[PATCH v3 2/3] power: reset: Add PSCRR NVMEM recorder

[Faruque Ansari]

Record the power state change reason into a NVMEM cell (a PMIC
SDAM byte, RTC scratch register, or EEPROM) and read it back on the
next boot, so the cause survives a power cycle.

The recorder is implemented as an NVMEM layout driver: it owns the
single cell described by its layout binding and adds it to the
parent NVMEM device, rather than acting as a standalone consumer of
someone else's cell.

Co-developed-by: Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>
Signed-off-by: Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>
Signed-off-by: Faruque Ansari <faruque.ansari@xxxxxxxxxxxxxxxx>
---
 drivers/power/reset/pscrr/Kconfig       |  14 ++
 drivers/power/reset/pscrr/Makefile      |   1 +
 drivers/power/reset/pscrr/pscrr-nvmem.c | 269 ++++++++++++++++++++++++++++++++
 3 files changed, 284 insertions(+)

diff --git a/drivers/power/reset/pscrr/Kconfig b/drivers/power/reset/pscrr/Kconfig
index 72de82731b53..459b05ba32d3 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_LAYOUTS
+	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..64f67f0521e0
--- /dev/null
+++ b/drivers/power/reset/pscrr/pscrr-nvmem.c
@@ -0,0 +1,269 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * pscrr-nvmem.c - NVMEM layout driver for PSCRR
+ *
+ * Copyright (c) Qualcomm Technologies, Inc. and/or its subsidiaries.
+ * Copyright (C) 2025 Pengutronix, Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>
+ */
+
+#include <linux/bitops.h>
+#include <linux/module.h>
+#include <linux/nvmem-consumer.h>
+#include <linux/nvmem-provider.h>
+#include <linux/of.h>
+#include <linux/pscrr.h>
+#include <linux/slab.h>
+
+struct pscrr_nvmem {
+	struct nvmem_device *nvmem;
+	u32 offset;		/* byte offset of the cell, from reg[0] */
+	u32 bit_offset;		/* 0 when the cell has no "bits" property */
+	u32 nbits;		/* BITS_PER_BYTE when the cell has no "bits" property */
+	enum psc_reason reason;
+};
+
+static inline u8 pscrr_nvmem_mask(struct pscrr_nvmem *priv)
+{
+	return (u8)GENMASK(priv->bit_offset + priv->nbits - 1, priv->bit_offset);
+}
+
+static int pscrr_nvmem_raw_read(struct pscrr_nvmem *priv, u8 *raw)
+{
+	int ret;
+
+	ret = nvmem_device_read(priv->nvmem, priv->offset, sizeof(*raw), raw);
+
+	return ret < 0 ? ret : 0;
+}
+
+static int pscrr_nvmem_raw_write(struct pscrr_nvmem *priv, u8 raw)
+{
+	int ret;
+
+	ret = nvmem_device_write(priv->nvmem, priv->offset, sizeof(raw), &raw);
+
+	return ret < 0 ? ret : 0;
+}
+
+static int pscrr_nvmem_cell_read(struct pscrr_nvmem *priv, u8 *out)
+{
+	u8 raw, mask = pscrr_nvmem_mask(priv);
+	int ret;
+
+	ret = pscrr_nvmem_raw_read(priv, &raw);
+	if (ret)
+		return ret;
+
+	*out = (raw & mask) >> priv->bit_offset;
+
+	return 0;
+}
+
+static int pscrr_nvmem_cell_write(struct pscrr_nvmem *priv, u8 val)
+{
+	u8 raw, mask = pscrr_nvmem_mask(priv);
+	int ret;
+
+	/* Sub-byte cell: preserve the neighbouring bits, read-modify-write. */
+	if (priv->nbits < BITS_PER_BYTE) {
+		ret = pscrr_nvmem_raw_read(priv, &raw);
+		if (ret)
+			return ret;
+	} else {
+		raw = 0;
+	}
+
+	raw = (raw & ~mask) | ((val << priv->bit_offset) & mask);
+
+	return pscrr_nvmem_raw_write(priv, raw);
+}
+
+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 struct device_node *pscrr_nvmem_get_cell(struct device *dev,
+						struct nvmem_device *nvmem,
+						u32 reg[2])
+{
+	struct device_node *layout_np, *cell_np = NULL;
