bootcount: add a DM RTC backing store for bootcount

This implements a driver using a RTC-based backing store for the DM
bootcount implementation.  The node configuring this feature will be
compatible with 'u-boot,bootcount-rtc' and the underlying RTC device
shall be reference through the property 'rtc'. An offset into the RTC
device's register space can be provided through the 'offset' property.

Tested on a RK3399-Q7 on a Flamingo carrier board using the SRAM area
of the carrier board's RV3029 RTC.
Signed-off-by: NPhilipp Tomsich <philipp.tomsich@theobroma-systems.com>

drivers/bootcount/Kconfig

@@ -80,6 +80,26 @@ config DM_BOOTCOUNT
 
 endchoice
 
+if DM_BOOTCOUNT
+
+menu "Backing stores for device-model backed bootcount"
+config DM_BOOTCOUNT_RTC
+	bool "Support RTC devices as a backing store for bootcount"
+	depends on DM_RTC
+	help
+	  Enabled reading/writing the bootcount in a DM RTC device.
+	  The wrapper device is to be specified with the compatible string
+	  'u-boot,bootcount-rtc' and the 'rtc'-property (a phandle pointing
+	  to the underlying RTC device) and an optional 'offset' property
+	  are supported.
+
+	  Accesses to the backing store are performed using the write16
+	  and read16 ops of DM RTC devices.
+
+endmenu
+
+endif
+
 config BOOTCOUNT_BOOTLIMIT
 	int "Maximum number of reboot cycles allowed"
 	default 0

drivers/bootcount/Makefile

@@ -9,3 +9,4 @@ obj-$(CONFIG_BOOTCOUNT_I2C)	+= bootcount_i2c.o
 obj-$(CONFIG_BOOTCOUNT_EXT)	+= bootcount_ext.o
 
 obj-$(CONFIG_DM_BOOTCOUNT)      += bootcount-uclass.o
+obj-$(CONFIG_DM_BOOTCOUNT_RTC)  += rtc.o

drivers/bootcount/rtc.c

+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * (C) Copyright 2018 Theobroma Systems Design und Consulting GmbH
+ */
+
+#include <common.h>
+#include <bootcount.h>
+#include <dm.h>
+#include <rtc.h>
+
+static const u8 bootcount_magic = 0xbc;
+
+struct bootcount_rtc_priv {
+	struct udevice *rtc;
+	u32 offset;
+};
+
+static int bootcount_rtc_set(struct udevice *dev, const u32 a)
+{
+	struct bootcount_rtc_priv *priv = dev_get_priv(dev);
+	const u16 val = bootcount_magic << 8 | (a & 0xff);
+
+	if (rtc_write16(priv->rtc, priv->offset, val) < 0) {
+		debug("%s: rtc_write16 failed\n", __func__);
+		return -EIO;
+	}
+
+	return 0;
+}
+
+static int bootcount_rtc_get(struct udevice *dev, u32 *a)
+{
+	struct bootcount_rtc_priv *priv = dev_get_priv(dev);
+	u16 val;
+
+	if (rtc_read16(priv->rtc, priv->offset, &val) < 0) {
+		debug("%s: rtc_write16 failed\n", __func__);
+		return -EIO;
+	}
+
+	if (val >> 8 == bootcount_magic) {
+		*a = val & 0xff;
+		return 0;
+	}
+
+	debug("%s: bootcount magic does not match on %04x\n", __func__, val);
+	return -EIO;
+}
+
+static int bootcount_rtc_probe(struct udevice *dev)
+{
+	struct ofnode_phandle_args phandle_args;
+	struct bootcount_rtc_priv *priv = dev_get_priv(dev);
+	struct udevice *rtc;
+
+	if (dev_read_phandle_with_args(dev, "rtc", NULL, 0, 0, &phandle_args)) {
+		debug("%s: rtc backing device not specified\n", dev->name);
+		return -ENOENT;
+	}
+
+	if (uclass_get_device_by_ofnode(UCLASS_RTC, phandle_args.node, &rtc)) {
+		debug("%s: could not get backing device\n", dev->name);
+		return -ENODEV;
+	}
+
+	priv->rtc = rtc;
+	priv->offset = dev_read_u32_default(dev, "offset", 0);
+
+	return 0;
+}
+
+static const struct bootcount_ops bootcount_rtc_ops = {
+	.get = bootcount_rtc_get,
+	.set = bootcount_rtc_set,
+};
+
+static const struct udevice_id bootcount_rtc_ids[] = {
+	{ .compatible = "u-boot,bootcount-rtc" },
+	{ }
+};
+
+U_BOOT_DRIVER(bootcount_rtc) = {
+	.name	= "bootcount-rtc",
+	.id	= UCLASS_BOOTCOUNT,
+	.priv_auto_alloc_size = sizeof(struct bootcount_rtc_priv),
+	.probe	= bootcount_rtc_probe,
+	.of_match = bootcount_rtc_ids,
+	.ops	= &bootcount_rtc_ops,
+};