linux-kernel - [PATCH tty-next] serial: 8250: of: Check for CONFIG_SERIAL_8250

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Message-Id: <20210423183206.3917725-1-f.fainelli@gmail.com>
Date:   Fri, 23 Apr 2021 11:32:04 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     linux-serial@...r.kernel.org
Cc:     alcooperx@...il.com, bcm-kernel-feedback-list@...adcom.com,
        andy.shevchenko@...il.com, Jim Quinlan <jim2101024@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Jonathan Neuschäfer <j.neuschaefer@....net>,
        linux-kernel@...r.kernel.org (open list)
Subject: [PATCH tty-next] serial: 8250: of: Check for CONFIG_SERIAL_8250_BCM7271

From: Jim Quinlan <jim2101024@...il.com>

Our SoC's have always had a NS16650A UART core and older SoC's would
have a compatible string of: 'compatible = ""ns16550a"' and use the
8250_of driver. Our newer SoC's have added enhancements to the base
core to add support for DMA and accurate high speed baud rates and use
this newer 8250_bcm7271 driver. The Device Tree node for our enhanced
UARTs has a compatible string of: 'compatible = "brcm,bcm7271-uart",
"ns16550a"''. With both drivers running and the link order setup so
that the 8250_bcm7217 driver is initialized before the 8250_of driver,
we should bind the 8250_bcm7271 driver to the enhanced UART, or for
upstream kernels that don't have the 8250_bcm7271 driver, we bind to
the 8250_of driver.

The problem is that when both the 8250_of and 8250_bcm7271 drivers
were running, occasionally the 8250_of driver would be bound to the
enhanced UART instead of the 8250_bcm7271 driver. This was happening
because we use SCMI based clocks which come up late in initialization
and cause probe DEFER's when the two drivers get their clocks.

Occasionally the SCMI clock would become ready between the 8250_bcm7271
probe and the 8250_of probe and the 8250_of driver would be bound. To
fix this we decided to config only our 8250_bcm7271 driver and added
"ns16665a0" to the compatible string so the driver would work on our
older system.

This commit has of_platform_serial_probe() check specifically for the
"brcm,bcm7271-uart" and whether its companion driver is enabled. If it
is the case, and the clock provider is not ready, we want to make sure
that when the 8250_bcm7271.c driver returns EPROBE_DEFER, we are not
getting the UART registered via 8250_of.c.

Signed-off-by: Jim Quinlan <jim2101024@...il.com>
Signed-off-by: Al Cooper <alcooperx@...il.com>
Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
---
 drivers/tty/serial/8250/8250_of.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_of.c b/drivers/tty/serial/8250/8250_of.c
index 0b077b45d6a9..bce28729dd7b 100644
--- a/drivers/tty/serial/8250/8250_of.c
+++ b/drivers/tty/serial/8250/8250_of.c
@@ -192,6 +192,10 @@ static int of_platform_serial_probe(struct platform_device *ofdev)
 	u32 tx_threshold;
 	int ret;

+	if (IS_ENABLED(CONFIG_SERIAL_8250_BCM7271) &&
+	    of_device_is_compatible(ofdev->dev.of_node, "brcm,bcm7271-uart"))
+		return -ENODEV;
+
 	port_type = (unsigned long)of_device_get_match_data(&ofdev->dev);
 	if (port_type == PORT_UNKNOWN)
 		return -EINVAL;
-- 
2.25.1