[PATCH v9 06/10] 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.

NVMEM is registered for a block device whose OF node describes an NVMEM
layout, either via an "nvmem-layout" child or by being a "fixed-layout"
node itself (e.g. an eMMC boot partition associated through its mmc-card
node).

The NVMEM device is a read-only side channel: the block device stays
fully usable, reads are performed with a non-exclusive BLK_OPEN_READ
open, and no synchronization is done against concurrent writes to the
same region. This is acceptable for the intended use case of write-once,
factory-provisioned data.

Signed-off-by: Loic Poulain <[email protected]>
Reviewed-by: Bartosz Golaszewski <[email protected]>
---
 block/Kconfig             |  11 +++++
 block/Makefile            |   1 +
 block/blk-nvmem.c         | 104 ++++++++++++++++++++++++++++++++++++++++++++++
 block/blk.h               |   8 ++++
 block/genhd.c             |   4 ++
 block/partitions/core.c   |   5 +++
 include/linux/blk_types.h |   4 ++
 7 files changed, 137 insertions(+)

diff --git a/block/Kconfig b/block/Kconfig
index 70e4a66d941ff66ecb0ec34f4f7eff7fd1e4be40..4899ad572c71ffdbb62460d57623216254893ddc 100644
--- a/block/Kconfig
+++ b/block/Kconfig
@@ -209,6 +209,17 @@ 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,
+	  exposing factory-provisioned data such as MAC addresses or Wi-Fi
+	  calibration blobs to the drivers that consume them. This is
+	  typically used on embedded devices where such data is stored in a
+	  dedicated area of an eMMC, instead of a separate EEPROM or OTP.
+
 source "block/partitions/Kconfig"
 
 config BLK_PM
diff --git a/block/Makefile b/block/Makefile
index e7bd320e3d6971a8abf584f5de42776bb54e8216..8200297fbcbab304beb4239d0c33da07011c3ed9 100644
--- a/block/Makefile
+++ b/block/Makefile
@@ -39,3 +39,4 @@ 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..05bc35b9b8f373ccb606bdca35d211c41d2764fe
--- /dev/null
+++ b/block/blk-nvmem.c
@@ -0,0 +1,104 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * block device NVMEM provider
+ *
+ * Copyright (c) 2024 Daniel Golle <[email protected]>
+ * Copyright (c) Qualcomm Technologies, Inc. and/or its subsidiaries.
+ *
+ * Useful on devices using a whole disk or a partition (e.g. an eMMC boot
+ * partition) to store MAC addresses, Bluetooth addresses or Wi-Fi
+ * calibration EEPROM data.
+ *
+ * The NVMEM device is a side channel onto a block device that stays fully
+ * usable. This is somewhat mitigated by opening the device exclusively.
+ */
+
+#include <linux/cleanup.h>
+#include <linux/device.h>
+#include <linux/file.h>
+#include <linux/fs.h>
+#include <linux/nvmem-provider.h>
+#include <linux/nvmem-consumer.h>
+#include <linux/of.h>
+#include <linux/property.h>
+
+#include "blk.h"
+
+static int blk_nvmem_reg_read(void *priv, unsigned int from,
+			      void *val, size_t bytes)
+{
+	struct block_device *bdev = priv;
+	struct file *bdev_file;
+	loff_t pos = from;
+	ssize_t ret;
+
+	/* open and prevent other exclusive openers */
+	bdev_file = bdev_file_open_by_dev(bdev->bd_dev,
+					  BLK_OPEN_READ | BLK_OPEN_EXCL,
+					  blk_nvmem_reg_read, NULL);
+	if (IS_ERR(bdev_file))
+		return PTR_ERR(bdev_file);
+
+	ret = kernel_read(bdev_file, val, bytes, &pos);
+	if (ret >= 0 && ret != bytes)
+		ret = -EIO;
+
+	fput(bdev_file);
+
+	return ret < 0 ? ret : 0;
+}
+
+int blk_nvmem_add(struct block_device *bdev)
+{
+	struct device *dev = &bdev->bd_device;
+	struct device_node *np = dev_of_node(dev);
+	struct nvmem_config config = {};
+	struct nvmem_device *nvmem;
+
+	/* skip devices which do not have a device tree node */
+	if (!np)
+		return 0;
+
+	/*
+	 * The layout is described either by an "nvmem-layout" child node or
+	 * by the device node itself being a "fixed-layout" container.
+	 */
+	struct device_node *child __free(device_node) =
+			of_get_child_by_name(np, "nvmem-layout");
+	if (!child && !of_device_is_compatible(np, "fixed-layout"))
+		return 0;
+
+	if (bdev_nr_bytes(bdev) > INT_MAX) {
+		dev_warn(dev, "block device too large to be an NVMEM\n");
+		return 0;
+	}
+
+	config.id = NVMEM_DEVID_NONE;
+	config.dev = dev;
+	config.name = dev_name(dev);
+	config.owner = THIS_MODULE;
+	config.priv = bdev;
+	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 = np;
+
+	nvmem = nvmem_register(&config);
+	if (IS_ERR(nvmem))
+		return dev_err_probe(dev, PTR_ERR(nvmem),
+				     "Failed to register NVMEM device\n");
+
+	bdev->bd_nvmem = nvmem;
+
+	return 0;
+}
+
+void blk_nvmem_del(struct block_device *bdev)
+{
+	nvmem_unregister(bdev->bd_nvmem);
+	bdev->bd_nvmem = NULL;
+}
diff --git a/block/blk.h b/block/blk.h
index 25af8ac5ef0f77c09b4d747827b275c32e9f2972..dad8f0d7a218edbafa9b6a03aed673657f3e8cbb 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -810,4 +810,12 @@ static inline void blk_debugfs_unlock(struct request_queue *q,
 	memalloc_noio_restore(memflags);
 }
 
