misc: Add support for nvmem cells

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. New drivers corresponding
to a Linux-style nvmem device should be implemented as one of the
previously-mentioned uclasses. The nvmem API acts as a compatibility
layer to adapt the (slightly different) APIs of these uclasses. It also
handles the lookup of nvmem cells.

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@700000 {
		#address-cells = <1>;
		#size-cells = <1>;
		reg = <0x00700000 0x100000>;

		/* ... */

		tsens_calibration: calib@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@seco.com>

1 files changed, 134 insertions, 0 deletions

diff --git a/include/nvmem.h b/include/nvmem.h
new file mode 100644
index 00000000000..822e698bdd4
--- /dev/null
+++ b/include/nvmem.h
@@ -0,0 +1,134 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright (c) 2022 Sean Anderson <sean.anderson@seco.com>
+ */
+
+#ifndef NVMEM_H
+#define NVMEM_H
+
+/**
+ * DOC: Design
+ *
+ * The NVMEM subsystem is a "meta-uclass" in that it abstracts over several
+ * different uclasses all with read/write APIs. One approach to implementing
+ * this could be to add a new sub-device for each nvmem-style device of
+ * UCLASS_NVMEM. This subsystem has taken the approach of using the existing
+ * access methods (i2c_eeprom_write, misc_write, etc.) directly. This has the
+ * advantage of not requiring an extra device/driver, saving on binary size and
+ * runtime memory usage. On the other hand, it is not idiomatic. Similar
+ * efforts should generally use a new uclass.
+ */
+
+/**
+ * struct nvmem_cell - One datum within non-volatile memory
+ * @nvmem: The backing storage device
+ * @offset: The offset of the cell from the start of @nvmem
+ * @size: The size of the cell, in bytes
+ */
+struct nvmem_cell {
+	struct udevice *nvmem;
+	unsigned int offset;
+	size_t size;
+};
+
+struct udevice;
+
+#if CONFIG_IS_ENABLED(NVMEM)
+
+/**
+ * nvmem_cell_read() - Read the value of an nvmem cell
+ * @cell: The nvmem cell to read
+ * @buf: The buffer to read into
+ * @size: The size of @buf
+ *
+ * Return:
+ * * 0 on success
+ * * -EINVAL if @buf is not the same size as @cell.
+ * * -ENOSYS if CONFIG_NVMEM is disabled
+ * * A negative error if there was a problem reading the underlying storage
+ */
+int nvmem_cell_read(struct nvmem_cell *cell, void *buf, size_t size);
+
+/**
+ * nvmem_cell_write() - Write a value to an nvmem cell
+ * @cell: The nvmem cell to write
+ * @buf: The buffer to write from
+ * @size: The size of @buf
+ *
+ * Return:
+ * * 0 on success
+ * * -EINVAL if @buf is not the same size as @cell
+ * * -ENOSYS if @cell is read-only, or if CONFIG_NVMEM is disabled
+ * * A negative error if there was a problem writing the underlying storage
+ */
+int nvmem_cell_write(struct nvmem_cell *cell, const void *buf, size_t size);
+
+/**
+ * nvmem_cell_get_by_index() - Get an nvmem cell from a given device and index
+ * @dev: The device that uses the nvmem cell
+ * @index: The index of the cell in nvmem-cells
+ * @cell: The cell to initialize
+ *
+ * Look up the nvmem cell referenced by the phandle at @index in nvmem-cells in
+ * @dev.
+ *
+ * Return:
+ * * 0 on success
+ * * -EINVAL if the regs property is missing, empty, or undersized
+ * * -ENODEV if the nvmem device is missing or unimplemented
+ * * -ENOSYS if CONFIG_NVMEM is disabled
+ * * A negative error if there was a problem reading nvmem-cells or getting the
+ *   device
+ */
+int nvmem_cell_get_by_index(struct udevice *dev, int index,
+			    struct nvmem_cell *cell);
+
+/**
+ * nvmem_cell_get_by_name() - Get an nvmem cell from a given device and name
+ * @dev: The device that uses the nvmem cell
+ * @name: The name of the nvmem cell
+ * @cell: The cell to initialize
+ *
+ * Look up the nvmem cell referenced by @name in the nvmem-cell-names property
+ * of @dev.
+ *
+ * Return:
+ * * 0 on success
+ * * -EINVAL if the regs property is missing, empty, or undersized
+ * * -ENODEV if the nvmem device is missing or unimplemented
+ * * -ENODATA if @name is not in nvmem-cell-names
+ * * -ENOSYS if CONFIG_NVMEM is disabled
+ * * A negative error if there was a problem reading nvmem-cell-names,
+ *   nvmem-cells, or getting the device
+ */
+int nvmem_cell_get_by_name(struct udevice *dev, const char *name,
+			   struct nvmem_cell *cell);
+
+#else /* CONFIG_NVMEM */
+
+static inline int nvmem_cell_read(struct nvmem_cell *cell, void *buf, int size)
+{
+	return -ENOSYS;
+}
+
+static inline int nvmem_cell_write(struct nvmem_cell *cell, const void *buf,
+				   int size)
+{
+	return -ENOSYS;
+}
+
+static inline int nvmem_cell_get_by_index(struct udevice *dev, int index,
+					  struct nvmem_cell *cell)
+{
+	return -ENOSYS;
+}
+
+static inline int nvmem_cell_get_by_name(struct udevice *dev, const char *name,
+					 struct nvmem_cell *cell)
+{
+	return -ENOSYS;
+}
+
+#endif /* CONFIG_NVMEM */
+
+#endif /* NVMEM_H */