[PATCH] mtd: spi-nor: clear Extended Address Reg on switch to 3-byte addressing.

[NeilBrown <neil@brown.name>]

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According to section
   8.2.7 Write Extended Address Register (C5h)

of the Winbond W25Q256FV data sheet (256M-BIT SPI flash)

   The Extended Address Register is only effective when the device is
   in the 3-Byte Address Mode.  When the device operates in the 4-Byte
   Address Mode (ADS=1), any command with address input of A31-A24
   will replace the Extended Address Register values. It is
   recommended to check and update the Extended Address Register if
   necessary when the device is switched from 4-Byte to 3-Byte Address
   Mode.

This patch adds code to implement that recommendation.  Without this,
my GNUBEE-PC1 will not successfully reboot, as the Extended Address
Register is left with a value of '1'. When the SOC attempts to read
(in 3-byte address mode) the boot loader, it reads from the wrong
location.

Signed-off-by: NeilBrown <neil@brown.name>
---
 drivers/mtd/spi-nor/spi-nor.c | 10 ++++++++++
 include/linux/mtd/spi-nor.h   |  2 ++
 2 files changed, 12 insertions(+)

diff --git a/drivers/mtd/spi-nor/spi-nor.c b/drivers/mtd/spi-nor/spi-nor.c
index d445a4d3b770..c303bf0d2982 100644
--- a/drivers/mtd/spi-nor/spi-nor.c
+++ b/drivers/mtd/spi-nor/spi-nor.c
@@ -269,6 +269,7 @@ static inline int set_4byte(struct spi_nor *nor, const struct flash_info *info,
 	int status;
 	bool need_wren = false;
 	u8 cmd;
+	u8 val;
 
 	switch (JEDEC_MFR(info)) {
 	case SNOR_MFR_MICRON:
@@ -283,6 +284,15 @@ static inline int set_4byte(struct spi_nor *nor, const struct flash_info *info,
 		status = nor->write_reg(nor, cmd, NULL, 0);
 		if (need_wren)
 			write_disable(nor);
+		if (!status && !enable &&
+		    nor->read_reg(nor, SPINOR_OP_RDXA, &val, 1) == 0 &&
+		    val != 0) {
+			/* need to reset the Extended Address Register */
+			write_enable(nor);
+			val = 0;
+			nor->write_reg(nor, SPINOR_OP_WRXA, &val, 1);
+			write_disable(nor);
+		}
 
 		return status;
 	default:
diff --git a/include/linux/mtd/spi-nor.h b/include/linux/mtd/spi-nor.h
index de36969eb359..42954419cfdf 100644
--- a/include/linux/mtd/spi-nor.h
+++ b/include/linux/mtd/spi-nor.h
@@ -62,6 +62,8 @@
 #define SPINOR_OP_RDCR		0x35	/* Read configuration register */
 #define SPINOR_OP_RDFSR		0x70	/* Read flag status register */
 #define SPINOR_OP_CLFSR		0x50	/* Clear flag status register */
+#define SPINOR_OP_RDXA		0xc8	/* Read Extended Address Register */
+#define SPINOR_OP_WRXA		0xc5	/* Write Extended Address Register */
 
 /* 4-byte address opcodes - used on Spansion and some Macronix flashes. */
 #define SPINOR_OP_READ_4B	0x13	/* Read data bytes (low frequency) */
-- 
2.14.0.rc0.dirty


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