[U-Boot] [PATCH v2] mpc83xx: update LCRR register handling

[Kim Phillips]

On Thu, 27 Aug 2009 13:11:25 +0200
Detlev Zundel <dzu at denx.de> wrote:

> Hi Kim,
> 
> > o LCRR_PDYP, granted dangerous in your case, is obviously a writeable
> > bit (not read-only), and documented as such in later documentation.  In
> > fact, there are no non-writeable bits in LCRR.
> 
> Well, "reserved" != "non-writable" (usually there is a comment that
> writing reserved bits produces undefined behaviour) so I agree with
> Heiko that as long the documentation that we have access to, designates
> bits as reserved, it makes sense to have such a mask.

I think we should allow board-configurable writes to the DBYP bit, which
is documented as "reserved" on some 83xx, on the 83xx parts that /do/
implement it.  So instead of having a mask, perhaps setting absolute
values for CONFIG_SYS_LCRR should be replaced with a better scheme that
allows board configs to just set LCRR bits by field, such as what the
SCCR setting code does.  I.e, deprecate CONFIG_SYS_LCRR and replace with
individually-specified CONFIG_SYS_LCRR_{CLKDIV,EADC,ECL,BUDCMDC,DBYP}
values.

This will allow the reserved bits, whether 1 on reset or
not, to be preserved across all 83xx (and 85xx for that matter).

> > o the user loses visibility into what is going on if they
> > decide to drop/add sensitive bits such as LCRR_DBYP in their board's
> > CONFIG_SYS_LCRR settings, and there's a mask lurking in the background.
> >
> > o let's be practical here - in a board port, LCRR settings have to be
> > paid attention to, no matter what hidden behaviours or new bits there
> > are lying underneath - perhaps the form of 'protection' you seek is in
> > the form of a comment in the code?
> 
> So what is it that you propose?  That Heiko uses a LCRR in his board
> config (over-)writing reserved bits?

that's what other boards do (like the 8323erdb), but I do see the
problem for the new board porter - they don't see the bit in their
documentation, so they omit it.

but I presume the above fix will allow Heiko and other new board
porters happy?

I'll send a patch out this weekend unless someone beats me to it.

Kim