+
+	layout_np = of_nvmem_layout_get_container(nvmem);
+	if (!layout_np)
+		return ERR_PTR(-ENOENT);
+
+	for_each_child_of_node_scoped(layout_np, child) {
+		if (of_property_read_u32_array(child, "reg", reg, 2))
+			continue;
+
+		cell_np = of_node_get(child);
+		break;
+	}
+	of_node_put(layout_np);
+
+	return cell_np ?: ERR_PTR(dev_err_probe(dev, -ENOENT,
+						"missing pscr nvmem cell\n"));
+}
+
+static int pscrr_nvmem_add_cells(struct nvmem_layout *layout)
+{
+	struct device *dev = &layout->dev;
+	struct pscrr_nvmem *priv;
+	struct device_node *cell_np;
+	struct nvmem_cell_info info = { };
+	u32 reg[2], bits[2];
+	u8 val;
+	int ret;
+
+	priv = devm_kzalloc(dev, sizeof(*priv), GFP_KERNEL);
+	if (!priv)
+		return -ENOMEM;
+
+	priv->nvmem = layout->nvmem;
+
+	cell_np = pscrr_nvmem_get_cell(dev, priv->nvmem, reg);
+	if (IS_ERR(cell_np))
+		return PTR_ERR(cell_np);
+
+	/* The reason is a single byte; a bit cell still occupies one. */
+	if (reg[1] != sizeof(val)) {
+		of_node_put(cell_np);
+		return dev_err_probe(dev, -EINVAL,
+				     "unsupported pscr nvmem cell size\n");
+	}
+	priv->offset = reg[0];
+
+	if (!of_property_read_u32_array(cell_np, "bits", bits, ARRAY_SIZE(bits))) {
+		priv->bit_offset = bits[0];
+		priv->nbits = bits[1];
+	} else {
+		priv->bit_offset = 0;
+		priv->nbits = BITS_PER_BYTE;
+	}
+
+	if (priv->bit_offset + priv->nbits > BITS_PER_BYTE * reg[1]) {
+		of_node_put(cell_np);
+		return dev_err_probe(dev, -EINVAL, "bits exceed cell size\n");
+	}
+
+	/* PSCR_REASON_COUNT must fit in the allotted bits. */
+	if (PSCR_REASON_COUNT > (1U << priv->nbits)) {
+		of_node_put(cell_np);
+		return dev_err_probe(dev, -EINVAL,
+				     "too few bits for the reason range\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) {
+		of_node_put(cell_np);
+		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) {
+		of_node_put(cell_np);
+		return dev_err_probe(dev, ret,
+				     "failed to clear the pscr nvmem cell\n");
+	}
+
+	info.name = kasprintf(GFP_KERNEL, "%pOFn", cell_np);
+	if (!info.name) {
+		of_node_put(cell_np);
+		return -ENOMEM;
+	}
+	info.offset = reg[0];
+	info.bytes = reg[1];
+	info.bit_offset = priv->bit_offset;
+	info.nbits = priv->nbits;
+	info.np = cell_np;
+
+	ret = nvmem_add_one_cell(priv->nvmem, &info);
+	kfree(info.name);
+	if (ret) {
+		of_node_put(cell_np);
+		return ret;
+	}
+
+	return PTR_ERR_OR_ZERO(devm_pscrr_provider_register(dev, "nvmem",
+							    &pscrr_nvmem_ops, NULL,
+							    priv));
+}
+
+static int pscrr_nvmem_probe(struct nvmem_layout *layout)
+{
+	layout->add_cells = pscrr_nvmem_add_cells;
+
+	return nvmem_layout_register(layout);
+}
+
+static void pscrr_nvmem_remove(struct nvmem_layout *layout)
+{
+	nvmem_layout_unregister(layout);
+}
+
+static const struct of_device_id pscrr_nvmem_of_match[] = {
+	{ .compatible = "pscrr-nvmem" },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, pscrr_nvmem_of_match);
+
+static struct nvmem_layout_driver pscrr_nvmem_layout = {
+	.driver	= {
+		.name		= "pscrr-nvmem",
+		.of_match_table	= pscrr_nvmem_of_match,
+	},
+	.probe	= pscrr_nvmem_probe,
+	.remove	= pscrr_nvmem_remove,
+};
+module_nvmem_layout_driver(pscrr_nvmem_layout);
+
+MODULE_AUTHOR("Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>");
+MODULE_AUTHOR("Faruque Ansari <faruque.ansari@xxxxxxxxxxxxxxxx>");
+MODULE_DESCRIPTION("NVMEM recorder provider for PSCRR");
+MODULE_LICENSE("GPL");

-- 
2.34.1