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

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Date:   Mon, 20 May 2019 09:33:36 +0000
From:   Leonard Crestez <leonard.crestez@....com>
To:     Peng Fan <peng.fan@....com>,
        "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>
Subject: Re: [RFC 1/2] dt-bindings: imx-ocotp: Add fusable-node property

On 20.05.2019 06:06, Peng Fan wrote:
> 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.

Couldn't firmware parse nvmem-cells? Even without a full nvmem subsystem 
it would be possible for imx-specific code to walk the entire device 
tree, parse nvmem-cells and disable nodes which are disabled by fuse.

--
Regards,
Leonard