Re: OF compatible MTD platform RAM driver ?

Sergei Shtylyov
2008-03-25T17:36:20Z

Laurent Pinchart wrote:

>>>>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.

>>Yeah. I forgeot that "flash" means EEPROM. Actually, the main facts about
>>the NVRAM that I'd want to be stated in the "compatible" property is that
>>it's non-volatile and directly/lineraly mapped... Just "nvram" doesn't seem
>>enopugh, maybe "linear-nvram" is.

> Direct mapping is a hard requirement for the nvram if we want to use it with
> the MTD subsystem.

I thought we're currently talking about a driver controlling the directly
mapped NVRAM. The "compatible" property wouldn't allow us to specify the
compatibility to *any* NVRAM since there could be no "least common
denominator" driver anyway.

> Regarding non-volatility nothing prevents a user from
> using a volatile RAM as an MTD device, but there's little point in doing so.

Indeed, if just for testing... we could specify non-volatility as the
device's prop, though...

> Would it be acceptable for the "linear-nvram" specification not to include
> volatile RAM ? ROM chips would be excluded too. Is that an issue ?

Well, I think we need a separate "compatible" prop for ROMs. Or we'll end
up with the "compatible" being just "memory" with the memory "attributes"
(R/O, N/V) being described by other "properties"... :-)

>>And we can specify "device_type" of "nvram" indeed (and #size).

> I suppose you meant #bytes.

Of course. :-)

> What about sub-partitions support ? Nothing prevents RAM-based MTD devices

Hm... I remember that the knowledge of MTD partitions turned me away from
"nvram" device type when I started spec'ing the flash binding -- it's not
uncommon to have a flash partition devoted to and labelled as "nvram".
Therefore, that sole partition would have been a "nvram" device for OF...

> from being partioned. Would it be acceptable to reference the CFI/JEDEC flash
> section in Documentation/powerpc/booting-without-of.txt in the description of
> the nvram node ?

I don't see why not.

> Best regards,

WBR, Sergei