linux-kernel - Re: [PATCH tty-next v3 5/6] serial: 8250: Switch to nbcon console

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Message-ID: <419edf13-3f5e-4d30-90d8-88118f77f704@kernel.org>
Date: Wed, 30 Oct 2024 07:33:24 +0100
From: Jiri Slaby <jirislaby@...nel.org>
To: John Ogness <john.ogness@...utronix.de>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: 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>,
 Geert Uytterhoeven <geert+renesas@...der.be>, Arnd Bergmann <arnd@...db.de>,
 Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
 Tony Lindgren <tony@...mide.com>, Rengarajan S <rengarajan.s@...rochip.com>,
 Peter Collingbourne <pcc@...gle.com>, Serge Semin <fancer.lancer@...il.com>,
 Lino Sanfilippo <l.sanfilippo@...bus.com>
Subject: Re: [PATCH tty-next v3 5/6] serial: 8250: Switch to nbcon console

On 25. 10. 24, 12:57, John Ogness wrote:
> Implement the necessary callbacks to switch the 8250 console driver
> to perform as an nbcon console.
> 
> Add implementations for the nbcon console callbacks (write_atomic,
> write_thread, device_lock, device_unlock) and add CON_NBCON to the
> initial flags.
> 
> All register access in the callbacks are within unsafe sections.
> The write callbacks allow safe handover/takeover per byte and add
> a preceding newline if they take over mid-line.
> 
> For the write_atomic() case, a new irq_work is used to defer modem
> control since it may be a context that does not allow waking up
> tasks.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> ---
>   drivers/tty/serial/8250/8250_core.c |  35 +++++-
>   drivers/tty/serial/8250/8250_port.c | 159 ++++++++++++++++++++++------
>   include/linux/serial_8250.h         |   7 +-
>   3 files changed, 164 insertions(+), 37 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
> index 5f9f06911795..7184100129bd 100644
> --- a/drivers/tty/serial/8250/8250_core.c
> +++ b/drivers/tty/serial/8250/8250_core.c
> @@ -388,12 +388,34 @@ void __init serial8250_register_ports(struct uart_driver *drv, struct device *de
>   
>   #ifdef CONFIG_SERIAL_8250_CONSOLE
>   
> -static void univ8250_console_write(struct console *co, const char *s,
> -				   unsigned int count)
> +static void univ8250_console_write_atomic(struct console *co,

Once 'co'.

> +					  struct nbcon_write_context *wctxt)
>   {
>   	struct uart_8250_port *up = &serial8250_ports[co->index];
>   
> -	serial8250_console_write(up, s, count);
> +	serial8250_console_write(up, wctxt, true);
> +}
> +
> +static void univ8250_console_write_thread(struct console *co,

Second time co.

> +					  struct nbcon_write_context *wctxt)
> +{
> +	struct uart_8250_port *up = &serial8250_ports[co->index];
> +
> +	serial8250_console_write(up, wctxt, false);
> +}
> +
> +static void univ8250_console_device_lock(struct console *con, unsigned long *flags)

And suddenly, it is 'con'.

> +{
> +	struct uart_port *up = &serial8250_ports[con->index].port;
> +
> +	__uart_port_lock_irqsave(up, flags);
> +}
> +
> +static void univ8250_console_device_unlock(struct console *con, unsigned long flags)
> +{
> +	struct uart_port *up = &serial8250_ports[con->index].port;
> +
> +	__uart_port_unlock_irqrestore(up, flags);
>   }
>   

...
>   static void serial8250_console_putchar(struct uart_port *port, unsigned char ch)
>   {
> +	struct uart_8250_port *up = up_to_u8250p(port);
> +
>   	serial_port_out(port, UART_TX, ch);
> +
> +	if (ch == '\n')
> +		up->console_line_ended = true;
> +	else
> +		up->console_line_ended = false;

So simply:
    up->console_line_ended = ch == '\n';
?

...
> -void serial8250_console_write(struct uart_8250_port *up, const char *s,
> -			      unsigned int count)
> +void serial8250_console_write(struct uart_8250_port *up,
> +			      struct nbcon_write_context *wctxt,
> +			      bool is_atomic)
>   {
>   	struct uart_8250_em485 *em485 = up->em485;
>   	struct uart_port *port = &up->port;
> -	unsigned long flags;
> -	unsigned int ier, use_fifo;
> -	int locked = 1;
> -
> -	touch_nmi_watchdog();
> +	unsigned int ier;
> +	bool use_fifo;
>   
> -	if (oops_in_progress)
> -		locked = uart_port_trylock_irqsave(port, &flags);
> -	else
> -		uart_port_lock_irqsave(port, &flags);
> +	if (!nbcon_enter_unsafe(wctxt))
> +		return;
>   
>   	/*
> -	 *	First save the IER then disable the interrupts
> +	 * First save IER then disable the interrupts. The special variant

When you are at it:
"First, save the IER, then"

(BTW why did you remove the "the"?)

> +	 * to clear IER is used because console printing may occur without
> +	 * holding the port lock.
>   	 */
>   	ier = serial_port_in(port, UART_IER);
> -	serial8250_clear_IER(up);
> +	__serial8250_clear_IER(up);
>   
>   	/* check scratch reg to see if port powered off during system sleep */
>   	if (up->canary && (up->canary != serial_port_in(port, UART_SCR))) {

> @@ -3497,6 +3593,9 @@ int serial8250_console_setup(struct uart_port *port, char *options, bool probe)
>   	if (!port->iobase && !port->membase)
>   		return -ENODEV;
>   
> +	up->console_line_ended = true;
> +	up->modem_status_work = IRQ_WORK_INIT(modem_status_handler);

Looks weird ^^^.

Do:
   init_irq_work(&up->modem_status_work, modem_status_handler)

thanks,
-- 
js
suse labs