[PATCH v3 2/9] nvmem: rmem: add write and protect support

[Oleksij Rempel <o.rempel@pengutronix.de>]

Add reg_write and reg_protect operations to the rmem NVMEM driver.
This makes rmem devices writable (they were previously read-only via the
NVMEM interface due to lacking reg_write) and allows specific regions
to be marked read-only.

The primary motivation is to improve testing of NVMEM consumer code that
handles write protection, by enabling rmem to emulate such hardware,
particularly in sandbox environments.

Key changes:

- reg_write implemented: Enables writes. Writes to protected regions
  return -EROFS.

- reg_protect implemented:
  - PROTECT_DISABLE_WRITE: Marks range read-only.

  - PROTECT_ENABLE_WRITE: Makes range writable.

  - Probe function updated for new ops and list initialization.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
---
changes v3:
- use bitmap helpers
changes v2:
- use a bitmap instead of lists of ranges.
---
 drivers/nvmem/rmem.c | 85 ++++++++++++++++++++++++++++++++++++++++++++
 include/driver.h     |  3 ++
 2 files changed, 88 insertions(+)

diff --git a/drivers/nvmem/rmem.c b/drivers/nvmem/rmem.c
index afa0dd78c8f4..fd742b69861b 100644
--- a/drivers/nvmem/rmem.c
+++ b/drivers/nvmem/rmem.c
@@ -5,12 +5,15 @@
 
 #include <io.h>
 #include <driver.h>
+#include <linux/bitmap.h>
 #include <linux/nvmem-provider.h>
 #include <init.h>
 
 struct rmem {
 	struct device *dev;
 	const struct resource *mem;
+	unsigned long *protection_bitmap;
+	size_t size;
 };
 
 static int rmem_read(void *context, unsigned int offset,
@@ -21,6 +24,82 @@ static int rmem_read(void *context, unsigned int offset,
 			bytes, offset, 0);
 }
 
+/**
+ * rmem_write() - Write data to the NVMEM device.
+ * @context:	Pointer to the rmem private data (struct rmem).
+ * @offset:	Offset within the NVMEM device to write to.
+ * @val:	Buffer containing the data to write.
+ * @bytes:	Number of bytes to write.
+ *
+ * This function is called by the NVMEM core to write data to the
+ * reserved memory region. It first checks the protection bitmap to ensure
+ * the target range is not write-protected. If writable, it uses the
+ * custom 'mem_copy' function.
+ *
+ * Return: 0 on success, or a negative error code on failure.
+ */
+static int rmem_write(void *context, unsigned int offset, const void *val,
+		      size_t bytes)
+{
+	unsigned long protected_offset;
+	struct rmem *rmem = context;
+
+	protected_offset = find_next_bit(rmem->protection_bitmap,
+					 offset + bytes, offset);
+	if (protected_offset < offset + bytes)
+		return -EROFS;
+
+	/*
+	 * The last two arguments to mem_copy (0, 0) are specific to
+	 * the custom mem_copy implementation.
+	 */
+	return mem_copy(rmem->dev, (void *)rmem->mem->start + offset, val,
+			bytes, 0, 0);
+}
+
+/**
+ * rmem_protect() - NVMEM callback to change protection status of a range.
+ * @context:	Pointer to the rmem private data (struct rmem).
+ * @offset:	Starting offset of the range.
+ * @bytes:	Length of the range in bytes.
+ * @prot_mode:	Protection mode to apply (PROTECT_DISABLE_WRITE or
+ * PROTECT_ENABLE_WRITE).
+ *
+ * This function is called by the NVMEM core to enable or disable
+ * write protection for a specified memory range.
+ *
+ * Return: 0 on success, or a negative error code on failure.
+ */
+static int rmem_protect(void *context, unsigned int offset, size_t bytes,
+			int prot_mode)
+{
+	struct rmem *rmem = context;
+	int ret;
+
+	if (offset + bytes > rmem->size)
+		return -EINVAL;
+
+	switch (prot_mode) {
+	case PROTECT_DISABLE_WRITE: /* Make read-only */
+		bitmap_set(rmem->protection_bitmap, offset, bytes);
+		break;
+	case PROTECT_ENABLE_WRITE: /* Make writable */
+		bitmap_clear(rmem->protection_bitmap, offset, bytes);
+		break;
+	default:
+		dev_warn(rmem->dev, "%s: Invalid protection mode %d\n",
+			 __func__, prot_mode);
+		ret = -EINVAL;
+		break;
+	}
+
+	dev_dbg(rmem->dev,
+		"Protection op complete [0x%x, len %zu], mode %d\n", offset,
+		bytes, prot_mode);
+
+	return 0;
+}
+
 static int rmem_probe(struct device *dev)
 {
 	struct nvmem_config config = { };
@@ -41,7 +120,13 @@ static int rmem_probe(struct device *dev)
 	config.priv = priv;
 	config.name = "rmem";
 	config.size = resource_size(mem);
+	priv->size = config.size;
+
+	priv->protection_bitmap = bitmap_xzalloc(priv->size);
+
 	config.reg_read = rmem_read;
+	config.reg_write = rmem_write;
+	config.reg_protect = rmem_protect;
 
 	return PTR_ERR_OR_ZERO(nvmem_register(&config));
 }
diff --git a/include/driver.h b/include/driver.h
index dd50a7aa3cce..8b62087a0c42 100644
--- a/include/driver.h
+++ b/include/driver.h
@@ -442,6 +442,9 @@ int platform_driver_register(struct driver *drv);
 
 int platform_device_register(struct device *new_device);
 
+#define PROTECT_ENABLE_WRITE		0
+#define PROTECT_DISABLE_WRITE		1
+
 struct cdev_operations {
 	/*! Called in response of reading from this device. Required */
 	ssize_t (*read)(struct cdev*, void* buf, size_t count, loff_t offset, ulong flags);
-- 
2.39.5