linux-kernel - Re: [PATCH v3 1/3] nvmem: Update the OF binding to use a subnode for the cells list

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Date:   Thu, 7 Jun 2018 18:41:55 +0200
From:   Alban <albeu@...e.fr>
To:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
Cc:     Aban Bedel <albeu@...e.fr>, linux-kernel@...r.kernel.org,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        David Woodhouse <dwmw2@...radead.org>,
        Brian Norris <computersforpeace@...il.com>,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Marek Vasut <marek.vasut@...il.com>,
        Richard Weinberger <richard@....at>,
        Cyrille Pitchen <cyrille.pitchen@...ev4u.fr>,
        devicetree@...r.kernel.org, linux-mtd@...ts.infradead.org
Subject: Re: [PATCH v3 1/3] nvmem: Update the OF binding to use a subnode
 for the cells list

On Tue, 1 May 2018 17:49:03 +0100
Srinivas Kandagatla <srinivas.kandagatla@...aro.org> wrote:

> On 18/04/18 14:34, Alban wrote:
> > On Wed, 18 Apr 2018 13:53:56 +0100
> > Srinivas Kandagatla <srinivas.kandagatla@...aro.org> wrote:
> >   
> >> On 18/04/18 13:32, Alban wrote:  
> >>>> I was also suggesting you to use nvmem-cell subnode, but make it a
> >>>> proper nvmem provider device, rather than reusing its parent device.
> >>>>
> >>>> You would end up some thing like this in dt.
> >>>>
> >>>> flash@0 {
> >>>> 	#address-cells = <1>;
> >>>> 	#size-cells = <1>;
> >>>> 	compatible = "s25sl064a";
> >>>> 	reg = <0>;
> >>>>
> >>>> 	nvmem-cells {
> >>>> 		compatible = "mtd-nvmem";
> >>>> 		#address-cells = <1>;
> >>>> 		#size-cells = <1>;
> >>>>
> >>>> 		calibration: calib@404 {
> >>>> 			reg = <0x404 0x10>;
> >>>> 		};
> >>>> 	};
> >>>> };  
> >>> But the root cause is in the nvmem binding, this conflict could exists  
> >> No, the root cause is because of passing wrong device instance to nvmem
> >> core. And trying to workaround is the actual issue.  
> > 
> > The data is stored on the MTD, so the nvmem provider is the MTD device.
> > I don't think it is a good idea to have a virtual device in the DT to
> > accommodate the nvmem API.
> >   
> Yep, I agree! this is same issue if we make nvmem-cells a child of nvmem 
> provider too.
> 
> However, I would like to see this moving forward.
> 
> I can think of one possible solution here, which is, adding 
> "nvmem-mtd-cell" or "nvmem-cell" compatible string to each cell.

I would definitely use "nvmem-cell", from the binding point of view it
doesn't matter what the underlaying storage is.

> The problem you mentioned regarding #address-cells and #size-cells with 
> provider need to be addressed in nvmem core.
> 
> Currently nvmem core only support offsets of 32 bits, if you are 
> expecting a 64 bit offsets then we should add that as a feature to nvmem 
> core.
> 
> nvmem core as it is today should work fine with 32 bit offsets for mtd 
> cases.

That's not what I meant, 32 bit should be more that enough for now.
What I meant is that if a binding already has children nodes using
unit-address, then we would end up with two different uses of the same
"address space".

> what do you think?

AFAIU the only thing that we disagree on now is if the nodes
representing the cells should be direct children of the provider
or in a dedicated subnode. For the MTD case both solution would solve
the binding clash. I would really appreciate if the DT people could
chip in so that we can settle this and get the MTD support merged.

Alban

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