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

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Date:   Mon, 22 Jun 2020 17:36:21 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     kgdb-bugreport@...ts.sourceforge.net, linux-serial@...r.kernel.org,
        gregkh@...uxfoundation.org, jason.wessel@...driver.com,
        dianders@...omium.org, jslaby@...e.com, linux@...linux.org.uk,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] serial: kgdb_nmi: Add support for interrupt based
 fallback

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.


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
>