[PATCH v2 4/8] block: implement NVMEM provider

[Loic Poulain <loic.poulain@oss.qualcomm.com>]

From: Daniel Golle <daniel@makrotopia.org>

On embedded devices using an eMMC it is common that one or more partitions
on the eMMC are used to store MAC addresses and Wi-Fi calibration EEPROM
data. Allow referencing the partition in device tree for the kernel and
Wi-Fi drivers accessing it via the NVMEM layer.

To safely defer the freeing of the provider private data until all
consumers have released their cells, a nvmem_dev() accessor is added to
the NVMEM core to expose the struct device embedded in struct nvmem_device.
This allows registering a devm action on the nvmem device itself, ensuring
the private data outlives any active cell references.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Co-developed-by: Loic Poulain <loic.poulain@oss.qualcomm.com>
Signed-off-by: Loic Poulain <loic.poulain@oss.qualcomm.com>
---
 block/Kconfig                  |   9 ++
 block/Makefile                 |   1 +
 block/blk-nvmem.c              | 188 +++++++++++++++++++++++++++++++++++++++++
 drivers/nvmem/core.c           |  13 +++
 include/linux/nvmem-consumer.h |   6 ++
 5 files changed, 217 insertions(+)

diff --git a/block/Kconfig b/block/Kconfig
index 15027963472d7b40e27b9097a5993c457b5b3054..0b33747e16dc33473683706f75c92bdf8b648f7c 100644
--- a/block/Kconfig
+++ b/block/Kconfig
@@ -209,6 +209,15 @@ config BLK_INLINE_ENCRYPTION_FALLBACK
 	  by falling back to the kernel crypto API when inline
 	  encryption hardware is not present.
 
+config BLK_NVMEM
+	bool "Block device NVMEM provider"
+	depends on OF
+	depends on NVMEM
+	help
+	  Allow block devices (or partitions) to act as NVMEM providers,
+	  typically used with eMMC to store MAC addresses or Wi-Fi
+	  calibration data on embedded devices.
+
 source "block/partitions/Kconfig"
 
