linux-kernel - Re: [patch RFC 00/29] printk: A new approach

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Date:   Sun, 11 Sep 2022 00:56:11 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Helge Deller <deller@....de>,
        Jason Wessel <jason.wessel@...driver.com>,
        Daniel Thompson <daniel.thompson@...aro.org>
Subject: Re: [patch RFC 00/29] printk: A new approach - WIP

On Sun, Sep 11 2022 at 00:27, Thomas Gleixner wrote:
> For testing we used the most simple driver: a hacked up version of the
> early uart8250 console as we wanted to concentrate on validating the core
> mechanisms of friendly handover and hostile takeovers instead of dealing
> with the horrors of port locks or whatever at the same time. That's the
> next challenge. Hack patch will be provided in a reply.

Here you go.

---
Subject: serial: 8250: Use 8250 serial for exploring noBKL consoles
From: John Ogness <jogness@...utronix.de>
Date: Sat, 10 Sep 2022 01:05:34 +0200

From: John Ogness <jogness@...utronix.de>

Utilize 8250 early console - the only console in the kernel which is
reentrancy and NMI safe - to explore the noBKL console infrastructure.

Not-Signed-off-by: John Ogness <jogness@...utronix.de>
Not-Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 drivers/tty/serial/8250/8250_early.c |   32 ++++++++++++++++++++++++++++++++
 drivers/tty/serial/8250/Kconfig      |   11 ++++++++++-
 drivers/tty/serial/8250/Makefile     |    2 +-
 3 files changed, 43 insertions(+), 2 deletions(-)

--- a/drivers/tty/serial/8250/8250_early.c
+++ b/drivers/tty/serial/8250/8250_early.c
@@ -107,6 +107,34 @@ static void early_serial8250_write(struc
 	uart_console_write(port, s, count, serial_putc);
 }
 
+static bool __early_serial8250_write(struct console *con, struct cons_write_context *wctxt,
+				     unsigned char c)
+{
+	struct earlycon_device *device = con->data;
+	struct uart_port *port = &device->port;
+	unsigned char *s = wctxt->outbuf;
+
+	serial_putc(port, c);
+
+	for (; wctxt->pos < wctxt->len; wctxt->pos++, s++) {
+		if (!console_can_proceed(wctxt))
+			return false;
+
+		uart_console_write(port, s, 1, serial_putc);
+	}
+	return true;
+}
+
+static bool early_serial8250_write_thread(struct console *con, struct cons_write_context *wctxt)
+{
+	return __early_serial8250_write(con, wctxt, 'T');
+}
+
+static bool early_serial8250_write_atomic(struct console *con, struct cons_write_context *wctxt)
+{
+	return __early_serial8250_write(con, wctxt, 'A');
+}
+
 #ifdef CONFIG_CONSOLE_POLL
 static int early_serial8250_read(struct console *console,
 				 char *s, unsigned int count)
@@ -170,6 +198,10 @@ int __init early_serial8250_setup(struct
 
 	device->con->write = early_serial8250_write;
 	device->con->read = early_serial8250_read;
+	device->con->flags &= ~CON_BOOT;
+	device->con->flags |= CON_NO_BKL;
+	device->con->write_thread = early_serial8250_write_thread;
+	device->con->write_atomic = early_serial8250_write_atomic;
 	return 0;
 }
 EARLYCON_DECLARE(uart8250, early_serial8250_setup);
--- a/drivers/tty/serial/8250/Kconfig
+++ b/drivers/tty/serial/8250/Kconfig
@@ -82,9 +82,18 @@ config SERIAL_8250_FINTEK
 
 	  If unsure, say N.
 
+config SERIAL_8250_CONSOLE_EARLY
+	bool "Console on 8250/16550 and compatible noBKL console mockup"
+	default SERIAL_8250
+	select SERIAL_CORE_CONSOLE
+	select SERIAL_EARLYCON
+	help
+	  Mockup to demonstrate the core capabilities for noBKL consoles.
+	  OTOH, the _only_ reliable reentrant and NMI safe console...
+
 config SERIAL_8250_CONSOLE
 	bool "Console on 8250/16550 and compatible serial port"
-	depends on SERIAL_8250=y
+	depends on SERIAL_8250=y && !SERIAL_8250_CONSOLE_EARLY
 	select SERIAL_CORE_CONSOLE
 	select SERIAL_EARLYCON
 	help
--- a/drivers/tty/serial/8250/Makefile
+++ b/drivers/tty/serial/8250/Makefile
@@ -20,7 +20,7 @@ obj-$(CONFIG_SERIAL_8250_CS)		+= serial_
 obj-$(CONFIG_SERIAL_8250_ACORN)		+= 8250_acorn.o
 obj-$(CONFIG_SERIAL_8250_ASPEED_VUART)	+= 8250_aspeed_vuart.o
 obj-$(CONFIG_SERIAL_8250_BCM2835AUX)	+= 8250_bcm2835aux.o
-obj-$(CONFIG_SERIAL_8250_CONSOLE)	+= 8250_early.o
+obj-$(CONFIG_SERIAL_8250_CONSOLE_EARLY)	+= 8250_early.o
 obj-$(CONFIG_SERIAL_8250_FOURPORT)	+= 8250_fourport.o
 obj-$(CONFIG_SERIAL_8250_ACCENT)	+= 8250_accent.o
 obj-$(CONFIG_SERIAL_8250_BOCA)		+= 8250_boca.o