mtd: spi-nor: add generic flash driver

Our SFDP parsing is everything we need to support all basic operations
of a flash device. If the flash isn't found in our in-kernel flash
database, gracefully fall back to a driver described solely by its SFDP
tables.
Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Tested-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Link: https://lore.kernel.org/r/20220810220654.1297699-7-michael@walle.cc

drivers/mtd/spi-nor/core.c

@@ -1634,6 +1634,16 @@ static const struct spi_nor_manufacturer *manufacturers[] = {
 	&spi_nor_xmc,
 };
 
+static const struct flash_info spi_nor_generic_flash = {
+	.name = "spi-nor-generic",
+	/*
+	 * JESD216 rev A doesn't specify the page size, therefore we need a
+	 * sane default.
+	 */
+	.page_size = 256,
+	.parse_sfdp = true,
+};
+
 static const struct flash_info *spi_nor_match_id(struct spi_nor *nor,
 						 const u8 *id)
 {
@@ -1672,6 +1682,14 @@ static const struct flash_info *spi_nor_detect(struct spi_nor *nor)
 		return ERR_PTR(-ENOMEM);
 
 	info = spi_nor_match_id(nor, id);
+
+	/* Fallback to a generic flash described only by its SFDP data. */
+	if (!info) {
+		ret = spi_nor_check_sfdp_signature(nor);
+		if (!ret)
+			info = &spi_nor_generic_flash;
+	}
+
 	if (!info) {
 		dev_err(nor->dev, "unrecognized JEDEC id bytes: %*ph\n",
 			SPI_NOR_MAX_ID_LEN, id);
@@ -2098,8 +2116,12 @@ static int spi_nor_select_pp(struct spi_nor *nor,
  * spi_nor_select_uniform_erase() - select optimum uniform erase type
  * @map:		the erase map of the SPI NOR
  * @wanted_size:	the erase type size to search for. Contains the value of
- *			info->sector_size or of the "small sector" size in case
- *			CONFIG_MTD_SPI_NOR_USE_4K_SECTORS is defined.
+ *			info->sector_size, the "small sector" size in case
+ *			CONFIG_MTD_SPI_NOR_USE_4K_SECTORS is defined or 0 if
+ *			there is no information about the sector size. The
+ *			latter is the case if the flash parameters are parsed
+ *			solely by SFDP, then the largest supported erase type
+ *			is selected.
  *
  * Once the optimum uniform sector erase command is found, disable all the
  * other.

drivers/mtd/spi-nor/core.h

@@ -701,6 +701,7 @@ int spi_nor_controller_ops_read_reg(struct spi_nor *nor, u8 opcode,
 int spi_nor_controller_ops_write_reg(struct spi_nor *nor, u8 opcode,
 				     const u8 *buf, size_t len);
 
+int spi_nor_check_sfdp_signature(struct spi_nor *nor);
 int spi_nor_parse_sfdp(struct spi_nor *nor);
 
 static inline struct spi_nor *mtd_to_spi_nor(struct mtd_info *mtd)

drivers/mtd/spi-nor/sfdp.c

@@ -1256,6 +1256,33 @@ static void spi_nor_post_sfdp_fixups(struct spi_nor *nor)
 		nor->info->fixups->post_sfdp(nor);
 }
 
+/**
+ * spi_nor_check_sfdp_signature() - check for a valid SFDP signature
+ * @nor:	pointer to a 'struct spi_nor'
+ *
+ * Used to detect if the flash supports the RDSFDP command as well as the
+ * presence of a valid SFDP table.
+ *
+ * Return: 0 on success, -errno otherwise.
+ */
+int spi_nor_check_sfdp_signature(struct spi_nor *nor)
+{
+	u32 signature;
+	int err;
+
+	/* Get the SFDP header. */
+	err = spi_nor_read_sfdp_dma_unsafe(nor, 0, sizeof(signature),
+					   &signature);
+	if (err < 0)
+		return err;
+
+	/* Check the SFDP signature. */
+	if (le32_to_cpu(signature) != SFDP_SIGNATURE)
+		return -EINVAL;
+
+	return 0;
+}
+
 /**
  * spi_nor_parse_sfdp() - parse the Serial Flash Discoverable Parameters.
  * @nor:		pointer to a 'struct spi_nor'