linux-kernel - Re: [RFC PATCH] iio: core: provide a default value `label` property

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Message-ID: <4f0c786f-e29c-4838-59e3-236a908e4431@vaisala.com>
Date:   Tue, 22 Feb 2022 09:42:12 +0200
From:   Nandor Han <nandor.han@...sala.com>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     lars@...afoo.de, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] iio: core: provide a default value `label` property

On 2/20/22 15:18, Jonathan Cameron wrote:
> On Wed, 16 Feb 2022 15:56:04 +0200
> Nandor Han <nandor.han@...sala.com> wrote:
> 

Thanks for reviewing the patch and provide feed back.

>> The label property is used to correctly identify the same IIO device
>> over reboots. The implementation requires that a value will be provided
>> through device-tree. This sometime could requires many changes to
>> device-trees when multiple devices want to use the label property.
>> In order to prevent this, we could use the device-tree node
>> name as default value. The device-tree node name is unique and
>> also reflects the device which makes it a good choice as default value.
>> This change is backward compatible since doesn't affect the users that
>> do configure a label using the device-tree or the ones that are not
>> using the labels at all.
>>
>> Use the device-tree node name as a default value for `label` property,
>> in case there isn't one configured through device-tree.
> 
> Interesting idea.  However a few concerns come to mind.
> 1) If we start having a default for this, then it will get used as ABI
>     and if a label is applied later to the DT then we will end up breaking
>     userspace scripts.

When a label is explicitly configured means that the userspace expects 
to have that value available. Therefore, I don't see this as ABI change, 
given that this affects the property label content and not for example 
the property name.

> 2) If we do this it should be firmware agnostics (we need to fix
>     the existing code to be such as well).

Not sure I understand this. If you could explain a bit more I would 
really appriciate.

> 3) Is the node name always unique (think multiple accelerometers on
>     different i2c masters)?
> 3) I'm fairly sure this information is readily available anyway.
>     either via the of_node link for the iio\:deviceX
>     So why not have your usespace use that instead of label?
>     I'm not a fan of duplicating information that is readily available
>     anyway - be it as name and reg in the of_node directory.
> 

The node name supposed to be unique AFAIK and you're right it is 
available already in the userspace.
My point with this patch is to provide a default value for the label 
content and I'm open to suggestions related to content. The of_node name 
was something that I thought that is unique and easy to use, but if 
somebody has better suggestions I'm really open to these.

> Thanks,
> 
> Jonathan
> 

<snip>


Thanks again and regards,
    Nandor