[PATCH v3 05/11] serial: sc16is7xx: fix broken port 0 uart init

From: Hugo Villeneuve
Date: Thu May 25 2023 - 00:06:26 EST

From: Hugo Villeneuve <hvilleneuve@xxxxxxxxxxxx>

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
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)

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.

Fixes: dfeae619d781 ("serial: sc16is7xx")
Signed-off-by: Hugo Villeneuve <hvilleneuve@xxxxxxxxxxxx>
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 af7e66db54b4..8a2fc6f89d36 100644
--- a/drivers/tty/serial/sc16is7xx.c
+++ b/drivers/tty/serial/sc16is7xx.c
@@ -1443,6 +1443,7 @@ 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;