serial: bcm2835: add driver for bcm2835-aux-uart

The bcm2835 SOC contains an auxiliary uart, which is very close
to the ns16550 with some differences.

The big difference is that the uart HW is not using an internal divider
of 16 but 8, which results in an effictive baud-rate being twice
the requested baud-rate.

This driver handles this device correctly and handles the difference in
the HW divider by scaling up the clock by a factor of 2.

The approach to write a separate (wrapper) driver instead of using a
multiplying clock and "ns16550" as compatibility in the device-tree
has been recommended by Stephen Warren.
Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_bcm2835aux.c

+/*
+ * Serial port driver for BCM2835AUX UART
+ *
+ * Copyright (C) 2016 Martin Sperl <kernel@martin.sperl.org>
+ *
+ * Based on 8250_lpc18xx.c:
+ * Copyright (C) 2015 Joachim Eastwood <manabian@gmail.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ */
+
+#include <linux/clk.h>
+#include <linux/io.h>
+#include <linux/module.h>
+#include <linux/of.h>
+#include <linux/platform_device.h>
+
+#include "8250.h"
+
+struct bcm2835aux_data {
+	struct uart_8250_port uart;
+	struct clk *clk;
+	int line;
+};
+
+static int bcm2835aux_serial_probe(struct platform_device *pdev)
+{
+	struct bcm2835aux_data *data;
+	struct resource *res;
+	int ret;
+
+	/* allocate the custom structure */
+	data = devm_kzalloc(&pdev->dev, sizeof(*data), GFP_KERNEL);
+	if (!data)
+		return -ENOMEM;
+
+	/* initialize data */
+	spin_lock_init(&data->uart.port.lock);
+	data->uart.capabilities = UART_CAP_FIFO;
+	data->uart.port.dev = &pdev->dev;
+	data->uart.port.regshift = 2;
+	data->uart.port.type = PORT_16550;
+	data->uart.port.iotype = UPIO_MEM;
+	data->uart.port.fifosize = 8;
+	data->uart.port.flags = UPF_SHARE_IRQ |
+				UPF_FIXED_PORT |
+				UPF_FIXED_TYPE |
+				UPF_SKIP_TEST;
+
+	/* get the clock - this also enables the HW */
+	data->clk = devm_clk_get(&pdev->dev, NULL);
+	ret = PTR_ERR_OR_ZERO(data->clk);
+	if (ret) {
+		dev_err(&pdev->dev, "could not get clk: %d\n", ret);
+		return ret;
+	}
+
+	/* get the interrupt */
+	data->uart.port.irq = platform_get_irq(pdev, 0);
+	if (data->uart.port.irq < 0) {
+		dev_err(&pdev->dev, "irq not found - %i",
+			data->uart.port.irq);
+		return data->uart.port.irq;
+	}
+
+	/* map the main registers */
+	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
+	if (!res) {
+		dev_err(&pdev->dev, "memory resource not found");
+		return -EINVAL;
+	}
+	data->uart.port.membase = devm_ioremap_resource(&pdev->dev, res);
+	ret = PTR_ERR_OR_ZERO(data->uart.port.membase);
+	if (ret)
+		return ret;
+
+	/* Check for a fixed line number */
+	ret = of_alias_get_id(pdev->dev.of_node, "serial");
+	if (ret >= 0)
+		data->uart.port.line = ret;
+
+	/* enable the clock as a last step */
+	ret = clk_prepare_enable(data->clk);
+	if (ret) {
+		dev_err(&pdev->dev, "unable to enable uart clock - %d\n",
+			ret);
+		return ret;
+	}
+
+	/* the HW-clock divider for bcm2835aux is 8,
+	 * but 8250 expects a divider of 16,
+	 * so we have to multiply the actual clock by 2
+	 * to get identical baudrates.
+	 */
+	data->uart.port.uartclk = clk_get_rate(data->clk) * 2;
+
+	/* register the port */
+	ret = serial8250_register_8250_port(&data->uart);
+	if (ret < 0) {
+		dev_err(&pdev->dev, "unable to register 8250 port - %d\n",
+			ret);
+		goto dis_clk;
+	}
+	data->line = ret;
+
+	platform_set_drvdata(pdev, data);
+
+	return 0;
+
+dis_clk:
+	clk_disable_unprepare(data->clk);
+	return ret;
+}
+
+static int bcm2835aux_serial_remove(struct platform_device *pdev)
+{
+	struct bcm2835aux_data *data = platform_get_drvdata(pdev);
+
+	serial8250_unregister_port(data->uart.port.line);
+	clk_disable_unprepare(data->clk);
+
+	return 0;
+}
+
+static const struct of_device_id bcm2835aux_serial_match[] = {
+	{ .compatible = "brcm,bcm2835-aux-uart" },
+	{ },
+};
+MODULE_DEVICE_TABLE(of, bcm2835aux_serial_match);
+
+static struct platform_driver bcm2835aux_serial_driver = {
+	.driver = {
+		.name = "bcm2835-aux-uart",
+		.of_match_table = bcm2835aux_serial_match,
+	},
+	.probe  = bcm2835aux_serial_probe,
+	.remove = bcm2835aux_serial_remove,
+};
+module_platform_driver(bcm2835aux_serial_driver);
+
+MODULE_DESCRIPTION("BCM2835 auxiliar UART driver");
+MODULE_AUTHOR("Martin Sperl <kernel@martin.sperl.org>");
+MODULE_LICENSE("GPL v2");

drivers/tty/serial/8250/Kconfig

@@ -272,6 +272,30 @@ config SERIAL_8250_ACORN
 	  system, say Y to this option.  The driver can handle 1, 2, or 3 port
 	  cards.  If unsure, say N.
 
+config SERIAL_8250_BCM2835AUX
+	tristate "BCM2835 auxiliar mini UART support"
+	depends on ARCH_BCM2835 || COMPILE_TEST
+	depends on SERIAL_8250 && SERIAL_8250_SHARE_IRQ
+	help
+	  Support for the BCM2835 auxiliar mini UART.
+
+	  Features and limitations of the UART are
+	    Registers are similar to 16650 registers,
+              set bits in the control registers that are unsupported
+	      are ignored and read back as 0
+	    7/8 bit operation with 1 start and 1 stop bit
+	    8 symbols deep fifo for rx and tx
+	    SW controlled RTS and SW readable CTS
+	    Clock rate derived from system clock
+	    Uses 8 times oversampling (compared to 16 times for 16650)
+	    Missing break detection (but break generation)
+	    Missing framing error detection
+	    Missing parity bit
+	    Missing receive time-out interrupt
+	    Missing DCD, DSR, DTR and RI signals
+
+	  If unsure, say N.
+
 config SERIAL_8250_FSL
 	bool
 	depends on SERIAL_8250_CONSOLE

drivers/tty/serial/8250/Makefile

@@ -12,6 +12,7 @@ obj-$(CONFIG_SERIAL_8250_PCI)		+= 8250_pci.o
 obj-$(CONFIG_SERIAL_8250_HP300)		+= 8250_hp300.o
 obj-$(CONFIG_SERIAL_8250_CS)		+= serial_cs.o
 obj-$(CONFIG_SERIAL_8250_ACORN)		+= 8250_acorn.o
+obj-$(CONFIG_SERIAL_8250_BCM2835AUX)	+= 8250_bcm2835aux.o
 obj-$(CONFIG_SERIAL_8250_CONSOLE)	+= 8250_early.o
 obj-$(CONFIG_SERIAL_8250_FOURPORT)	+= 8250_fourport.o
 obj-$(CONFIG_SERIAL_8250_ACCENT)	+= 8250_accent.o