block: implement NVMEM provider

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.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

block/Kconfig

@@ -206,6 +206,15 @@ 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 prodivers,
+	  typically used with eMMC to store MAC addresses or Wi-Fi
+	  calibration data on embedded devices.
+
 source "block/partitions/Kconfig"
 
 config BLK_MQ_PCI

block/Makefile

@@ -34,6 +34,7 @@ obj-$(CONFIG_BLK_DEV_ZONED)	+= blk-zoned.o
 obj-$(CONFIG_BLK_WBT)		+= blk-wbt.o
 obj-$(CONFIG_BLK_DEBUG_FS)	+= blk-mq-debugfs.o
 obj-$(CONFIG_BLK_DEBUG_FS_ZONED)+= blk-mq-debugfs-zoned.o
+obj-$(CONFIG_BLK_NVMEM)		+= blk-nvmem.o
 obj-$(CONFIG_BLK_SED_OPAL)	+= sed-opal.o
 obj-$(CONFIG_BLK_PM)		+= blk-pm.o
 obj-$(CONFIG_BLK_INLINE_ENCRYPTION)	+= blk-crypto.o blk-crypto-profile.o \

block/blk-nvmem.c

@@ -0,0 +1,163 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * block device NVMEM provider
+ *
+ * Copyright (c) 2023 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/of.h>
+#include <linux/pagemap.h>
+#include <linux/property.h>
+
+/* List of all NVMEM devices */
+static LIST_HEAD(nvmem_devices);
+static DEFINE_MUTEX(devices_mutex);
+
+struct blk_nvmem {
+	struct nvmem_device *nvmem;
+	struct block_device *bdev;
+	struct list_head list;
+};
+
+static int blk_nvmem_reg_read(void *priv, unsigned int from,
+			      void *val, size_t bytes)
+{
+	pgoff_t f_index = from >> PAGE_SHIFT;
+	struct address_space *mapping;
+	struct blk_nvmem *bnv = priv;
+	unsigned long offs, to_read;
+	size_t bytes_left = bytes;
+	struct folio *folio;
+	void *p;
+
+	if (!bnv->bdev)
+		return -ENODEV;
+
+	offs = from & ~PAGE_MASK;
+	mapping = bnv->bdev->bd_inode->i_mapping;
+
+	while (bytes_left) {
+		folio = read_mapping_folio(mapping, f_index++, NULL);
+		if (IS_ERR(folio))
+			return PTR_ERR(folio);
+
+		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);
+	}
+
+	return 0;
+}
+
+static int blk_nvmem_register(struct device *dev)
+{
+	struct block_device *bdev = dev_to_bdev(dev);
+	struct nvmem_config config = {};
+	struct blk_nvmem *bnv;
+
+	/*
+	 * skip devices which don't have GENHD_FL_NVMEM set
+	 *
+	 * This flag is used for mtdblock and ubiblock devices because
+	 * both, MTD and UBI already implement their own NVMEM provider.
+	 * To avoid registering multiple NVMEM providers for the same
+	 * device node, skip the block NVMEM provider.
+	 */
+	if (!(bdev->bd_disk->flags & GENHD_FL_NVMEM))
+		return 0;
+
+	if (!fwnode_device_is_compatible(dev->fwnode, "nvmem-cells"))
+		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(sizeof(struct blk_nvmem), GFP_KERNEL);
+	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->bdev = bdev;
+	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);
+	}
+
+	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 block_device *bdev = dev_to_bdev(dev);
+	struct blk_nvmem *bnv_c, *bnv = NULL;
+
+	mutex_lock(&devices_mutex);
+	list_for_each_entry(bnv_c, &nvmem_devices, list) {
+		if (bnv_c->bdev == bdev) {
+			bnv = bnv_c;
+			break;
+		}
+	}
+
+	if (!bnv) {
+		mutex_unlock(&devices_mutex);
+		return;
+	}
+
+	list_del(&bnv->list);
+	mutex_unlock(&devices_mutex);
+	nvmem_unregister(bnv->nvmem);
+	kfree(bnv);
+}
+
+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);