[PATCH 06/14] misc: Add support for nvmem cells

[Simon Glass]

Hi Sean,

On Mon, 7 Feb 2022 at 16:42, Sean Anderson <sean.anderson at seco.com> wrote:
>
> This adds support for "nvmem cells" as seen in Linux. The nvmem device
> class in Linux is used for various assorted ROMs and EEPROMs. In this
> sense, it is similar to UCLASS_MISC, but also includes
> UCLASS_I2C_EEPROM, UCLASS_RTC, and UCLASS_MTD. While nvmem devices can
> be accessed directly, they are most often used by reading/writing
> contiguous values called "cells". Cells typically hold information like
> calibration, versions, or configuration (such as mac addresses).
>
> nvmem devices can specify "cells" in their device tree:
>
>         qfprom: eeprom at 700000 {
>                 #address-cells = <1>;
>                 #size-cells = <1>;
>                 reg = <0x00700000 0x100000>;
>
>                 /* ... */
>
>                 tsens_calibration: calib at 404 {
>                         reg = <0x404 0x10>;
>                 };
>         };
>
> which can then be referenced like:
>
>         tsens {
>                 /* ... */
>                 nvmem-cells = <&tsens_calibration>;
>                 nvmem-cell-names = "calibration";
>         };
>
> The tsens driver could then read the calibration value like:
>
>         struct nvmem_cell cal_cell;
>         u8 cal[16];
>         nvmem_cell_get_by_name(dev, "calibration", &cal_cell);
>         nvmem_cell_read(&cal_cell, cal, sizeof(cal));
>
> Because nvmem devices are not all of the same uclass, supported uclasses
> must register a nvmem_interface struct. This allows CONFIG_NVMEM to be
> enabled without depending on specific uclasses. At the moment,
> nvmem_interface is very bare-bones, and assumes that no initialization
> is necessary. However, this could be amended in the future.
>
> Although I2C_EEPROM and MISC are quite similar (and could likely be
> unified), they present different read/write function signatures. To
> abstract over this, NVMEM uses the same read/write signature as Linux.
> In particular, short read/writes are not allowed, which is allowed by
> MISC.
>
> The functionality implemented by nvmem cells is very similar to that
> provided by i2c_eeprom_partition. "fixed-partition"s for eeproms does
> not seem to have made its way into Linux or into any device tree other
> than sandbox. It is possible that with the introduction of this API it
> would be possible to remove it.
>
> Signed-off-by: Sean Anderson <sean.anderson at seco.com>
> ---
>
>  MAINTAINERS           |   7 ++
>  doc/api/index.rst     |   1 +
>  doc/api/nvmem.rst     |   7 ++
>  drivers/misc/Kconfig  |  16 ++++
>  drivers/misc/Makefile |   1 +
>  drivers/misc/nvmem.c  | 109 +++++++++++++++++++++++++
>  include/nvmem.h       | 185 ++++++++++++++++++++++++++++++++++++++++++
>  7 files changed, 326 insertions(+)
>  create mode 100644 doc/api/nvmem.rst
>  create mode 100644 drivers/misc/nvmem.c
>  create mode 100644 include/nvmem.h

Here I think it would be better to add a new uclass so that drivers
which support it can add a child device in that uclass. This is done
in lots of places in U-Boot.

Regards,
Simon