Re: [PATCH 2/6] PowerPC 440EPx: Sequoia DTS

[Segher Boessenkool <segher@kernel.crashing.org>]

>> To be honest, I'm not sure that describing the mapping is really the
>> job of the compatible property.  That the flash is mapped into the
>> address space is kind of implicit in the way the reg interacts with
>> the parents' ranges property.
>
>     Ah, I keep forgetting about implied 1:1 parent/child address
> correspondence... :-<

It's not implied, it is explicitly specified.

>     But does it really imply how the (low) address bits of the *child* 
> bus
> ("ebc" in this case) are connected to the chip?  I don't think so...

The currently proposed binding assumes linear mapping.

Strange setups will need to extend this binding; this
does mean that the burden for that is not on the "normal"
case.

There are so many weird ways in which people wire up memory
devices that there is no chance to define a "generic" binding
that works for all in less than 400 pages of documentation.

>     Well, "device-width" is not the thing that we care about either. 
> ;-)

You need to know the device-width to drive the chip (and no,
it isn't enough to know the exact chip model even) -- why do
you say you don't care about it?

>>>    Hmm... what does @2,0 mean? :-O
>
>> EBC chip select 2, offset 0.
>
>     Well, so this node is under some kind of local bus node -- that's 
> good.
> Didn't know that the spec allows commas after @...

Most characters are allowed in the unit-address...  The following
is just fine: "my-secret-base@the-moon".  ISA uses letters to
distinguish between its different address spaces, for example.

There is a direct correspondence between the first "reg"
address and the text representation of the unit address;
this correspondence is bus-dependent however.

David, can multiple devices sit on the same chip-select
on EBC, or on the same "minor" address?  If not, you can
simplify your unit address representation.

>> "direct-mapped" is simply not a sufficiently specific compatible
>> property, no two ways about it.
>
>     Yes, for example "direct-mapped-cfi" and "direct-mapped-jedec" 
> would have
> been better...

Nah.  These memory devices are meant to sit at some address/data
bus; whether it is direct-mapped or not is obvious from the node
hierarchy the flash node hangs under already; let's make the simple
case simple and the complicated cases possible.


Segher