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

[Rob Herring <robh@kernel.org>]

On Wed, Nov 13, 2019 at 06:15:04PM +0100, Miquel Raynal wrote:
> From: Bernhard Frauendienst <kernel@nospam.obeliks.de>
> 
> 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.

> 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.


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";
    ...
  };
};

Maybe a link back to the previous paritions too or a boolean to mark as 
a continuation.

No idea how well this works or not for the kernel, but that really 
shouldn't matter for the binding design.

Rob