linux-kernel - Re: [PATCH v12 2/7] nvmem: Clarify the situation when there is no DT node available

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Date:   Fri, 6 Oct 2023 18:32:29 +0200
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     Rafał Miłecki <rafal@...ecki.pl>
Cc:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Michael Walle <michael@...le.cc>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Robert Marko <robert.marko@...tura.hr>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Luka Perkov <luka.perkov@...tura.hr>,
        Randy Dunlap <rdunlap@...radead.org>,
        Chen-Yu Tsai <wenst@...omium.org>,
        Daniel Golle <daniel@...rotopia.org>
Subject: Re: [PATCH v12 2/7] nvmem: Clarify the situation when there is no
 DT node available

Hi Rafał,

rafal@...ecki.pl wrote on Fri, 06 Oct 2023 13:41:52 +0200:

> On 2023-10-05 17:59, Miquel Raynal wrote:
> > At a first look it might seem that the presence of the of_node pointer
> > in the nvmem device does not matter much, but in practice, after > looking
> > deep into the DT core, nvmem_add_cells_from_dt() will simply and always
> > return NULL if this field is not provided. As most mtd devices don't
> > populate this field (this could evolve later), it means none of their
> > children cells will be populated unless no_of_node is explicitly set to
> > false. In order to clarify the logic, let's add clear check at the
> > beginning of this helper.  
> 
> I'm somehow confused by above explanation and code too. I read it
> carefully 5 times but I can't see what exactly this change helps with.
> 
> At first look at nvmem_add_cells_from_legacy_of() I can see it uses
> "of_node" so I don't really agree with "it might seem that the presence
> of the of_node pointer in the nvmem device does not matter much".
> 
> You really don't need to look deep into DT core (actually you don't have
> to look into it at all) to understand that nvmem_add_cells_from_dt()
> will return 0 (nitpicking: not NULL) for a NULL pointer. It's all made
> of for_each_child_of_node(). Obviously it does nothing if there is
> nothing to loop over.

That was not obvious to me as I thought it would start from /, which I
think some other function do when you don't provide a start node.

> Given that for_each_child_of_node() is NULL-safe I think code from this
> patch is redundant.

I didn't say it was not safe, just not explicit.

> Later you mention "no_of_node" which I agree to be a very non-intuitive
> config option. As pointed in another thread I already sent:
> [PATCH] Revert "nvmem: add new config option"
> https://lore.kernel.org/lkml/ba3c419a-6511-480a-b5f2-6c418f9c02e7@gmail.com/t/

I actually wanted to find again that patch and could not get my hands on
it, but it is probably a much better fix than my other mtd patch, I
agree with you.

> Maybe with above patch finally things will get more clear and we don't
> need this PATCH after all?

Yes. Srinivas, what are your plans for the above patch?

Thanks,
Miquèl