[PATCH v2 2/4] power: reset: add PSCRR NVMEM recorder

[Faruque Ansari <faruque.ansari@oss.qualcomm.com>]

Record the power state change reason into a small NVMEM cell (an RTC
scratch register or EEPROM) and read it back on the next boot, so the
cause survives a power cycle. The cell is a plain nvmem consumer
reference and the reason value is stored directly.

Co-developed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Faruque Ansari <faruque.ansari@oss.qualcomm.com>
---
 drivers/power/reset/pscrr/Kconfig       |  14 +++
 drivers/power/reset/pscrr/Makefile      |   1 +
 drivers/power/reset/pscrr/pscrr-nvmem.c | 154 ++++++++++++++++++++++++++++++++
 3 files changed, 169 insertions(+)

diff --git a/drivers/power/reset/pscrr/Kconfig b/drivers/power/reset/pscrr/Kconfig
index 72de82731b53..bfcf04b0c8f5 100644
--- a/drivers/power/reset/pscrr/Kconfig
+++ b/drivers/power/reset/pscrr/Kconfig
@@ -31,3 +31,17 @@ menuconfig PSCRR
 	  unless hardware provides the reset cause.
 
 	  If unsure, say N.
+
+if PSCRR
+
+config PSCRR_NVMEM
+	tristate "PSCRR NVMEM recorder provider"
+	depends on NVMEM
+	help
+	  PSCRR recorder that stores the power state change reason in a
+	  small NVMEM cell (such as an RTC scratch register) and reads it
+	  back on the next boot, so the cause survives a power cycle.
+
+	  If unsure, say N.
+
+endif # PSCRR
diff --git a/drivers/power/reset/pscrr/Makefile b/drivers/power/reset/pscrr/Makefile
index e5530a858971..95c80c80c7da 100644
--- a/drivers/power/reset/pscrr/Makefile
+++ b/drivers/power/reset/pscrr/Makefile
@@ -1,2 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0
 obj-$(CONFIG_PSCRR) += pscrr.o
+obj-$(CONFIG_PSCRR_NVMEM) += pscrr-nvmem.o
diff --git a/drivers/power/reset/pscrr/pscrr-nvmem.c b/drivers/power/reset/pscrr/pscrr-nvmem.c
new file mode 100644
index 000000000000..1c3145c0d058
--- /dev/null
+++ b/drivers/power/reset/pscrr/pscrr-nvmem.c
@@ -0,0 +1,154 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * pscrr-nvmem.c - NVMEM recorder provider for PSCRR
+ *
+ * Copyright (c) Qualcomm Technologies, Inc. and/or its subsidiaries.
+ * Copyright (C) 2025 Pengutronix, Oleksij Rempel <o.rempel@pengutronix.de>
+ */
+
+#include <linux/module.h>
+#include <linux/nvmem-consumer.h>
+#include <linux/platform_device.h>
+#include <linux/pscrr.h>
+#include <linux/reboot.h>
+#include <linux/slab.h>
+
+struct pscrr_nvmem {
+	struct nvmem_cell *cell;
+	enum psc_reason reason;
+};
+
+static int pscrr_nvmem_cell_read(struct pscrr_nvmem *priv, u8 *out)
+{
+	size_t len;
+	void *buf;
+
+	buf = nvmem_cell_read(priv->cell, &len);
+	if (IS_ERR(buf))
+		return PTR_ERR(buf);
+
+	*out = len ? *(u8 *)buf : 0;
+	kfree(buf);
+
+	return 0;
+}
+
+static int pscrr_nvmem_cell_write(struct pscrr_nvmem *priv, u8 val)
+{
+	int ret;
+
+	/* nvmem_cell_write() returns the byte count on success, not 0. */
+	ret = nvmem_cell_write(priv->cell, &val, sizeof(val));
+
+	return ret < 0 ? ret : 0;
+}
+
+static int pscrr_nvmem_read_reasons(struct pscrr_provider *p,
+				    unsigned long *reasons)
