Re: [RFC PATCH 00/16] OMAP: GPMC: Restructure OMAP GPMC driver (NAND) : DT binding change proposal

[Roger Quadros <rogerq-l0cyMroinI0@public.gmane.org>]

Hi Ezequiel,

On 05/21/2014 07:08 PM, Ezequiel Garcia wrote:
> Hi Roger,
> 
> On 21 May 02:20 PM, Roger Quadros wrote:
>>
>> For DT boot:
>> - The GPMC controller node should have a chip select (CS) node for each used
>>   chip select. The CS node must have a child device node for each device
>>   attached to that chip select. Properties for that child are GPMC agnostic.
>>
>>   i.e.
>> 	gpmc {
>> 		cs0 {
>> 			nand0 {
>> 			}
>> 		};
>>
>> 		cs1 {
>> 			nor0 {
>> 			}
>> 		}
>> 		...
>> 	};
>>
> 
> While I agree that the GPMC driver is a bit messy, I'm not sure it's possible
> to go through such a complete devicetree binding re-design (breaking backwards
> compatibility) now that the binding is already in production.

Why not? especially if the existing bindings are poorly dones. Is anyone using these
bindings burning the DT into ROM and can't change it when they update the kernel?

I wouldn't bother much about backward compatibility but just focus on not breaking
functionality with all GPMC users while cleaning up the existing bindings.

> 
> AFAIK, TI's SDK 7.0 is released, with a v3.8.x kernel which uses this GPMC
> binding. And then you have the ISEE board too, using this binding.

How does this prevent them from not using the new bindings when they update the kernel?

> 
> Also, what's the problem with the current devicetree binding (not that I'm fan
> of it)?
> 

The existing binding uses this format

	gpmc {
		ranges <cs-num 0 IO_partition_start IO_partition_size,
			cs-num 0 IO_partition_start IO_partition_size,
			...>

		node-name0 {
			compatible = "<id for device on this chip select>";
			reg = <cs-num IO_offset IO_size>;

			<gpmc cs properties>;

			<device specific properties>;
		};

		node-name1 {
			...
		};
	};

with requirements that
- chip select number (cs-num) is encoded in range id
- child's node-name is used to identify device type (NAND, Onenand, etc) and driver expects that.

All this results in the following issues
- No way to define entire GPMC I/O map (typically 1st 1GB), without assigning them to a Chip select. i.e. incomplete hardware description.
  TI SoCs variants can have different GPMC I/O sizes, some can have 512MB others can have 1GB. There needs to be a way to specify that.
- No clean way to specify GPMC register map for use by child nodes. NAND controller which can be one of the children needs to use the GPMC register map.
- Tricky to define multiple devices within a single chip select region.
- Uses node name to identify device type like nand and onenand. Doesn't use compatible id for them.
- GPMC CS properties are mixed with device properties, resulting in unnecessary binding documents like
http://lxr.free-electrons.com/source/Documentation/devicetree/bindings/net/gpmc-eth.txt
http://lxr.free-electrons.com/source/Documentation/devicetree/bindings/mtd/gpmc-nor.txt

To solve these issues, I'm proposing the following format

	gpmc {
		ranges <0 0 IO_start IO_size		/* entire GPMC I/O space e.g. 1GB or 512MB */
			1 0 Reg_start Reg_size>;	/* GPMC register space */

		cs0 {
			ranges <0 0 IO_partition_start IO_partition_size>;	/* CS0 IO partition e.g. 16MB */

			gpmc,cs-num = <0>;		/* pass chip select number explicitly */
			<gpmc cs properties>;

			dev0 {
				compatible = "<id for device on this chip select>";
				reg = <0 IO_offset IO_size			/* Device IO region e.g. 1KB */
					1 Reg_offset Reg_size>;

				<device specific properties>;
			};

			dev1 {
			...
			};
		};
		
		cs1 {
			...
		};
	};

All I'm doing is splitting up the CS node and the device node and removing the cs-num encoding from the ranges property.
This results in a much cleaner DT binding and code.

The format is similar to the one used by the ti-aemif driver.
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/memory-controllers/ti-aemif.txt

Having a unified format for all TI memory controllers will make life much easier for us.

cheers,
-roger
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