Winbond spi-nor flash 32MB and larger have an 'Extended Address
Register' as one option for addressing beyond 16MB (Macronix
has the same concept, Spansion has EXTADD bits in the Bank Address

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

So the documentation suggests clearing the EAR after switching to
3-byte mode.  Experimentation shows that the EAR is *always* one after
the switch to 3-byte mode, so clearing the EAR is mandatory at
shutdown for a subsequent 3-byte-addressed reboot to work.

Note that some SOCs (e.g. MT7621) do not assert a reset line at normal
reboot, so we cannot rely on hardware reset.  The MT7621 does assert a
reset line at watchdog-reset.

Signed-off-by: NeilBrown <>

following a helpful discussion with Marek, I've revised the description
a little, and make the code change specific to winbond.
I've change the OP names to RDEAR and WREAR instead of RDXA and WRXA to
match names used in the Macronix documentation.  Winbond documentation
doesn't provide abbreviated OP names.


 drivers/mtd/spi-nor/spi-nor.c | 13 +++++++++++++
 include/linux/mtd/spi-nor.h   |  2 ++
 2 files changed, 15 insertions(+)

diff --git a/drivers/mtd/spi-nor/spi-nor.c b/drivers/mtd/spi-nor/spi-nor.c
index d445a4d3b770..0d0af0acf8b9 100644
--- a/drivers/mtd/spi-nor/spi-nor.c
+++ b/drivers/mtd/spi-nor/spi-nor.c
@@ -284,6 +284,19 @@ static inline int set_4byte(struct spi_nor *nor, const 
struct flash_info *info,
                if (need_wren)
+               if (!status && !enable &&
+                   JEDEC_MFR(info) == SNOR_MFR_WINBOND) {
+                       /* On Winbond W25Q256FV, leaving 4byte mode causes
+                        * the Extended Address Register to be set to 1, so all
+                        * 3-byte-address reads come from the second 16M.
+                        * We must clear the register to enable normal behavior.
+                        */
+                       write_enable(nor);
+                       nor->cmd_buf[0] = 0;
+                       nor->write_reg(nor, SPINOR_OP_WREAR, nor->cmd_buf, 1);
+                       write_disable(nor);
+               }
                return status;
                /* Spansion style */
diff --git a/include/linux/mtd/spi-nor.h b/include/linux/mtd/spi-nor.h
index de36969eb359..e60da0d34cc1 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_RDEAR                0xc8    /* Read Extended Address 
Register */
+#define SPINOR_OP_WREAR                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) */

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