Re: [PATCH] [POWERPC] get rid of `model = "UCC"' in the ucc nodes

On Tue, Feb 05, 2008 at 09:39:02AM -0700, Grant Likely wrote:

On 2/5/08, David Gibson [off-list ref] wrote:

quoted

On Fri, Feb 01, 2008 at 09:23:47AM -0700, Grant Likely wrote:

quoted

cell-index has been useful for things like clock controllers to know
what offset into a shared clock control register or something like
that and a driver would pass the cell-index value to the shared reg
driver when requesting service.

Right.  Except that if the shared resource is just a single register,
calling the routines to access it a "shared reg driver" gives a
misleading impression.  Depending on how the shared reg is used, even
a lock may not be necessary, so potentially the drivers for the
individual device instances using the shared resource can (safely)
directly access it.

Fair enough.  In the case of a single shared, or a homogeneous set of
shared registers (all of them use the same index) I can see the
argument for cell index.

Yes, that's the situation it was created for.

However, there are places where cell-index is being used where the
value of cell-index has no relation to the offset into a register.
But what about the case where the device uses multiple shared
registers, each one using a *different* offset.  cell-index doesn't
describe this situation well (or at least no better than just using
the value of reg instead, a translation is still required)

Absolutely.  This is not a suitable situation in which to use
cell-index.

[snip]

quoted

From booting-without-of (in the EMAC description):

- cell-index        : 1 cell, hardware index of the EMAC cell on a
given ASIC (typically 0x0 and 0x1 for EMAC0 and EMAC1 on each Axon
chip.

So, even if the intent was for cell-index to specify offsets into
shared regs, the description does not reflect that purpose.  And
reading thorough the rest of the document, cell-index is described
purely in terms of enumerating ip blocks, so that is clearly the
assumption that people are making when using it.

In other words, my point is this:  *If* cell-index is just a way to
encode the manufacturing assigned ip-block number (EMAC0, EMAC1, etc)
then there is probably little or no value in it.  The two arguments I
see for using cell-index in that mode are:

1) to offset into shared registers (but this doesn't hold because ip
block numbers often don't match register offsets and the reg property
would be just as suitable)

2) to logically identify ip blocks to the user (but cell-index was
never intended for this and /aliases is a better solution anyway)

Right.  The confusion arises because cell-index was invented on 4xx,
where it's common practice to index global registers by the ip block
number.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson