enable_nmi)
> > +             arch_kgdb_ops.enable_nmi(0);
> >
> >       ret = unregister_console(&kgdb_nmi_console);
> >       if (ret)
> > --
> > 2.7.4
> >