Add the necessary callbacks(write_atomic, write_thread, device_lock
and device_unlock) and CON_NBCON flag to switch the sifive console
driver to perform as nbcon console.

Both ->write_atomic() and ->write_thread() will check for console
ownership whenever they are accessing registers.

The ->device_lock()/unlock() will provide the additional serilization
necessary for ->write_thread() which is called from dedicated printing
thread.

Signed-off-by: Ryo Takakura <ryotkkr98@gmail.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
---

This patch is once again sent as a standalone patch for converting sifive
driver to nbcon. It used to be sent as a part of series. The related fixes
along the way can be found [0].

Sincerely,
Ryo Takakura

[0] https://lore.kernel.org/all/20250405043833.397020-1-ryotkkr98@gmail.com/

---
 drivers/tty/serial/sifive.c | 87 +++++++++++++++++++++++++++++++------
 1 file changed, 74 insertions(+), 13 deletions(-)

diff --git a/drivers/tty/serial/sifive.c b/drivers/tty/serial/sifive.c
index 054a8e630..8f7f770ec 100644
--- a/drivers/tty/serial/sifive.c
+++ b/drivers/tty/serial/sifive.c
@@ -151,6 +151,7 @@ struct sifive_serial_port {
 	unsigned long		baud_rate;
 	struct clk		*clk;
 	struct notifier_block	clk_notifier;
+	bool			console_line_ended;
 };
 
 /*
@@ -785,33 +786,88 @@ static void sifive_serial_console_putchar(struct uart_port *port, unsigned char
 
 	__ssp_wait_for_xmitr(ssp);
 	__ssp_transmit_char(ssp, ch);
+
+	ssp->console_line_ended = (ch == '\n');
+}
+
+static void sifive_serial_device_lock(struct console *co, unsigned long *flags)
+{
+	struct uart_port *up = &sifive_serial_console_ports[co->index]->port;
+
+	__uart_port_lock_irqsave(up, flags);
+}
+
+static void sifive_serial_device_unlock(struct console *co, unsigned long flags)
+{
+	struct uart_port *up = &sifive_serial_console_ports[co->index]->port;
+
+	__uart_port_unlock_irqrestore(up, flags);
 }
 
-static void sifive_serial_console_write(struct console *co, const char *s,
-					unsigned int count)
+static void sifive_serial_console_write_atomic(struct console *co,
+					       struct nbcon_write_context *wctxt)
 {
 	struct sifive_serial_port *ssp = sifive_serial_console_ports[co->index];
-	unsigned long flags;
+	struct uart_port *port = &ssp->port;
 	unsigned int ier;
-	int locked = 1;
 
 	if (!ssp)
 		return;
 
-	if (oops_in_progress)
-		locked = uart_port_trylock_irqsave(&ssp->port, &flags);
-	else
-		uart_port_lock_irqsave(&ssp->port, &flags);
+	if (!nbcon_enter_unsafe(wctxt))
+		return;
 
 	ier = __ssp_readl(ssp, SIFIVE_SERIAL_IE_OFFS);
 	__ssp_writel(0, SIFIVE_SERIAL_IE_OFFS, ssp);
 
-	uart_console_write(&ssp->port, s, count, sifive_serial_console_putchar);
+	if (!ssp->console_line_ended)
+		uart_console_write(port, "\n", 1, sifive_serial_console_putchar);
+	uart_console_write(port, wctxt->outbuf, wctxt->len,
+			   sifive_serial_console_putchar);
 
 	__ssp_writel(ier, SIFIVE_SERIAL_IE_OFFS, ssp);
 
-	if (locked)
-		uart_port_unlock_irqrestore(&ssp->port, flags);
+	nbcon_exit_unsafe(wctxt);
+}
+
+static void sifive_serial_console_write_thread(struct console *co,
+					       struct nbcon_write_context *wctxt)
+{
+	struct sifive_serial_port *ssp = sifive_serial_console_ports[co->index];
+	struct uart_port *port = &ssp->port;
+	unsigned int ier;
+
+	if (!ssp)
+		return;
+
+	if (!nbcon_enter_unsafe(wctxt))
+		return;
+
+	ier = __ssp_readl(ssp, SIFIVE_SERIAL_IE_OFFS);
+	__ssp_writel(0, SIFIVE_SERIAL_IE_OFFS, ssp);
+
+	if (nbcon_exit_unsafe(wctxt)) {
+		int len = READ_ONCE(wctxt->len);
+		int i;
+
+		for (i = 0; i < len; i++) {
+			if (!nbcon_enter_unsafe(wctxt))
+				break;
+
+			uart_console_write(port, wctxt->outbuf + i, 1,
+					   sifive_serial_console_putchar);
+
+			if (!nbcon_exit_unsafe(wctxt))
+				break;
+		}
+	}
+
+	while (!nbcon_enter_unsafe(wctxt))
+		nbcon_reacquire_nobuf(wctxt);
+
+	__ssp_writel(ier, SIFIVE_SERIAL_IE_OFFS, ssp);
+
+	nbcon_exit_unsafe(wctxt);
 }
 
 static int sifive_serial_console_setup(struct console *co, char *options)
@@ -829,6 +885,8 @@ static int sifive_serial_console_setup(struct console *co, char *options)
 	if (!ssp)
 		return -ENODEV;
 
