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SubjectRe: [PATCH v5 4/4] mtd: Add driver for concatenating devices
Hi Rob,

Boris Brezillon <boris.brezillon@collabora.com> wrote on Mon, 9 Dec
2019 11:35:06 +0100:

> On Wed, 27 Nov 2019 11:55:22 +0100
> Miquel Raynal <miquel.raynal@bootlin.com> wrote:
>
> > Introduce a generic way to define concatenated MTD devices. This may
> > be very useful in the case of ie. stacked SPI-NOR. Partitions to
> > concatenate are described in an additional property of the partitions
> > subnode:
> >
> > flash0 {
> > partitions {
> > compatible = "fixed-partitions";
> > part-concat = <&flash0_part1>, <&flash1_part0>;
> >
> > part0@0 {
> > label = "part0_0";
> > reg = <0x0 0x800000>;
> > };
> >
> > flash0_part1: part1@800000 {
> > label = "part0_1";
> > reg = <0x800000 0x800000>;
>
> So, flash0_part1 and flash0_part2 will be created even though the user
> probably doesn't need them?
>
> > };
> > };
> > };
> >
> > flash1 {
> > partitions {
> > compatible = "fixed-partitions";
> >
> > flash0_part1: part1@0 {
> > label = "part1_0";
> > reg = <0x0 0x800000>;
> > };
> >
> > part0@800000 {
> > label = "part1_1";
> > reg = <0x800000 0x800000>;
> > };
> > };
> > };
>
> IMHO this representation is far from intuitive. At first glance it's not
> obvious which partitions are linked together and what's the name of the
> resulting concatenated part. I definitely prefer the solution where we
> have a virtual device describing the concatenation. I also understand
> that this goes against the #1 DT rule: "DT only decribes HW blocks, not
> how they should be used/configured", but maybe we can find a compromise
> here, like moving this description to the /chosen node?
>
> chosen {
> flash-arrays {
> /*
> * my-flash-array is the MTD name if label is
> * not present.
> */
> my-flash-array {
> /*
> * We could have
> * compatible = "flash-array";
> * but we can also do without it.
> */
> label = "foo";
> flashes = <&flash1 &flash2 ...>;
> partitions {
> /* usual partition description. */
> ...
> };
> };
> };
> };
>
> Rob, what do you think?

Rob, I would really welcome your thoughts on this solution, having
something like a flash-array node in the /chosen/ node would avoid
creating dummy devices, keep the declarations of the physical nodes
tidy and have a very simple description.

Hope this compromise could fit!

>
> >
> > This is useful for boards where memory range has been extended with
> > the use of multiple flash chips as memory banks of a single MTD
> > device, with partitions spanning chip borders.
> >
> > Suggested-by: Bernhard Frauendienst <kernel@nospam.obeliks.de>
> > Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>

Thanks,
Miquèl

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