 config BLK_PM
diff --git a/block/Makefile b/block/Makefile
index 7dce2e44276c4274c11a0a61121c83d9c43d6e0c..d7ac389e71902bc091a8800ea266190a43b3e63d 100644
--- a/block/Makefile
+++ b/block/Makefile
@@ -36,3 +36,4 @@ obj-$(CONFIG_BLK_INLINE_ENCRYPTION)	+= blk-crypto.o blk-crypto-profile.o \
 					   blk-crypto-sysfs.o
 obj-$(CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK)	+= blk-crypto-fallback.o
 obj-$(CONFIG_BLOCK_HOLDER_DEPRECATED)	+= holder.o
+obj-$(CONFIG_BLK_NVMEM)                += blk-nvmem.o
diff --git a/block/blk-nvmem.c b/block/blk-nvmem.c
new file mode 100644
index 0000000000000000000000000000000000000000..96c0ffc51b1862a75644f3f94add35d59577d86b
--- /dev/null
+++ b/block/blk-nvmem.c
@@ -0,0 +1,188 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * block device NVMEM provider
+ *
+ * Copyright (c) 2024 Daniel Golle <daniel@makrotopia.org>
+ *
+ * Useful on devices using a partition on an eMMC for MAC addresses or
+ * Wi-Fi calibration EEPROM data.
+ */
+
+#include "blk.h"
+#include <linux/nvmem-provider.h>
+#include <linux/nvmem-consumer.h>
+#include <linux/of.h>
+#include <linux/pagemap.h>
+#include <linux/property.h>
+
+static void blk_nvmem_free(void *data)
+{
+	kfree(data);
+}
+
+/* List of all NVMEM devices */
+static LIST_HEAD(nvmem_devices);
+static DEFINE_MUTEX(devices_mutex);
+
+struct blk_nvmem {
+	struct nvmem_device	*nvmem;
+	dev_t			devt;
+	bool			removed;
+	struct list_head	list;
+};
+
+static int blk_nvmem_reg_read(void *priv, unsigned int from,
+			      void *val, size_t bytes)
+{
+	blk_mode_t mode = BLK_OPEN_READ | BLK_OPEN_RESTRICT_WRITES;
+	unsigned long offs = from & ~PAGE_MASK, to_read;
+	pgoff_t f_index = from >> PAGE_SHIFT;
+	struct blk_nvmem *bnv = priv;
+	size_t bytes_left = bytes;
+	struct file *bdev_file;
+	struct folio *folio;
+	void *p;
+	int ret = 0;
+
+	if (bnv->removed)
+		return -ENODEV;
+
+	bdev_file = bdev_file_open_by_dev(bnv->devt, mode, priv, NULL);
+	if (!bdev_file)
+		return -ENODEV;
+
+	if (IS_ERR(bdev_file))
+		return PTR_ERR(bdev_file);
+
+	while (bytes_left) {
+		folio = read_mapping_folio(bdev_file->f_mapping, f_index++, NULL);
+		if (IS_ERR(folio)) {
+			ret = PTR_ERR(folio);
+			goto err_release_bdev;
+		}
+		to_read = min_t(unsigned long, bytes_left, PAGE_SIZE - offs);
+		p = folio_address(folio) + offset_in_folio(folio, offs);
+		memcpy(val, p, to_read);
+		offs = 0;
+		bytes_left -= to_read;
+		val += to_read;
+		folio_put(folio);
+	}
+
+err_release_bdev:
+	fput(bdev_file);
+
+	return ret;
+}
+
+static int blk_nvmem_register(struct device *dev)
+{
+	struct device_node *np = dev_of_node(dev);
+	struct block_device *bdev = dev_to_bdev(dev);
+	struct nvmem_config config = {};
+	struct blk_nvmem *bnv;
+
+	/* skip devices which do not have a device tree node */
+	if (!np)
+		return 0;
+
+	/* skip devices without an nvmem layout defined */
+	if (!of_get_child_by_name(np, "nvmem-layout"))
+		return 0;
+
+	/*
+	 * skip block device too large to be represented as NVMEM devices
+	 * which are using an 'int' as address
+	 */
+	if (bdev_nr_bytes(bdev) > INT_MAX)
+		return -EFBIG;
+
+	bnv = kzalloc_obj(*bnv);
+	if (!bnv)
+		return -ENOMEM;
+
+	config.id = NVMEM_DEVID_NONE;
+	config.dev = &bdev->bd_device;
+	config.name = dev_name(&bdev->bd_device);
+	config.owner = THIS_MODULE;
+	config.priv = bnv;
+	config.reg_read = blk_nvmem_reg_read;
+	config.size = bdev_nr_bytes(bdev);
+	config.word_size = 1;
+	config.stride = 1;
+	config.read_only = true;
+	config.root_only = true;
+	config.ignore_wp = true;
+	config.of_node = to_of_node(dev->fwnode);
+
+	bnv->devt = bdev->bd_device.devt;
+	bnv->nvmem = nvmem_register(&config);
+	if (IS_ERR(bnv->nvmem)) {
+		dev_err_probe(&bdev->bd_device, PTR_ERR(bnv->nvmem),
+			      "Failed to register NVMEM device\n");
+
+		kfree(bnv);
+		return PTR_ERR(bnv->nvmem);
+	}
+
+	/*
+	 * Free bnv only when the nvmem device is fully released (i.e. when
+	 * its kref hits zero), not at unregister time. This prevents a
+	 * use-after-free if a consumer still holds an nvmem_cell reference
+	 * when the block device is removed: nvmem_unregister() only does a
+	 * kref_put(), so reg_read could still be called with bnv as priv
+	 * until the last consumer drops its cell.
+	 */
+	if (devm_add_action(nvmem_dev(bnv->nvmem), blk_nvmem_free, bnv)) {
+		nvmem_unregister(bnv->nvmem);
+		kfree(bnv);
+		return -ENOMEM;
+	}
+
+	mutex_lock(&devices_mutex);
+	list_add_tail(&bnv->list, &nvmem_devices);
+	mutex_unlock(&devices_mutex);
+
+	return 0;
+}
+
+static void blk_nvmem_unregister(struct device *dev)
+{
+	struct blk_nvmem *bnv_c, *bnv = NULL;
+
+	mutex_lock(&devices_mutex);
+	list_for_each_entry(bnv_c, &nvmem_devices, list) {
+		if (bnv_c->devt == dev_to_bdev(dev)->bd_device.devt) {
+			bnv = bnv_c;
+			break;
+		}
+	}
+
+	if (!bnv) {
+		mutex_unlock(&devices_mutex);
+		return;
+	}
+
+	list_del(&bnv->list);
+	mutex_unlock(&devices_mutex);
+	bnv->removed = true;
+	nvmem_unregister(bnv->nvmem);
+}
+
+static struct class_interface blk_nvmem_bus_interface __refdata = {
+	.class = &block_class,
+	.add_dev = &blk_nvmem_register,
+	.remove_dev = &blk_nvmem_unregister,
+};
+
+static int __init blk_nvmem_init(void)
+{
+	int ret;
+
+	ret = class_interface_register(&blk_nvmem_bus_interface);
+	if (ret)
+		return ret;
+
+	return 0;
+}
+device_initcall(blk_nvmem_init);
diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index 311cb2e5a5c02d2c6979d7c9bbb7f94abdfbdad1..ee3481229c20b3063c86d0dd66aabbf6b5e29169 100644
--- a/drivers/nvmem/core.c
+++ b/drivers/nvmem/core.c
@@ -2154,6 +2154,19 @@ const char *nvmem_dev_name(struct nvmem_device *nvmem)
 }
 EXPORT_SYMBOL_GPL(nvmem_dev_name);
 
+/**
+ * nvmem_dev() - Get the struct device of a given nvmem device.
+ *
+ * @nvmem: nvmem device.
+ *
+ * Return: pointer to the struct device of the nvmem device.
+ */
+struct device *nvmem_dev(struct nvmem_device *nvmem)
+{
+	return &nvmem->dev;
+}
+EXPORT_SYMBOL_GPL(nvmem_dev);
+
 /**
  * nvmem_dev_size() - Get the size of a given nvmem device.
  *
diff --git a/include/linux/nvmem-consumer.h b/include/linux/nvmem-consumer.h
index 34c0e58dfa26636d2804fcc7e0bc4a875ee73dae..ce387c89dc8e4bc1241f3b6f36be8c6c95e282ed 100644
--- a/include/linux/nvmem-consumer.h
+++ b/include/linux/nvmem-consumer.h
@@ -82,6 +82,7 @@ int nvmem_device_cell_write(struct nvmem_device *nvmem,
 
 const char *nvmem_dev_name(struct nvmem_device *nvmem);
 size_t nvmem_dev_size(struct nvmem_device *nvmem);
+struct device *nvmem_dev(struct nvmem_device *nvmem);
 
 void nvmem_add_cell_lookups(struct nvmem_cell_lookup *entries,
 			    size_t nentries);
@@ -220,6 +221,11 @@ static inline const char *nvmem_dev_name(struct nvmem_device *nvmem)
 	return NULL;
 }
 
+static inline struct device *nvmem_dev(struct nvmem_device *nvmem)
+{
+	return NULL;
+}
+
 static inline void
 nvmem_add_cell_lookups(struct nvmem_cell_lookup *entries, size_t nentries) {}
 static inline void

-- 
2.34.1