+	ssp->console_line_ended = true;
+
 	if (options)
 		uart_parse_options(options, &baud, &parity, &bits, &flow);
 
@@ -839,10 +897,13 @@ static struct uart_driver sifive_serial_uart_driver;
 
 static struct console sifive_serial_console = {
 	.name		= SIFIVE_TTY_PREFIX,
-	.write		= sifive_serial_console_write,
+	.write_atomic	= sifive_serial_console_write_atomic,
+	.write_thread	= sifive_serial_console_write_thread,
+	.device_lock	= sifive_serial_device_lock,
+	.device_unlock	= sifive_serial_device_unlock,
 	.device		= uart_console_device,
 	.setup		= sifive_serial_console_setup,
-	.flags		= CON_PRINTBUFFER,
+	.flags		= CON_PRINTBUFFER | CON_NBCON,
 	.index		= -1,
 	.data		= &sifive_serial_uart_driver,
 };
-- 
2.34.1

Hi Ryo,

kernel test robot noticed the following build warnings:

[auto build test WARNING on tty/tty-testing]
[also build test WARNING on tty/tty-next tty/tty-linus linus/master v6.14 next-20250404]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Ryo-Takakura/serial-sifive-Switch-to-nbcon-console/20250405-230051
base:   https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git tty-testing
patch link:    https://lore.kernel.org/r/20250405145915.493173-1-ryotkkr98%40gmail.com
patch subject: [PATCH v2] serial: sifive: Switch to nbcon console
config: arm-randconfig-002-20250406 (https://download.01.org/0day-ci/archive/20250406/202504060816.EWk91sJS-lkp@intel.com/config)
compiler: arm-linux-gnueabi-gcc (GCC) 7.5.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250406/202504060816.EWk91sJS-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202504060816.EWk91sJS-lkp@intel.com/

All warnings (new ones prefixed by >>):

>> drivers/tty/serial/sifive.c:155: warning: Function parameter or struct member 'console_line_ended' not described in 'sifive_serial_port'


vim +155 drivers/tty/serial/sifive.c

45c054d0815b15 Paul Walmsley 2019-04-12  131  
45c054d0815b15 Paul Walmsley 2019-04-12  132  /*
45c054d0815b15 Paul Walmsley 2019-04-12  133   *
45c054d0815b15 Paul Walmsley 2019-04-12  134   */
45c054d0815b15 Paul Walmsley 2019-04-12  135  
45c054d0815b15 Paul Walmsley 2019-04-12  136  /**
180bb243de730c Lee Jones     2020-11-04  137   * struct sifive_serial_port - driver-specific data extension to struct uart_port
45c054d0815b15 Paul Walmsley 2019-04-12  138   * @port: struct uart_port embedded in this struct
45c054d0815b15 Paul Walmsley 2019-04-12  139   * @dev: struct device *
45c054d0815b15 Paul Walmsley 2019-04-12  140   * @ier: shadowed copy of the interrupt enable register
45c054d0815b15 Paul Walmsley 2019-04-12  141   * @baud_rate: UART serial line rate (e.g., 115200 baud)
180bb243de730c Lee Jones     2020-11-04  142   * @clk: reference to this device's clock
45c054d0815b15 Paul Walmsley 2019-04-12  143   * @clk_notifier: clock rate change notifier for upstream clock changes
45c054d0815b15 Paul Walmsley 2019-04-12  144   *
45c054d0815b15 Paul Walmsley 2019-04-12  145   * Configuration data specific to this SiFive UART.
45c054d0815b15 Paul Walmsley 2019-04-12  146   */
45c054d0815b15 Paul Walmsley 2019-04-12  147  struct sifive_serial_port {
45c054d0815b15 Paul Walmsley 2019-04-12  148  	struct uart_port	port;
45c054d0815b15 Paul Walmsley 2019-04-12  149  	struct device		*dev;
45c054d0815b15 Paul Walmsley 2019-04-12  150  	unsigned char		ier;
45c054d0815b15 Paul Walmsley 2019-04-12  151  	unsigned long		baud_rate;
45c054d0815b15 Paul Walmsley 2019-04-12  152  	struct clk		*clk;
45c054d0815b15 Paul Walmsley 2019-04-12  153  	struct notifier_block	clk_notifier;
328ee9dbff3941 Ryo Takakura  2025-04-05  154  	bool			console_line_ended;
45c054d0815b15 Paul Walmsley 2019-04-12 @155  };
45c054d0815b15 Paul Walmsley 2019-04-12  156  

-- 
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https://github.com/intel/lkp-tests/wiki