Re: [PATCH v4 3/4] dt-bindings: mtd: Describe mtd-concat devices

[Rob Herring <robh@xxxxxxxxxx>]

On Mon, Nov 25, 2019 at 8:15 AM Miquel Raynal <miquel.raynal@xxxxxxxxxxx> wrote:
>
> Hi Rob,
>
> Rob Herring <robh@xxxxxxxxxx> wrote on Mon, 18 Nov 2019 16:13:41 -0600:
>
> > On Wed, Nov 13, 2019 at 06:15:04PM +0100, Miquel Raynal wrote:
> > > From: Bernhard Frauendienst <kernel@xxxxxxxxxxxxxxxxx>
> > >
> > > The main use case to concatenate MTD devices is probably SPI-NOR
> > > flashes where the number of address bits is limited to 24, which can
> > > access a range of 16MiB. Board manufacturers might want to double the
> > > SPI storage size by adding a second flash asserted thanks to a second
> > > chip selects which enhances the addressing capabilities to 25 bits,
> > > 32MiB. Having two devices for twice the size is great but without more
> > > glue, we cannot define partition boundaries spread across the two
> > > devices. This is the gap mtd-concat intends to address.
> > >
> > > There are two options to describe concatenated devices:
> > > 1/ One flash chip is described in the DT with two CS;
> > > 2/ Two flash chips are described in the DT with one CS each, a virtual
> > > device is also created to describe the concatenation.
> > >
> > > Solution 1/ presents at least 3 issues:
> > > * The hardware description is abused;
> > > * The concatenation only works for SPI devices (while it could be
> > >   helpful for any MTD);
> > > * It would require a lot of rework in the SPI core as most of the
> > >   logic assumes there is and there always will be only one CS per
> > >   chip.
> >
> > This seems ok if all the devices are identical.
>
> This is not an option for Mark and I agree with him as we are faking
> the reality: the two devices we want to virtually concatenate may be
> two physically different devices. Binding them as one is lying.
>
> > > Solution 2/ also has caveats:
> > > * The virtual device has no hardware reality;
> > > * Possible optimizations at the hardware level will be hard to enable
> > >   efficiently (ie. a common direct mapping abstracted by a SPI
> > >   memories oriented controller).
> >
> > Something like this may be necessary if data is interleaved rather than
> > concatinated.
>
> This is something that is gonna happen too, it is called "dual
> parallel".

Then it would be good to think about how that should look. Maybe
there's overlap or maybe not.

> > Solution 3
> > Describe each device and partition separately and add link(s) from one
> > partition to the next
> >
> > flash0 {
> >   partitions {
> >     compatible = "fixed-partitions";
> >     concat-partition = <&flash1_partitions>;
> >     ...
> >   };
> > };
> >
> > flash1 {
> >   flash1_partition: partitions {
> >     compatible = "fixed-partitions";
> >     ...
> >   };
> > };
>
> I honestly don't see how this is different as solution 2/?

It's a single new property rather than a whole binding for a virtual
device. It's a minimal change to the DT. It would work with existing
bootloaders (and other OSs and older kernels) without change except
for the one concatenated partition.

> In one case
> we describe the partition concatenation in one subnode as a "link", in
> the other we create a separate node to describe the link. Are you
> strongly opposed as solution 2/?

I'd prefer to not have virtual devices without good reason.

> From a pure conceptual point of view,
> is it really different than 3/?
>
>
> Thanks,
> Miquèl

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