[v3] serial: amba-pl011: Implement nbcon console

Implement the callbacks required for an NBCON console [0] on the
amba-pl011 console driver.

Referred to the NBCON implementation work for 8250 [1] and imx [2].

The normal-priority write_thread checks for console ownership
each time a character is printed.
write_atomic holds the console ownership until the entire string
is printed.

UART register operations are protected from other contexts by
uart_port_lock, except for a final flush(nbcon_atomic_flush_unsafe)
on panic.

The patch has been verified to correctly handle the output and
competition of messages with different priorities and flushing
panic message to console after nmi panic using ARM64 QEMU and
a physical machine(A64FX).

Stress testing was conducted on a physical machine(A64FX).
The results are as follows:

- The output speed and volume of messages using the NBCON console were
  comparable to the legacy console (data suggests a slight improvement).

- When inducing a panic (sysrq-triggered or NMI) under heavy contention
  on the serial console output,
  the legacy console resulted in the loss of some or all crash messages.
  However, using the NBCON console, no message loss occurred.

This testing referenced the NBCON console work for 8250 [3].

[0] https://lore.kernel.org/all/ZuRRTbapH0DCj334@pathway.suse.cz/
[1] https://lore.kernel.org/all/20240913140538.221708-1-john.ogness@linutronix.de/T/
[2] https://lore.kernel.org/linux-arm-kernel/20240913-serial-imx-nbcon-v3-1-4c627302335b@geanix.com/T/
[3] https://lore.kernel.org/lkml/ZsdoD6PomBRsB-ow@debarbos-thinkpadt14sgen2i.remote.csb/#t

Signed-off-by: Toshiyuki Sato <fj6611ie@aa.jp.fujitsu.com>
---
This patch removes the legacy console code.
Please comment if you have any concerns.

Changes in v3:
- Add stress test results to patch comments.
- based on tty-next branch
  https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git
v2: https://lore.kernel.org/all/20250115052749.3205675-1-fj6611ie@aa.jp.fujitsu.com/

Changes in v2:
- Remove the module parameter used to switch between legacy and NBCON.
- Remove codes for legacy console.
- Fix build errors detected by the test robot.
- based on 6.13-rc7
v1: https://lore.kernel.org/all/20250108004730.2302996-1-fj6611ie@aa.jp.fujitsu.com/

Thanks Greg for the comment.
---
 drivers/tty/serial/amba-pl011.c | 143 ++++++++++++++++++++++----------
 1 file changed, 97 insertions(+), 46 deletions(-)


base-commit: 2014c95afecee3e76ca4a56956a936e23283f05b

Dear Reviewers,

It has been about a month since this patch was merged into tty-next -> linux-next.
I would appreciate any comments or feedback on this patch.

