Subject: Representation of external memory-mapped devices in DT (gpmc)

From: [email-masked] (Rob Herring)
Date: Mon, 29 Oct 2012 16:49:23 -0500

On 10/29/2012 12:22 PM, Daniel Mack wrote:
> On 29.10.2012 16:09, Rob Herring wrote:
>> On 10/29/2012 09:39 AM, Daniel Mack wrote:
>>> Hi,
>>>
>>> we're currently working on a DT binding for the GPMC bus that is found
>>> on SoCs by TI. The implementation is based on CS lines and an 8, 16 or
>>> 32 bit parallel interface. That IP is quite flexible, and it can for
>>> example be used for physmap flash, external peripherals or even NAND.
>>>
>>> Depending on which CS is used to control the device, different memory
>>> regions are reserved, and there's code to calculate the location and
>>> size of them, given a CS number (see arch/arm/mach-omap2/gpmc.c).
>>
>> I don't know the details of the h/w, but I would think the DT core code
>> should be able work out the addresses. This can be done using ranges
>> property which defines the mapping of a child bus into the parent bus
>> addresses.
> 
> In this case, the controller is @0x50000000 while the external device is
> mapped to address 0x0.

Okay, so it's not fixed or some sub-range of the memory map.

>>> The binding will use one top-level node to describe the GPMC controller
>>> itself and register the actual devices as sub-nodes to it. The NAND type
>>> is the only one that is currently supported. This is how it currently looks:
>>>
>>> 	gpmc: gpmc at 50000000 {
>>> 		compatible = "ti,gpmc";
>>> 		ti,hwmods = "gpmc";
>>> 		reg = <0x50000000 0x2000>;
>>> 		interrupt-parent = <&intc>;
>>> 		interrupts = <100>;
>>> 		#address-cells = <1>;
>>> 		#size-cells = <0>;
>>>
>>> 		nand at 0 {
>>
>> You may want a CS0 node with nand as a child node of that.
> 
> Hmm, I don't see what that would buy us. The question is which way is
> feasible for storing both the memory region and the cs number in the
> device tree. The CS number should certainly go to the child node, no?

I was thinking of if you had per CS properties you needed to setup each
CS. So something like this (using non-zero address to show the
translation better):

gpmc {
	compatible = "ti,gpmc", "simple-bus";
	ti,hwmods = "gpmc";
	reg = <0x50000000 0x2000>;

	CS0 {
		// map 1MB @ gpmc offset 0 to host address 0x10000000
		ranges = <0x10000000 0x0 <0x100000>;
		// other gpmc specific per CS properties.
		nand {
			reg = <0x0 0x100000>;
		}
	};

The core code will look at parents' ranges properties to translate the
local offset of 0 into host address of 0x10000000 Then the nand node's
reg address will get translated to 0x10000000.

The gpmc code can then use the ranges properties to setup the mapping of
each chip select to the host address. You don't need a new property to
describe the cs mapping.

Rob

> 
> IOW, would it be a good idea to have something like the following layout?
> 
> 	gpmc: gpmc at 50000000 {
> 		compatible = "ti,gpmc";
> 		ti,hwmods = "gpmc";
> 		reg = <0x50000000 0x2000>;
> 
> 		/* cs-reg stores the setup of the controller's
> 		   memory map */
> 
> 			/* offset	size */
> 		cs-reg = <0x0		0x1000000
> 			  ....		.....
> 			  ....		.....>;
> 
> 		nand: child at 0 {
> 			/* timings */
> 			/* peripheral specifics */
> 		};
> 	};
> 
> I would actually much prefer that approach.
> 
> Afzal, because because that way, we can leave the code as-is for now and
> add the "cs-reg" property once the code is switched to dynamic handling.
> What do you think?
> 
> 
> Thanks,
> Daniel
>