Re: [PATCH v5 3/7] mtd: spi-nor: sfdp: expose the SFDP as a read-only NVMEM device

The SPI NOR core already reads the SFDP tables during enumeration and
caches them in nor->sfdp->dwords (see spi_nor_parse_sfdp()). Re-expose
that cached data as a read-only NVMEM device, in on-flash byte order,
rooted at the flash's SFDP child node (compatible "jedec,sfdp").

This lets NVMEM cells reference any SFDP data: a fixed-layout for
parameters at a known offset, or an nvmem-layout parser for vendor data
whose location must be discovered at runtime.The device is only registered
when an "sfdp" node is present in the device tree.

Signed-off-by: Manikandan Muralidharan <manikandan.m@microchip.com>
---
 drivers/mtd/spi-nor/core.c |  8 ++++
 drivers/mtd/spi-nor/core.h |  1 +
 drivers/mtd/spi-nor/sfdp.c | 86 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 95 insertions(+)

diff --git a/drivers/mtd/spi-nor/core.c b/drivers/mtd/spi-nor/core.c
index ccf4396cdcd0..b833d8ec2d65 100644
--- a/drivers/mtd/spi-nor/core.c
+++ b/drivers/mtd/spi-nor/core.c

@@ -3204,6 +3204,14 @@ static int spi_nor_init_params(struct spi_nor *nor)
 		spi_nor_init_params_deprecated(nor);
 	}
 
+	/*
+	 * Expose the SFDP table as an NVMEM device only when
+	 * the flash actually provides one
+	 */
+	ret = spi_nor_register_sfdp_nvmem(nor);
+	if (ret)
+		return ret;
+
 	ret = spi_nor_late_init_params(nor);
 	if (ret)
 		return ret;

diff --git a/drivers/mtd/spi-nor/core.h b/drivers/mtd/spi-nor/core.h
index ba2d1a862c9d..0a6484298c5c 100644
--- a/drivers/mtd/spi-nor/core.h
+++ b/drivers/mtd/spi-nor/core.h

@@ -698,6 +698,7 @@ int spi_nor_controller_ops_write_reg(struct spi_nor *nor, u8 opcode,
 
 int spi_nor_check_sfdp_signature(struct spi_nor *nor);
 int spi_nor_parse_sfdp(struct spi_nor *nor);
+int spi_nor_register_sfdp_nvmem(struct spi_nor *nor);

 static inline struct spi_nor *mtd_to_spi_nor(struct mtd_info *mtd)
 {

diff --git a/drivers/mtd/spi-nor/sfdp.c b/drivers/mtd/spi-nor/sfdp.c
index 4600983cb579..704799fe92ae 100644
--- a/drivers/mtd/spi-nor/sfdp.c
+++ b/drivers/mtd/spi-nor/sfdp.c

@@ -6,6 +6,8 @@
 
 #include <linux/bitfield.h>
 #include <linux/mtd/spi-nor.h>
+#include <linux/nvmem-provider.h>
+#include <linux/of.h>
 #include <linux/slab.h>
 #include <linux/sort.h>
 

@@ -1612,3 +1614,87 @@ int spi_nor_parse_sfdp(struct spi_nor *nor)
 	kfree(param_headers);
 	return err;
 }
+
+static int spi_nor_sfdp_reg_read(void *priv, unsigned int offset,
+				 void *val, size_t bytes)
+{
+	struct spi_nor *nor = priv;
+	struct sfdp *sfdp = nor->sfdp;
+	size_t sfdp_size = sfdp->num_dwords * sizeof(*sfdp->dwords);
+
+	if (offset >= sfdp_size || bytes > sfdp_size - offset)
+		return -EINVAL;
+
+	/* The cached SFDP is kept in on-flash (little-endian) byte order. */
+	memcpy(val, (u8 *)sfdp->dwords + offset, bytes);
+
+	return 0;
+}
+
+static void spi_nor_sfdp_nvmem_put_np(void *data)
+{
+	of_node_put(data);
+}
+
+/**
+ * spi_nor_register_sfdp_nvmem() - expose the SFDP as a read-only NVMEM device
+ * @nor:	pointer to a 'struct spi_nor'
+ *
+ * Expose the whole SFDP, in on-flash byte order, as a read-only NVMEM device
+ * rooted at the flash's SFDP child node (compatible "jedec,sfdp"). This lets
+ * generic (fixed-layout) or vendor (nvmem-layout) cells reference any SFDP
+ * data. The device is only registered when a child node with the "jedec,sfdp"
+ * compatible is described in the device tree.
+ *
+ * Return: 0 on success or if there is nothing to do, -errno otherwise.
+ */
+int spi_nor_register_sfdp_nvmem(struct spi_nor *nor)