[PATCH v4 2/2] doc: dt: mtd: add a DT property to enable the use of 4byte-address op codes

[Cyrille Pitchen]

This patch adds a new optional DT property which enables an alternative
way of supporting memory size above 16MiB (128Mib). This new mechanism
translates the regular 3byte-address op codes into their 4byte-address
version whereas the old/default mecanism makes the SPI memory enter its
4byte-address mode, which has annoying side effects for early bootloaders.

We cannot discover at run time whether the SPI NOR memory supports the
4byte-address op codes. For instance both Macronix MX25L25635E and
MX25L25673G share the same JEDEC ID (C22019 without any extension byte).
However the first one doesn't support 4byte-address op codes whereas the
second one does.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@xxxxxxxxx>
---
 Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt b/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt
index 2c91c03e7eb0..3cf1b3cdafe8 100644
--- a/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt
+++ b/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt
@@ -66,6 +66,17 @@ Optional properties:
                    Refer to your chips' datasheet to check if this is supported
                    by your chip.
 
+- spi-nor-4byte-opcodes: For memory size above 16MiB (128Mib), use the dedicated
+                         4byte-address opcodes instead of entering the 4byte
+                         address mode. This mode changes the internal state of
+                         the chip so may conflict with some early boot loaders,
+                         which expect to use the regular (Fast) Read opcodes
+                         with 3byte address.
+                         However 4byte-address opcodes are not supported by all
+                         chips and support for them cannot be detected at
+                         runtime. Refer to you chip's datasheet to check if this
+                         is supported by your chip.
+
 Example:
 
 	flash: m25p80@0 {
-- 
1.8.2.2