linux-kernel - [RFC PATCH] serial: sc16is7xx: fix broken port 0 uart init

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Date:   Wed, 17 May 2023 15:44:07 -0400
From:   Hugo Villeneuve <hugo@...ovil.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>
Cc:     hugo@...ovil.com, Hugo Villeneuve <hvilleneuve@...onoff.com>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [RFC PATCH] serial: sc16is7xx: fix broken port 0 uart init

From: Hugo Villeneuve <hvilleneuve@...onoff.com>

While experimenting with rs485 configuration on a SC16IS752 dual UART,
I found that the sc16is7xx_config_rs485() function was called only for
the second port (index 1, channel B), causing initialization problems
for the first port.

For the sc16is7xx driver, port->membase and port->mapbase are not set,
and their default values are 0. And we set port->iobase to the device
index. This means that when the first device is registered using the
uart_add_one_port() function, the following values will be in the port
structure:
    port->membase = 0
    port->mapbase = 0
    port->iobase  = 0

Therefore, the function uart_configure_port() in serial_core.c will
exit early because of the following check:
	/*
	 * If there isn't a port here, don't do anything further.
	 */
	if (!port->iobase && !port->mapbase && !port->membase)
		return;

Typically, I2C and SPI drivers do not set port->membase and
port->mapbase. But I found that the max310x driver sets
port->membase to ~0 (all ones). By implementing the same change in our
driver, uart_configure_port() is now correctly executed.

Signed-off-by: Hugo Villeneuve <hvilleneuve@...onoff.com>
---

I am not sure if this change is the best long-term solution to this
problem, and maybe uart_configure_port() itself could be modified to
take into account the fact that some devices have all three *base
values set to zero?

Also, many drivers use port->iobase as an index, is it the correct way
to use it?

For example, for our driver, there was
commit 5da6b1c079e6 ("sc16is7xx: Set iobase to device index") with the
following explanation:
    "Set the .iobase value to the relative index within the device to allow
    infering the order through sysfs."

 drivers/tty/serial/sc16is7xx.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/tty/serial/sc16is7xx.c b/drivers/tty/serial/sc16is7xx.c
index 1a3143331c1f..3e0e63498052 100644
--- a/drivers/tty/serial/sc16is7xx.c
+++ b/drivers/tty/serial/sc16is7xx.c
@@ -1432,6 +1432,7 @@ static int sc16is7xx_probe(struct device *dev,
 		s->p[i].port.fifosize	= SC16IS7XX_FIFO_SIZE;
 		s->p[i].port.flags	= UPF_FIXED_TYPE | UPF_LOW_LATENCY;
 		s->p[i].port.iobase	= i;
+		s->p[i].port.membase	= (void __iomem *)~0;
 		s->p[i].port.iotype	= UPIO_PORT;
 		s->p[i].port.uartclk	= freq;
 		s->p[i].port.rs485_config = sc16is7xx_config_rs485;
-- 
2.30.2