Re: [parisc-linux] [PATCH] PDC cleanup and encapsulation

[Bjorn Helgaas <bjorn_helgaas@hp.com>]

On Wednesday 04 April 2001 11:20, Matthew Wilcox wrote:
> On Wed, Apr 04, 2001 at 09:57:25AM -0700, Bjorn Helgaas wrote:
> > - What are the guidelines for #ifdef __LP64__ vs CONFIG_PA20?  You
> > replaced __LP64__ with CONFIG_PA20 in some places but not others and
> > I'm not clear on the difference.
>
> Not all PA-2.0 processors are run in 64-bit mode.  For example, a C3000
> can be run in either 32 or 64 bit mode.  At compile-time you can choose
> to optimise for a particular processor (and risk it not working on
> other machines).

Ok, so let me see if I have this right:

CONFIG_PA20 =>
	PA2.0 architecture, in either narrow or wide mode.
	Defined iff CONFIG_PA8X00 selected, not directly user-configurable.

CONFIG_PARISC64 =>
	Only possible when CONFIG_PA20 selected
	Build 64-bit kernel by using $(CROSS_COMPILE) = hppa64-linux.
	cpp predefines __LP64__.

So is there a reason for testing __LP64__ rather than CONFIG_PARISC64 in 
the code?  There are a zillion tests for the former and none for the 
latter, even though the latter seems more direct and obvious.

It seems to me that __LP64__ is overloaded for several things.  
Fundamentally, it is a property of the kernel, i.e., it is compiled to run 
in wide mode.  But we also use it to decide whether to use PDC PAT, which 
seems like a slightly different question.  If you run a narrow kernel on a 
C3000, the firmware still supports PDC PAT (although I suppose we would 
have to do some work to call it in wide mode).