Regards.
Toshiyuki Sato


> Implement the callbacks required for an NBCON console [0] on the
> amba-pl011 console driver.
> 
> Referred to the NBCON implementation work for 8250 [1] and imx [2].
> 
> The normal-priority write_thread checks for console ownership each time a
> character is printed.
> write_atomic holds the console ownership until the entire string is printed.
> 
> UART register operations are protected from other contexts by uart_port_lock,
> except for a final flush(nbcon_atomic_flush_unsafe) on panic.
> 
> The patch has been verified to correctly handle the output and competition of
> messages with different priorities and flushing panic message to console after
> nmi panic using ARM64 QEMU and a physical machine(A64FX).
> 
> Stress testing was conducted on a physical machine(A64FX).
> The results are as follows:
> 
> - The output speed and volume of messages using the NBCON console were
>   comparable to the legacy console (data suggests a slight improvement).
> 
> - When inducing a panic (sysrq-triggered or NMI) under heavy contention
>   on the serial console output,
>   the legacy console resulted in the loss of some or all crash messages.
>   However, using the NBCON console, no message loss occurred.
> 
> This testing referenced the NBCON console work for 8250 [3].
> 
> [0] https://lore.kernel.org/all/ZuRRTbapH0DCj334@pathway.suse.cz/
> [1]
> https://lore.kernel.org/all/20240913140538.221708-1-john.ogness@linutronix.d
> e/T/
> [2]
> https://lore.kernel.org/linux-arm-kernel/20240913-serial-imx-nbcon-v3-1-4c62
> 7302335b@geanix.com/T/
> [3]
> https://lore.kernel.org/lkml/ZsdoD6PomBRsB-ow@debarbos-thinkpadt14sgen2
> i.remote.csb/#t
> 
> Signed-off-by: Toshiyuki Sato <fj6611ie@aa.jp.fujitsu.com>
> ---
> This patch removes the legacy console code.
> Please comment if you have any concerns.
> 
> Changes in v3:
> - Add stress test results to patch comments.
> - based on tty-next branch
>   https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git
> v2:
> https://lore.kernel.org/all/20250115052749.3205675-1-fj6611ie@aa.jp.fujitsu.c
> om/
> 
> Changes in v2:
> - Remove the module parameter used to switch between legacy and NBCON.
> - Remove codes for legacy console.
> - Fix build errors detected by the test robot.
> - based on 6.13-rc7
> v1:
> https://lore.kernel.org/all/20250108004730.2302996-1-fj6611ie@aa.jp.fujitsu.c
> om/
> 
> Thanks Greg for the comment.
> ---
>  drivers/tty/serial/amba-pl011.c | 143
> ++++++++++++++++++++++----------
>  1 file changed, 97 insertions(+), 46 deletions(-)
> 
> diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
> index 04212c823..9a9a1d630 100644
> --- a/drivers/tty/serial/amba-pl011.c
> +++ b/drivers/tty/serial/amba-pl011.c
> @@ -272,6 +272,7 @@ struct uart_amba_port {
>  	enum pl011_rs485_tx_state	rs485_tx_state;
>  	struct hrtimer		trigger_start_tx;
>  	struct hrtimer		trigger_stop_tx;
> +	bool			console_line_ended;
>  #ifdef CONFIG_DMA_ENGINE
>  	/* DMA stuff */
>  	unsigned int		dmacr;		/* dma control reg */
> @@ -2366,50 +2367,7 @@ static void pl011_console_putchar(struct uart_port
> *port, unsigned char ch)
>  	while (pl011_read(uap, REG_FR) & UART01x_FR_TXFF)
>  		cpu_relax();
>  	pl011_write(ch, uap, REG_DR);
> -}
> -
> -static void
> -pl011_console_write(struct console *co, const char *s, unsigned int count) -{
> -	struct uart_amba_port *uap = amba_ports[co->index];
> -	unsigned int old_cr = 0, new_cr;
> -	unsigned long flags;
> -	int locked = 1;
> -
> -	clk_enable(uap->clk);
> -
> -	if (oops_in_progress)
> -		locked = uart_port_trylock_irqsave(&uap->port, &flags);
> -	else
> -		uart_port_lock_irqsave(&uap->port, &flags);
> -
> -	/*
> -	 *	First save the CR then disable the interrupts
> -	 */
> -	if (!uap->vendor->always_enabled) {
> -		old_cr = pl011_read(uap, REG_CR);
> -		new_cr = old_cr & ~UART011_CR_CTSEN;
> -		new_cr |= UART01x_CR_UARTEN | UART011_CR_TXE;
> -		pl011_write(new_cr, uap, REG_CR);
> -	}
> -
> -	uart_console_write(&uap->port, s, count, pl011_console_putchar);
> -
> -	/*
> -	 *	Finally, wait for transmitter to become empty and restore the
> -	 *	TCR. Allow feature register bits to be inverted to work around
> -	 *	errata.
> -	 */
> -	while ((pl011_read(uap, REG_FR) ^ uap->vendor->inv_fr)
> -						& uap->vendor->fr_busy)
> -		cpu_relax();
> -	if (!uap->vendor->always_enabled)
> -		pl011_write(old_cr, uap, REG_CR);
> -
> -	if (locked)
> -		uart_port_unlock_irqrestore(&uap->port, flags);
> -
