linux-kernel - [PATCH] serial: core: Report magic multiplier extra baud rates

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Date:   Sat, 26 Jun 2021 06:11:21 +0200 (CEST)
From:   "Maciej W. Rozycki" <macro@...am.me.uk>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>
cc:     linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] serial: core: Report magic multiplier extra baud rates

Report extra baud rates supported above the base rate for ports with the 
UPF_MAGIC_MULTIPLIER property, so that people have a way to find out 
that they can be used with their system, e.g.:

Serial: 8250/16550 driver, 5 ports, IRQ sharing enabled
printk: console [ttyS0] disabled
serial8250.0: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A
serial8250.0: ttyS0 extra baud rates supported: 230400, 460800
printk: console [ttyS0] enabled
printk: bootconsole [uart8250] disabled
serial8250.0: ttyS1 at I/O 0x2f8 (irq = 3, base_baud = 115200) is a 16550A
serial8250.0: ttyS1 extra baud rates supported: 230400, 460800
serial8250.0: ttyS2 at MMIO 0x1f000900 (irq = 20, base_baud = 230400) is a 16550A

Otherwise there is no clear way to figure this out, as the feature is 
only reported as an obscure TTY flag in bit 16:

$ cat /sys/class/tty/ttyS[0-2]/flags
0x10010040
0x10010040
0x90000040
$ 

Signed-off-by: Maciej W. Rozycki <macro@...am.me.uk>
---
 drivers/tty/serial/serial_core.c |    8 ++++++++
 1 file changed, 8 insertions(+)

linux-serial-core-magic-multiplier.diff
Index: linux-macro-ide-tty/drivers/tty/serial/serial_core.c
===================================================================
--- linux-macro-ide-tty.orig/drivers/tty/serial/serial_core.c
+++ linux-macro-ide-tty/drivers/tty/serial/serial_core.c
@@ -2314,6 +2314,14 @@ uart_report_port(struct uart_driver *drv
 	       port->dev ? ": " : "",
 	       port->name,
 	       address, port->irq, port->uartclk / 16, uart_type(port));
+
+	/* The magic multiplier feature is a bit obscure, so report it too.  */
+	if (port->flags & UPF_MAGIC_MULTIPLIER)
+		pr_info("%s%s%s extra baud rates supported: %d, %d",
+			port->dev ? dev_name(port->dev) : "",
+			port->dev ? ": " : "",
+			port->name,
+			port->uartclk / 8, port->uartclk / 4);
 }
 
 static void