Re: OF compatible MTD platform RAM driver ?

Laurent Pinchart
2008-03-25T16:44:48Z

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On Tuesday 25 March 2008 17:23, Sergei Shtylyov wrote:
> Laurent Pinchart wrote:
>
> >>>We're talking about a very specific type of RAM, used for permanent
storage
> >>>with a battery backup. The RAM is really meant to be used as an MTD
device
> >>>and as such I think it makes sense to describe it as an mtd-compatible
device
> >>>on the local bus.
>
> >>>What about the following definition for the RAM node ?
>
> >>> nvram@2,0000 {
>
> >> Note that there's a OF "device_type" of "nvram", so your (generic)
device
> >>name seems to add some mess. (IIRC, that OF device type didn't actually
> >>represent a "real" device, and only served to provide access to NVRAM for
OF).
>
> > Ok.
>
> Well, I might have gone too far here -- it should be a real device
> (spec'ed in Device Support Extensions recommended practice). It's just that
> the spec didn't mention "reg" property, only "#bytes" (the device capacity).
> So, it may be worth considering...

The nvram device descrived in the Device Support Extensions is probably meant
to describe the kind of nvram found in RTC chips. That memory isn't directly
accessible. As the spec doesn't mention this explicitely we could still reuse
the nvram device type for direct-mapped battery-backed ram. I have no strong
opinion for or against that.

> >>> compatible = "mtd,ram";
>
> >> The part before comma should be a company name or a stock ticker. What
did
> >>you mean here?
>
> > I didn't know that. Let's say I meant "mtd-ram" :-)
>
> >>> reg = <2 0x0000 0x00100000>;
> >>> bank-width = <2>;
> >>> };
>
> >>>Or should the node have a device-type property of either 'ram' or 'rom'
with
> >>>the compatible property just referencing MTD ?
>
> >> The "device_type" properties are not required and their further
creation
> >>has been discouraged on liunxppc-dev.
>
> > What about
>
> > mtdram@2,0000 {
> > compatible = "mtd-ram";
> > reg = <2 0x0000 0x00100000>;
> > bank-width = <2>;
> > };
>
> > ROMs could use "mtd-rom" for their compatible property.
>
> Heh, there was a whole company against mentioning "mtd" when we started
> working on this (of course, the first idea was to call the flash device type
> "mtd"). I don't think "mtd" looks good here -- I'd suggest "flash-ram" (if
> this is just a linearly mapped NVRAM).

I'm fine with "flash-ram" (even thought it looks a bit weird). I'll prepare a
patch.

Best regards,

--
Laurent Pinchart
CSE Semaphore Belgium

Chaussée de Bruxelles, 732A
B-1410 Waterloo
Belgium

T +32 (2) 387 42 59
F +32 (2) 387 42 75

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