Re: [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 <loic.poulain@oss.qualcomm.com>
Reviewed-by: Bartosz Golaszewski <redacted>
---
 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 <daniel@makrotopia.org>
+ * 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;

+	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;