linux-kernel - Re: [PATCH tty-next v4 5/6] serial: 8250: Switch to nbcon console

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Message-ID: <Z3B3e6ppZc94Pdck@smile.fi.intel.com>
Date: Sun, 29 Dec 2024 00:11:07 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	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, Matt Turner <mattst88@...il.com>,
	Tony Lindgren <tony@...mide.com>, Arnd Bergmann <arnd@...db.de>,
	Niklas Schnelle <schnelle@...ux.ibm.com>,
	Serge Semin <fancer.lancer@...il.com>
Subject: Re: [PATCH tty-next v4 5/6] serial: 8250: Switch to nbcon console

On Fri, Dec 27, 2024 at 11:51:21PM +0106, 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_atomic and ->write_therad() callbacks allow safe

->write_atomic()

> handover/takeover per byte and add a preceding newline if they
> take over from another context mid-line.
> 
> For the ->write_atomic() callback, a new irq_work is used to defer
> modem control since it may be called from a context that does not
> allow waking up tasks.
> 
> Note: A new __serial8250_clear_IER() is introduced for direct
> clearing of UART_IER. This will allow to restore the lockdep
> check to serial8250_clear_IER() in a follow-up commit.

...

>  		if (toggle_ier) {
> +			/*
> +			 * Port locked to synchronize UART_IER access
> +			 * against the console

Missing period in multi-line comment.

> +			 */
> +			lockdep_assert_held_once(&p->port.lock);
> +
>  			p->ier |= UART_IER_RLSI | UART_IER_RDI;
>  			serial_port_out(&p->port, UART_IER, p->ier);
>  		}

...

> -static void fifo_wait_for_lsr(struct uart_8250_port *up, unsigned int count)
> +static int fifo_wait_for_lsr(struct uart_8250_port *up,
> +			      struct nbcon_write_context *wctxt,
> +			      unsigned int count)
>  {
>  	unsigned int i;
>  
>  	for (i = 0; i < count; i++) {
> +		if (!nbcon_can_proceed(wctxt))
> +			return -EPERM;
> +
>  		if (wait_for_lsr(up, UART_LSR_THRE))
> -			return;
> +			return 0;
>  	}
> +
> +	return -ETIMEDOUT;
>  }

...

>  static void serial8250_console_fifo_write(struct uart_8250_port *up,
> -					  const char *s, unsigned int count)
> +					  struct nbcon_write_context *wctxt)
>  {
> -	const char *end = s + count;
>  	unsigned int fifosize = up->tx_loadsz;
>  	struct uart_port *port = &up->port;
> +	const char *s = wctxt->outbuf;
> +	const char *end = s + wctxt->len;
>  	unsigned int tx_count = 0;
>  	bool cr_sent = false;
>  	unsigned int i;
>  
>  	while (s != end) {
>  		/* Allow timeout for each byte of a possibly full FIFO */
> -		fifo_wait_for_lsr(up, fifosize);
> +		if (fifo_wait_for_lsr(up, wctxt, fifosize) == -EPERM)
> +			return;

Perhaps it was already discussed and I forgot about it or hadn't read,
but I don't get how per-byte check for NBCON permission can help if there
is something already in the HW FIFO?

I mean in _this_ case wouldn't be enough to check FIFO to drain and only after
that check the permission?

>  		for (i = 0; i < fifosize && s != end; ++i) {
>  			if (*s == '\n' && !cr_sent) {

>  	 * Allow timeout for each byte written since the caller will only wait
>  	 * for UART_LSR_BOTH_EMPTY using the timeout of a single character
>  	 */
> -	fifo_wait_for_lsr(up, tx_count);
> +	fifo_wait_for_lsr(up, wctxt, tx_count);
> +}


...

> +	if (up->msr_saved_flags) {
> +		/*
> +		 * For atomic, it must be deferred to irq_work because this
> +		 * may be a context that does not permit waking up tasks.
> +		 */
> +		if (is_atomic)
> +			irq_work_queue(&up->modem_status_work);
> +		else
> +			serial8250_modem_status(up);

Hmm... Shouldn't this be part of _modem_status() for all drivers using this call?

> +	}

...

> +	bool			console_line_ended;	/* line fully output */

Sorry, I didn't get the comment. Do you meant "line is fully printed out
[to HW]"?

-- 
With Best Regards,
Andy Shevchenko