+#ifdef CONFIG_BLK_NVMEM
+int blk_nvmem_add(struct block_device *bdev);
+void blk_nvmem_del(struct block_device *bdev);
+#else
+static inline int blk_nvmem_add(struct block_device *bdev) { return 0; }
+static inline void blk_nvmem_del(struct block_device *bdev) {}
+#endif
+
 #endif /* BLK_INTERNAL_H */
diff --git a/block/genhd.c b/block/genhd.c
index f84b6a355b574af88c870938a8c69aeb9f8f44ad..0c9c5e5ea7baffa326a2d45b075688cf3a0edf5a 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -421,6 +421,8 @@ static void add_disk_final(struct gendisk *disk)
 		 */
 		dev_set_uevent_suppress(ddev, 0);
 		disk_uevent(disk, KOBJ_ADD);
+
+		blk_nvmem_add(disk->part0);
 	}
 
 	blk_apply_bdi_limits(disk->bdi, &disk->queue->limits);
@@ -704,6 +706,8 @@ static void __del_gendisk(struct gendisk *disk)
 
 	disk_del_events(disk);
 
+	blk_nvmem_del(disk->part0);
+
 	/*
 	 * Prevent new openers by unlinked the bdev inode.
 	 */
diff --git a/block/partitions/core.c b/block/partitions/core.c
index 72f435dfeffc63e897913571f319e5edd8e7ba88..e4a41ffcce12a08051bbd90437312b9242bf600e 100644
--- a/block/partitions/core.c
+++ b/block/partitions/core.c
@@ -279,6 +279,8 @@ void drop_partition(struct block_device *part)
 	xa_erase(&part->bd_disk->part_tbl, bdev_partno(part));
 	kobject_put(part->bd_holder_dir);
 
+	blk_nvmem_del(part);
+
 	device_del(&part->bd_device);
 	put_device(&part->bd_device);
 }
@@ -396,6 +398,9 @@ static struct block_device *add_partition(struct gendisk *disk, int partno,
 	/* suppress uevent if the disk suppresses it */
 	if (!dev_get_uevent_suppress(ddev))
 		kobject_uevent(&pdev->kobj, KOBJ_ADD);
+
+	blk_nvmem_add(bdev);
+
 	return bdev;
 
 out_del:
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index 8808ee76e73c09e0ceaac41ba59e86fb0c4efc64..6ed173c649025b95cce9253b27f68f2c7dbab8eb 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -15,6 +15,7 @@
 struct bio_set;
 struct bio;
 struct bio_integrity_payload;
+struct nvmem_device;
 struct page;
 struct io_context;
 struct cgroup_subsys_state;
@@ -73,6 +74,9 @@ struct block_device {
 	int			bd_writers;
 #ifdef CONFIG_SECURITY
 	void			*bd_security;
+#endif
+#ifdef CONFIG_BLK_NVMEM
+	struct nvmem_device	*bd_nvmem;
 #endif
 	/*
 	 * keep this out-of-line as it's both big and not needed in the fast

-- 
2.34.1