> -	clk_disable(uap->clk);
> +	uap->console_line_ended = (ch == '\n');
>  }
> 
>  static void pl011_console_get_options(struct uart_amba_port *uap, int *baud,
> @@ -2472,6 +2430,8 @@ static int pl011_console_setup(struct console *co, char
> *options)
>  	if (ret)
>  		return ret;
> 
> +	uap->console_line_ended = true;
> +
>  	if (dev_get_platdata(uap->port.dev)) {
>  		struct amba_pl011_data *plat;
> 
> @@ -2555,14 +2515,105 @@ static int pl011_console_match(struct console *co,
> char *name, int idx,
>  	return -ENODEV;
>  }
> 
> +static void
> +pl011_console_write_atomic(struct console *co, struct
> +nbcon_write_context *wctxt) {
> +	struct uart_amba_port *uap = amba_ports[co->index];
> +	unsigned int old_cr = 0;
> +
> +	if (!nbcon_enter_unsafe(wctxt))
> +		return;
> +
> +	clk_enable(uap->clk);
> +
> +	if (!uap->vendor->always_enabled) {
> +		old_cr = pl011_read(uap, REG_CR);
> +		pl011_write((old_cr & ~UART011_CR_CTSEN) |
> (UART01x_CR_UARTEN | UART011_CR_TXE),
> +				uap, REG_CR);
> +	}
> +
> +	if (!uap->console_line_ended)
> +		uart_console_write(&uap->port, "\n", 1,
> pl011_console_putchar);
> +	uart_console_write(&uap->port, wctxt->outbuf, wctxt->len,
> +pl011_console_putchar);
> +
> +	while ((pl011_read(uap, REG_FR) ^ uap->vendor->inv_fr) &
> uap->vendor->fr_busy)
> +		cpu_relax();
> +
> +	if (!uap->vendor->always_enabled)
> +		pl011_write(old_cr, uap, REG_CR);
> +
> +	clk_disable(uap->clk);
> +
> +	nbcon_exit_unsafe(wctxt);
> +}
> +
> +static void
> +pl011_console_write_thread(struct console *co, struct
> +nbcon_write_context *wctxt) {
> +	struct uart_amba_port *uap = amba_ports[co->index];
> +	unsigned int old_cr = 0;
> +
> +	if (!nbcon_enter_unsafe(wctxt))
> +		return;
> +
> +	clk_enable(uap->clk);
> +
> +	if (!uap->vendor->always_enabled) {
> +		old_cr = pl011_read(uap, REG_CR);
> +		pl011_write((old_cr & ~UART011_CR_CTSEN) |
> (UART01x_CR_UARTEN | UART011_CR_TXE),
> +				uap, REG_CR);
> +	}
> +
> +	if (nbcon_exit_unsafe(wctxt)) {
> +		int i;
> +		unsigned int len = READ_ONCE(wctxt->len);
> +
> +		for (i = 0; i < len; i++) {
> +			if (!nbcon_enter_unsafe(wctxt))
> +				break;
> +			uart_console_write(&uap->port, wctxt->outbuf + i, 1,
> pl011_console_putchar);
> +			if (!nbcon_exit_unsafe(wctxt))
> +				break;
> +		}
> +	}
> +
> +	while (!nbcon_enter_unsafe(wctxt))
> +		nbcon_reacquire_nobuf(wctxt);
> +
> +	while ((pl011_read(uap, REG_FR) ^ uap->vendor->inv_fr) &
> uap->vendor->fr_busy)
> +		cpu_relax();
> +
> +	if (!uap->vendor->always_enabled)
> +		pl011_write(old_cr, uap, REG_CR);
> +
> +	clk_disable(uap->clk);
> +
> +	nbcon_exit_unsafe(wctxt);
> +}
> +
> +static void
> +pl011_console_device_lock(struct console *co, unsigned long *flags) {
> +	__uart_port_lock_irqsave(&amba_ports[co->index]->port, flags); }
> +
> +static void
> +pl011_console_device_unlock(struct console *co, unsigned long flags) {
> +	__uart_port_unlock_irqrestore(&amba_ports[co->index]->port, flags); }
> +
>  static struct uart_driver amba_reg;
>  static struct console amba_console = {
>  	.name		= "ttyAMA",
> -	.write		= pl011_console_write,
>  	.device		= uart_console_device,
>  	.setup		= pl011_console_setup,
>  	.match		= pl011_console_match,
> -	.flags		= CON_PRINTBUFFER | CON_ANYTIME,
> +	.write_atomic	= pl011_console_write_atomic,
> +	.write_thread	= pl011_console_write_thread,
> +	.device_lock	= pl011_console_device_lock,
> +	.device_unlock	= pl011_console_device_unlock,
> +	.flags		= CON_PRINTBUFFER | CON_ANYTIME |
> CON_NBCON,
>  	.index		= -1,
>  	.data		= &amba_reg,
>  };
> 
> base-commit: 2014c95afecee3e76ca4a56956a936e23283f05b
> --
> 2.34.1

On Mon, Mar 10, 2025 at 02:23:35AM +0000, Toshiyuki Sato (Fujitsu) wrote:
> Dear Reviewers,
> 
> It has been about a month since this patch was merged into tty-next -> linux-next.
> I would appreciate any comments or feedback on this patch.

What needs to be commented on?  It is accepted and will be merged in the
next -rc merge cycle.

confused,

greg k-h

Hi Greg, thanks for your comment.

> > Dear Reviewers,
> >
> > It has been about a month since this patch was merged into tty-next ->
> linux-next.
> > I would appreciate any comments or feedback on this patch.
> 
> What needs to be commented on?  It is accepted and will be merged in the next
> -rc merge cycle.
> 
> confused,
>
> greg k-h

I had thought that some kind of action such as "Reviewed-by" would be necessary.
I'm sorry for the confusion. Thanks for accepting this patch.

Regards,
Toshiyuki Sato