[PATCH v7 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 <[email protected]>
---
 drivers/mtd/spi-nor/core.c | 78 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 78 insertions(+)

diff --git a/drivers/mtd/spi-nor/core.c b/drivers/mtd/spi-nor/core.c
index ccf4396cdcd0..0425af6e898f 100644
--- a/drivers/mtd/spi-nor/core.c
+++ b/drivers/mtd/spi-nor/core.c
@@ -17,6 +17,7 @@
 #include <linux/mtd/mtd.h>
 #include <linux/mtd/spi-nor.h>
 #include <linux/mutex.h>
+#include <linux/nvmem-provider.h>
 #include <linux/of.h>
 #include <linux/regulator/consumer.h>
 #include <linux/sched/task_stack.h>
@@ -3001,6 +3002,75 @@ static void spi_nor_init_fixup_flags(struct spi_nor *nor)
 		nor->flags |= SNOR_F_IO_MODE_EN_VOLATILE;
 }
 
+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;
+}
+
+/**
+ * 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.
+ */
+static int spi_nor_register_sfdp_nvmem(struct spi_nor *nor)
+{
+	struct device *dev = nor->dev;
+	struct nvmem_config config = { };
+	struct nvmem_device *nvmem;
+	struct device_node *np;
+
+	if (!nor->sfdp)
+		return 0;
+
+	np = of_get_compatible_child(dev_of_node(dev), "jedec,sfdp");
+	if (!np)
+		return 0;
+
+	config.dev = dev;
+	config.of_node = np;
+	config.name = "sfdp";
+	config.id = NVMEM_DEVID_AUTO;
+	config.owner = THIS_MODULE;
+	config.read_only = true;
+	config.word_size = 1;
+	config.stride = 1;
+	config.size = (int)(nor->sfdp->num_dwords * sizeof(*nor->sfdp->dwords));
+	config.reg_read = spi_nor_sfdp_reg_read;
+	config.priv = nor;
+
+	nvmem = devm_nvmem_register(dev, &config);
+	of_node_put(np);
+	if (IS_ERR(nvmem)) {
+		/* NVMEM support is optional. */
+		if (PTR_ERR(nvmem) == -EOPNOTSUPP)
+			return 0;
+		return dev_err_probe(dev, PTR_ERR(nvmem),
+				     "failed to register SFDP NVMEM device\n");
+	}
+
+	dev_dbg(dev, "exposed %d-byte SFDP as an NVMEM device\n", config.size);
+
+	return 0;
+}
+
 /**
  * spi_nor_late_init_params() - Late initialization of default flash parameters.
  * @nor:	pointer to a 'struct spi_nor'
@@ -3204,6 +3274,14 @@ 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;
-- 
2.43.0