Re: [PATCH v5 1/3] mtd: nand: gpio: Add DT property to automatically determine bus width

[Ezequiel Garcia <ezequiel.garcia@free-electrons.com>]

Brian,

On Sat, Nov 30, 2013 at 01:15:56AM -0800, Brian Norris wrote:
> 
> > Can anyone help me understand if there's *any* valid use case where we
> > want to specify a-priori the bus width, considering it's completely
> > discoverable at run-time?
> 
> I think the primary use case should be to reflect a limitation in the
> hardware (besides just the flash chip). It can mean that the controller
> itself only supports one bus width, or that the board is only wired up
> for x8, for instance.
> 

What do you mean by "reflect a limitation" of the hardware?

Maybe I'm not really following you, but it sounds as you're mixing up
the NAND device buswidth and the memory controller buswidth. At least
I consider them as two different parameters for two different pieces
of hardware.

Consider OMAP, just as an example. We currently have two DT properties:

1. nand-bus-width: not related to OMAP but kernel wide. It's supposed
   to specify the devices buswidth.

2. gpmc,device-width: It specifies how the controller should be
   configured (it doesn't really specify hardware, but configuration,
   sadly).

I now realise maybe this piece of DT binding is not a good example of DT.

Anyway, once again: Why would we need to set "nand-bus-width" to specify
the flash device width given we can discover that as soon as the NAND
is detected?

Notice, that this discussion is independent of the discussion about removing
the NAND_BUSWIDTH_AUTO. It's just about removing a DT property for a
parameter that's runtime configurable.
-- 
Ezequiel García, Free Electrons
Embedded Linux, Kernel and Android Engineering
http://free-electrons.com