linux-kernel - [PATCH v3 2/2] doc: dt: mtd: add a DT property to enable the use of 4byte-address op codes

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Message-ID: <93b32638d08044de4907b46b2e4a21c645298a48.1458736784.git.cyrille.pitchen@atmel.com>
Date:	Wed, 23 Mar 2016 13:49:11 +0100
From:	Cyrille Pitchen <cyrille.pitchen@...el.com>
To:	<computersforpeace@...il.com>, <linux-mtd@...ts.infradead.org>
CC:	<nicolas.ferre@...el.com>, <boris.brezillon@...e-electrons.com>,
	<marex@...x.de>, <linux-kernel@...r.kernel.org>,
	<devicetree@...r.kernel.org>, <robh+dt@...nel.org>,
	<pawel.moll@....com>, <mark.rutland@....com>,
	<ijc+devicetree@...lion.org.uk>, <galak@...eaurora.org>,
	Cyrille Pitchen <cyrille.pitchen@...el.com>
Subject: [PATCH v3 2/2] doc: dt: mtd: add a DT property to enable the use of 4byte-address op codes

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@...el.com>
---
 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..8be610482089 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.
 
+- m25p,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 can not 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