Re: [PATCH] 64-bit struct resource fields

[Eric W. Biederman]

Matt Porter <[e-mail]> writes:

> On Mon, Nov 25, 2002 at 03:11:00PM -0800, Dave Hansen wrote:
> > We need some way to replicate the e820 tables for kexec.  This 
> > modifies struct resource to use u64's for its start and end fields. 
> > This way we can export the whole e820 table on PAE machines.
> > 
> > resource->flags seems to be used often to mask out things in 
> > resource->start/end, so I think it needs to be u64 too.  But, Is it 
> > all right to let things like pcibios_update_resource() truncate the 
> > resource addresses like they do?
> > 
> > With my config, it has no more warnings than it did before.
> 
> I could make use of this on my PPC440 systems which have all I/O
> (onboard and PCIX host bridge) above 4GB.  However, the patch
> I have been playing with typedefs a phys_addr_t so that only
> systems which are 32-bit/36-bit+ split like PAE ia32, AUxxxx (MIPS),
> and PPC440 have to do long long manipulation.  If you explicitly
> use u64 everywhere it forces all native 32-bit/32-bit systems to
> do unnecessary long long manipulation.

Except for the fact that if you have a 32bit pci bus, you can
plug in cards with 64bit bars.  And they can still legitimately do
64bit DAC to other pci cards.  It is a silly configuration, but
possible.
 
> In the past there has been quite a bit of resistance to even
> introducing a physical address typedef due to some claims of
> gcc not handling long longs very well [1].  I don't see how
> having _everybody_ that is 32-bit native handle long longs is
> going to be more acceptable but I could be surprised.

The primary concern has been efficiency and I do believe there is
anywhere the pci resource allocator is on the fast path, so that
should not be a problem.

There are some rare bugs with 2.95.2 and kin with handling long longs
but all it has been possible to reformulate the C code so it works
in all cases where the bugs have been observed.

And beyond that it was Linus idea to bring the resource allocator to
64bits which tends to help.

> That said, I think when we have existence of systems that require
> long long types and gcc is "buggy" in this respect, then using
> a phys_addr_t is the lesser of two evils (even though everybody hates
> typedefs).  We already have this type defined local to PPC because
> it is necessary to cleanly handle ioremap and local page mapping
> functionality.  going to u64 or phys_addr_t resources would be a
> huge improvement on a horribly kludgy hack we use to crate the
> most significant 32-bits for our 64-bit ioremaps.

A phys_addr_t may be a sane idea, or in this case it would need to be
a res_addr_t.
 
> BTW, since u64 is long long on 32-bit platforms and long on 64-bit
> platforms, you will get warnings from every printk that dumps
> resource infos.  My thought is to provide some macros to massage
> resource values to strings for display.
> 

> [1] I get feedback from many people using the PPC440 port and have 
>     yet to find any instances of gcc mishandling long longs. (though
>     this is just anecdotal evidence).

I have written code that trips it up, but I believe the bugs have been
fixed in recent compilers, and the bugs (not the inefficiencies) may
be specific to a specific port.

Eric