DT description of SRAM with power manageable banks

[Vladimir Zapolskiy]

Hi DT team,

let me ask you for advice how to extend a common "mmio-sram" compatible
binding description with information about power manageable banks,
which are specific to some SRAM. My main interest is to give a better
support of NXP LPC32xx SRAM, but as far as I know it concerns other
controllers from the LPC series, and maybe some other ones.

Taking a simple visual example let's consider NXP LPC3250 SRAM, which
has four banks, three of them has independent power controls:

	0x00000 - 0x0ffff (64KiB), always on-power bank,
	0x10000 - 0x1ffff (64KiB), power manageable bank
	0x20000 - 0x2ffff (64KiB), power manageable bank
	0x30000 - 0x3ffff (64KiB), power manageable bank

All banks together form a single continuous address space, and it seems
important to have just one SRAM device and not 4, since e.g. in normal
runtime operation continuous 100KiB from the whole SRAM area are reserved
for storing MAC RX/TX buffers, it won't fit into one bank, however in
sleep mode (or when there is no users of some SRAM banks in runtime)
it is desirable to turn off unused banks, thus there should be a connection
between each power manageable bank and some power domain like this:

	sram_pd: power-controller at 110 {
		compatible = "nxp,lpc3220-pos";
		reg = <0x110 0x8>;
		#power-domain-cells = <1>;
	};

...

	sram-bank at 08010000 {
		reg = <0x08010000 0x10000>;
		power-domains = <&sram_pd 0>;
	};

...

The ordinary SRAM device node looks like

	sram at 08000000 {
		compatible = "mmio-sram";
		reg = <0x08000000 0x40000>;
	};

and I would happy to have it just like that, and actually it works
perfectly, if the problem with delicate power management is ignored.
At least it demonstrates that a generic wide compatible "mmio-sram"
suits well here.

The problem I face is in setting a proper layout of "sram@" and three
"sram-bank@" device nodes (remember 1 of 4 banks has no power control).
Unfortunately "sram-bank@" device nodes can not be children of "sram@",
because children of "mmio-sram" compatible device node play a role
of reserved areas ("partitions"), and that's not what I want. Moreover
reserved areas and power manageable banks are orthogonal, for instance
one reserved area may overlap with two power manageable banks or
vice versa.

Questions I have concerning this case:

1) would it be correct to add the second compatible like this?

	sram at 08000000 {
		compatible = "mmio-sram", "nxp,lpc3220-sram";
		reg = <0x08000000 0x40000>;
	};

Or should I add the new compatible to device nodes of each of the power
manageable banks?

2) I assume that "sram-bank@" device nodes still should be descendants
of "sram@" node, but because children with "reg" property of "mmio-sram"
compatible device node can not be used to describe power manageable banks,
what name for an umbrella device node would you suggest? Would "banks" be
good enough? Should it have its own compatible?

3) Do you think that reserved areas nodes should be placed under their
own device node with probably own compatible? So, something like it
was done recently with MTD partitions (compatible = "fixed-partitions").

Thank you in advance.
-- 
With best wishes,
Vladimir