linux-kernel - Re: [PATCH 0/4] dt-bindings: imx: add nvmem property

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Date:   Thu, 31 Mar 2022 19:15:29 -0500
From:   Rob Herring <robh@...nel.org>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     "Peng Fan (OSS)" <peng.fan@....nxp.com>, ulf.hansson@...aro.org,
        krzk+dt@...nel.org, shawnguo@...nel.org, s.hauer@...gutronix.de,
        wg@...ndegger.com, mkl@...gutronix.de, davem@...emloft.net,
        kuba@...nel.org, pabeni@...hat.com, qiangqing.zhang@....com,
        devicetree@...r.kernel.org, Peng Fan <peng.fan@....com>,
        netdev@...r.kernel.org, linux-mmc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-can@...r.kernel.org,
        linux-imx@....com, kernel@...gutronix.de, festevam@...il.com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 0/4] dt-bindings: imx: add nvmem property

On Thu, Mar 24, 2022 at 12:11:04PM +0100, Uwe Kleine-König wrote:
> Hello,
> 
> On Thu, Mar 24, 2022 at 12:20:20PM +0800, Peng Fan (OSS) wrote:
> > From: Peng Fan <peng.fan@....com>
> > 
> > To i.MX SoC, there are many variants, such as i.MX8M Plus which
> > feature 4 A53, GPU, VPU, SDHC, FLEXCAN, FEC, eQOS and etc.
> > But i.MX8M Plus has many parts, one part may not have FLEXCAN,
> > the other part may not have eQOS or GPU.
> > But we use one device tree to support i.MX8MP including its parts,
> > then we need update device tree to mark the disabled IP status "disabled".
> > 
> > In NXP U-Boot, we hardcoded node path and runtime update device tree
> > status in U-Boot according to fuse value. But this method is not
> > scalable and need encoding all the node paths that needs check.
> > 
> > By introducing nvmem property for each node that needs runtime update
> > status property accoridng fuse value, we could use one Bootloader
> > code piece to support all i.MX SoCs.
> > 
> > The drawback is we need nvmem property for all the nodes which maybe
> > fused out.
> 
> I'd rather not have that in an official binding as the syntax is
> orthogonal to status = "..." but the semantic isn't. Also if we want
> something like that, I'd rather not want to adapt all bindings, but
> would like to see this being generic enough to be described in a single
> catch-all binding.
> 
> I also wonder if it would be nicer to abstract that as something like:
> 
> 	/ {
> 		fuse-info {
> 			compatible = "otp-fuse-info";
> 
> 			flexcan {
> 				devices = <&flexcan1>, <&flexcan2>;
> 				nvmem-cells = <&flexcan_disabled>;
> 				nvmem-cell-names = "disabled";
> 			};
> 
> 			m7 {
> 				....
> 			};
> 		};
> 	};
> 
> as then the driver evaluating this wouldn't need to iterate over the
> whole dtb but just over this node. But I'd still keep this private to
> the bootloader and not describe it in the generic binding.

There's been discussions (under the system DT umbrella mostly) about 
bindings for peripheral enable/disable control/status. Most of the time 
it is in context of device assignment to secure/non-secure world or 
partitions in a system (via a partitioning hypervisor).

This feels like the same thing and could use the same binding. But 
someone has to take into account all the uses and come up with 
something. One off solutions are a NAK.

Rob