+{
+	struct pscrr_nvmem *priv = p->priv;
+
+	/*
+	 * Report the reason latched at probe (or overwritten by a later
+	 * record); the cell itself was cleared at probe, so it is not read
+	 * live here.
+	 */
+	set_bit(READ_ONCE(priv->reason), reasons);
+
+	return 0;
+}
+
+static int pscrr_nvmem_write_reason(struct pscrr_provider *p,
+				    enum psc_reason reason)
+{
+	struct pscrr_nvmem *priv = p->priv;
+	int ret;
+
+	if (reason >= PSCR_REASON_COUNT)
+		return -EINVAL;
+
+	ret = pscrr_nvmem_cell_write(priv, reason);
+	if (ret)
+		return ret;
+
+	WRITE_ONCE(priv->reason, reason);
+
+	return 0;
+}
+
+static const struct pscrr_provider_ops pscrr_nvmem_ops = {
+	.read_reasons = pscrr_nvmem_read_reasons,
+	.write_reason = pscrr_nvmem_write_reason,
+};
+
+static int pscrr_nvmem_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct pscrr_nvmem *priv;
+	size_t bytes, bits;
+	u8 val;
+	int ret;
+
+	priv = devm_kzalloc(dev, sizeof(*priv), GFP_KERNEL);
+	if (!priv)
+		return -ENOMEM;
+
+	priv->cell = devm_nvmem_cell_get(dev, "pscr");
+	if (IS_ERR(priv->cell))
+		return dev_err_probe(dev, PTR_ERR(priv->cell),
+				     "failed to get the pscr nvmem cell\n");
+
+	ret = nvmem_cell_get_size(priv->cell, &bytes, &bits);
+	if (ret)
+		return ret;
+
+	/* The reason is a single byte; a bit cell still occupies one. */
+	if ((bytes ?: DIV_ROUND_UP(bits, 8)) != sizeof(val))
+		return dev_err_probe(dev, -EINVAL,
+				     "unsupported pscr nvmem cell size\n");
+
+	/*
+	 * The cell survives resets, so at boot it still holds whatever the
+	 * previous session recorded before it went down. Latch that as this
+	 * boot's reason, then clear the cell back to PSCR_UNKNOWN: if this
+	 * session is later cut short by an abrupt reset that never runs the
+	 * recorder (watchdog power-cycle, sudden power loss), the next boot
+	 * reads "unknown" instead of a stale reason left over from an earlier
+	 * cycle.
+	 */
+	ret = pscrr_nvmem_cell_read(priv, &val);
+	if (ret)
+		return dev_err_probe(dev, ret,
+				     "failed to read the pscr nvmem cell\n");
+
+	priv->reason = val < PSCR_REASON_COUNT ? val : PSCR_UNKNOWN;
+
+	ret = pscrr_nvmem_cell_write(priv, PSCR_UNKNOWN);
+	if (ret)
+		return dev_err_probe(dev, ret,
+				     "failed to clear the pscr nvmem cell\n");
+
+	return PTR_ERR_OR_ZERO(devm_pscrr_provider_register(dev, "nvmem",
+							    &pscrr_nvmem_ops, NULL,
+							    priv));
+}
+
+static const struct of_device_id pscrr_nvmem_of_match[] = {
+	{ .compatible = "pscrr-nvmem" },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, pscrr_nvmem_of_match);
+
+static struct platform_driver pscrr_nvmem_driver = {
+	.probe	= pscrr_nvmem_probe,
+	.driver	= {
+		.name		= "pscrr-nvmem",
+		.of_match_table	= pscrr_nvmem_of_match,
+	},
+};
+module_platform_driver(pscrr_nvmem_driver);
+
+MODULE_AUTHOR("Oleksij Rempel <o.rempel@pengutronix.de>");
+MODULE_AUTHOR("Faruque Ansari <faruque.ansari@oss.qualcomm.com>");
+MODULE_DESCRIPTION("NVMEM recorder provider for PSCRR");
+MODULE_LICENSE("GPL");

-- 
2.34.1