serial: bcm283x_mu: Detect disabled serial device

On the raspberry pi, you can disable the serial port to gain dynamic frequency
scaling which can get handy at times.

However, in such a configuration the serial controller gets its rx queue filled
up with zero bytes which then happily get transmitted on to whoever calls
getc() today.

This patch adds detection logic for that case by checking whether the RX pin is
mapped to GPIO15 and disables the mini uart if it is not mapped properly.

That way we can leave the driver enabled in the tree and can determine during
runtime whether serial is usable or not, having a single binary that allows for
uart and non-uart operation.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Simon Glass <sjg@chromium.org>

board/raspberrypi/rpi/rpi.c

@@ -50,7 +50,7 @@ U_BOOT_DEVICE(bcm2835_serials) = {
 	.platdata = &serial_platdata,
 };
 #else
-static const struct bcm283x_mu_serial_platdata serial_platdata = {
+static struct bcm283x_mu_serial_platdata serial_platdata = {
 	.base = 0x3f215040,
 	.clock = 250000000,
 	.skip_init = true,
@@ -452,6 +452,38 @@ int board_init(void)
 	return power_on_module(BCM2835_MBOX_POWER_DEVID_USB_HCD);
 }
 
+#ifndef CONFIG_PL01X_SERIAL
+static bool rpi_is_serial_active(void)
+{
+	int serial_gpio = 15;
+	struct udevice *dev;
+
+	/*
+	 * The RPi3 disables the mini uart by default. The easiest way to find
+	 * out whether it is available is to check if the RX pin is muxed.
+	 */
+
+	if (uclass_first_device(UCLASS_GPIO, &dev) || !dev)
+		return true;
+
+	if (bcm2835_gpio_get_func_id(dev, serial_gpio) != BCM2835_GPIO_ALT5)
+		return false;
+
+	return true;
+}
+#endif
+
+int board_early_init_f(void)
+{
+#ifndef CONFIG_PL01X_SERIAL
+	/* Disable mini-UART I/O if it's not pinmuxed to our pins */
+	if (!rpi_is_serial_active())
+		serial_platdata.disabled = true;
+#endif
+
+	return 0;
+}
+
 int board_mmc_init(bd_t *bis)
 {
 	ALLOC_CACHE_ALIGN_BUFFER(struct msg_get_clock_rate, msg_clk, 1);

configs/rpi_3_32b_defconfig

@@ -20,3 +20,5 @@ CONFIG_CMD_FAT=y
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_PHYS_TO_BUS=y
 CONFIG_OF_LIBFDT=y
+# CONFIG_REQUIRE_SERIAL_CONSOLE is not set
+CONFIG_SYS_MALLOC_F_LEN=0x2000

configs/rpi_3_defconfig

@@ -19,3 +19,5 @@ CONFIG_CMD_FAT=y
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_PHYS_TO_BUS=y
 CONFIG_OF_LIBFDT=y
+# CONFIG_REQUIRE_SERIAL_CONSOLE is not set
+CONFIG_SYS_MALLOC_F_LEN=0x2000

drivers/gpio/bcm2835_gpio.c

@@ -123,5 +123,6 @@ U_BOOT_DRIVER(gpio_bcm2835) = {
 	.id	= UCLASS_GPIO,
 	.ops	= &gpio_bcm2835_ops,
 	.probe	= bcm2835_gpio_probe,
+	.flags	= DM_FLAG_PRE_RELOC,
 	.priv_auto_alloc_size = sizeof(struct bcm2835_gpios),
 };

drivers/serial/serial_bcm283x_mu.c

@@ -73,6 +73,9 @@ static int bcm283x_mu_serial_probe(struct udevice *dev)
 	struct bcm283x_mu_serial_platdata *plat = dev_get_platdata(dev);
 	struct bcm283x_mu_priv *priv = dev_get_priv(dev);
 
+	if (plat->disabled)
+		return -ENODEV;
+
 	priv->regs = (struct bcm283x_mu_regs *)plat->base;
 
 	return 0;

include/configs/rpi.h

@@ -16,6 +16,7 @@
 
 /* Architecture, CPU, etc.*/
 #define CONFIG_ARCH_CPU_INIT
+#define CONFIG_BOARD_EARLY_INIT_F
 
 /* Use SoC timer for AArch32, but architected timer for AArch64 */
 #ifndef CONFIG_ARM64

include/dm/platform_data/serial_bcm283x_mu.h

@@ -19,6 +19,7 @@ struct bcm283x_mu_serial_platdata {
 	unsigned long base;
 	unsigned int clock;
 	bool skip_init;
+	bool disabled;
 };
 
 #endif