RFC nvmem: add block device NVMEM provider

On embedded devices using an eMMC it is common that one or more (hw/sw) partitions on the eMMC are used to store MAC addresses and Wi-Fi calibration EEPROM data.

Implement an NVMEM provider backed by block devices as typically the NVMEM framework is used to have kernel drivers read and use binary data from EEPROMs, efuses, flash memory (MTD), ...

In order to be able to reference hardware partitions on an eMMC, add code to bind each hardware partition to a specific firmware subnode.

This series is meant to open the discussion on how exactly the device tree schema for block devices and partitions may look like, and even if using the block layer to back the NVMEM device is at all the way to go -- to me it seemed to be a good solution because it will be reuable e.g. for (normal, software GPT or MBR) partitions of an NVMe SSD.

This series has previously been submitted on July 19th 2023[1] and most of the basic idea did not change since.

However, the recent introduction of bdev_file_open_by_dev() allow to get rid of most use of block layer internals which supposedly was the main objection raised by Christoph Hellwig back then.

Most of the other comments received for in the first RFC have also been addressed, however, what remains is the use of class_interface (lacking an alternative way to get notifications about addition or removal of block devices from the system). As this has been criticized in the past I'm specifically interested in suggestions on how to solve this in another way -- ideally without having to implement a whole new way for in-kernel notifications of appearing or disappearing block devices...

And, in a way just like in case of MTD and UBI, I believe acting as an NVMEM provider *is* a functionality which belongs to the block layer itself and, other than e.g. filesystems, is inconvenient to implement elsewhere.

[1]: https://patchwork.kernel.org/project/linux-block/list/?series=767565

Daniel Golle (8):
dt-bindings: block: add basic bindings for block devices
block: partitions: populate fwnode
block: add new genhd flag GENHD_FL_NVMEM
block: implement NVMEM provider
dt-bindings: mmc: mmc-card: add block device nodes
mmc: core: set card fwnode_handle
mmc: block: set fwnode of disk devices
mmc: block: set GENHD_FL_NVMEM

 .../bindings/block/block-device.yaml          |  22 +++
 .../devicetree/bindings/block/partition.yaml  |  51 +++++
 .../devicetree/bindings/block/partitions.yaml |  20 ++
 .../devicetree/bindings/mmc/mmc-card.yaml     |  45 +++++
 block/Kconfig                                 |   9 +
 block/Makefile                                |   1 +
 block/blk-nvmem.c                             | 175 ++++++++++++++++++
 block/partitions/core.c                       |  41 ++++
 drivers/mmc/core/block.c                      |   8 +
 drivers/mmc/core/bus.c                        |   2 +
 include/linux/blkdev.h                        |   2 +
 11 files changed, 376 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/block/block-device.yaml
 create mode 100644 Documentation/devicetree/bindings/block/partition.yaml
 create mode 100644 Documentation/devicetree/bindings/block/partitions.yaml
 create mode 100644 block/blk-nvmem.c

2.44.0