linux-kernel - Re: [PATCH] mux: mmio: use reg property when parent device is not a syscon

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Message-ID: <558ebfaf-bd7e-1760-5799-8ed430acad7a@ti.com>
Date:   Tue, 16 May 2023 11:29:58 -0500
From:   Andrew Davis <afd@...com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Peter Rosin <peda@...ntia.se>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Nishanth Menon <nm@...com>,
        Vignesh Raghavendra <vigneshr@...com>
CC:     <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mux: mmio: use reg property when parent device is not a
 syscon

On 5/16/23 11:19 AM, Krzysztof Kozlowski wrote:
> On 16/05/2023 17:18, Andrew Davis wrote:
>> On 5/15/23 4:14 PM, Peter Rosin wrote:
>>> Hi!
>>>
>>> 2023-05-15 at 21:19, Andrew Davis wrote:
>>>> The DT binding for the reg-mux compatible states it can be used when the
>>>> "parent device of mux controller is not syscon device". It also allows
>>>> for a reg property. When the parent device is indeed not a syscon device,
>>>> nor is it a regmap provider, we should fallback to using that reg
>>>> property to identify the address space to use for this mux.
>>>
>>> We should? Says who?
>>>
>>> Don't get me wrong, I'm not saying the change is bad or wrong, I would just
>>> like to see an example where it matters. Or, at least some rationale for why
>>> the code needs to change other than covering some case that looks like it
>>> could/should be possible based on the binding. I.e., why is it not better to
>>> "close the hole" in the binding instead?
>>>
>>
>> Sure, so this all stated when I was building a checker to make sure that drivers
>> are not mapping overlapping register spaces. I noticed syscon nodes are a source
>> of that so I'm trying to look into their usage.
>>
>> To start, IHMO there is only one valid use for syscon and that is when more than
>> one driver needs to access shared bits in a single register. DT has no way to
> 
> It has... what about all existing efuse/nvmem devices?
> 
>> describe down to the bit granular level, so one must give that register to
>> a "syscon node", then have the device node use a phandle to the syscon node:
>>
>> common_reg: syscon@...00 {
>> 	compatible = "syscon";
>> 	reg = <0x10000 0x4>;
>> };
>>
>> consumer@1 {
>> 	syscon-efuse = <&common_reg 0x1>;
>> };
>>
>> consumer@2 {
>> 	syscon-efuse = <&common_reg 0x2>;
>> };
>>
>> Something like that, then regmap will take care of synchronizing access.
> 
> Syscon is not for this.
> 

That is how it is used today, and in 5 other ways too and there is
no guidance on it. Let me know what syscon is for then.

>>
> 
> ...
> 
>>
>> Ideally DT nodes all describe their register space in a "reg"
>> property and all the "large collection of devices" spaces become
>> "simple-bus" nodes. "syscon" nodes can then be limited to only the
>> rare case when multiple devices share bits in a single register.
>>
>> If Rob and Krzysztof agree I can send a patch with the above
>> guidance to the Devicetree Specification repo also.
> 
> Agree on what?
> 

That we should provide the above guidance on when and how to use syscon
nodes. Right now it is a free for all and it is causing issues.

Andrew

> 
> Best regards,
> Krzysztof
>