linux-kernel - [RFC 1/2] dt-bindings: imx-ocotp: Add fusable-node property

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Date:   Mon, 20 May 2019 03:06:35 +0000
From:   Peng Fan <peng.fan@....com>
To:     "srinivas.kandagatla@...aro.org" <srinivas.kandagatla@...aro.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>
CC:     "festevam@...il.com" <festevam@...il.com>,
        dl-linux-imx <linux-imx@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Peng Fan <peng.fan@....com>
Subject: [RFC 1/2] dt-bindings: imx-ocotp: Add fusable-node property

Introduce fusable-node property for i.MX OCOTP driver.
The property will only be used by Firmware(eg. U-Boot) to
runtime disable the nodes.

Take i.MX6ULL for example, there are several parts that only
have limited modules enabled controlled by OCOTP fuse. It is
not flexible to provide several dts for the serval parts, instead
we could provide one device tree and let Firmware to runtime disable
the device tree nodes for those modules that are disable(fused).

Signed-off-by: Peng Fan <peng.fan@....com>
---

Currently NXP vendor use U-Boot to set status to disabled for devices
that could not function,
https://source.codeaurora.org/external/imx/uboot-imx/tree/arch/arm/mach-imx/mx6/module_fuse.c?h=imx_v2018.03_4.14.98_2.0.0_ga#n149
But this approach is will not work if kernel dts node path changed.

There are two approaches to resolve:

1. This patch is to add a fusable-node property, and Firmware will parse
   the property and read fuse to decide whether to disable or keeep enable
   the nodes.

2. There is another approach is that add nvmem-cells for all nodes that
   could be disabled(fused). Then in each linux driver to use nvmem
   api to detect fused or not, or in linux driver common code to check
   device functionable or not with nvmem API.

To make it easy to work, we choose [1] here. Please advise whether
it is acceptable, because the property is not used by linux driver in
approach [1]. Or you prefer [2] or please advise if any better solution.

Thanks.

 Documentation/devicetree/bindings/nvmem/imx-ocotp.txt | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Documentation/devicetree/bindings/nvmem/imx-ocotp.txt b/Documentation/devicetree/bindings/nvmem/imx-ocotp.txt
index 7a999a135e56..e9a998588dbd 100644
--- a/Documentation/devicetree/bindings/nvmem/imx-ocotp.txt
+++ b/Documentation/devicetree/bindings/nvmem/imx-ocotp.txt
@@ -21,6 +21,8 @@ Required properties:

 Optional properties:
 - read-only: disable write access
+- fusable-node: array of phandles with reg base and bit offset, this
+		property is used by Firmware to runtime disable nodes.

 Optional Child nodes:

@@ -42,4 +44,7 @@ Example:
 		tempmon_temp_grade: temp-grade@20 {
 			reg = <0x20 4>;
 		};
+
+		fusable-node = <&usdhc1 0x10 4
+				&usdhc2 0x10 5>;
 	};
-- 
2.16.4