Re: [patch 4/6] xip: support non-struct page backed memory

[Nick Piggin <npiggin@suse.de>]

On Mon, Mar 03, 2008 at 12:04:37PM -0800, Linus Torvalds wrote:
> 
> 
> On Mon, 3 Mar 2008, Jared Hulbert wrote:
> > 
> > By 1:1 you mean virtual + offset == physical + offset right?
> 
> Right. It's a special case, and it's an important special case because 
> it's the only one that is fast to do.
> 
> It's not very common, but it's common enough that it's worth doing.
> 
> That said, xip should probably never have used virt_to_phys() in the first 
> place. It should be limited to purely architecture-specific memory 
> management routines.

Actually, xip in your kernel doesn't, it was just a patch I proposed.
Basically I wanted to get a pfn from a kva, however that kva might be
ioremapped which I didn't actually worry about because only testing
a plain RAM backed system.

 
> [ There's a number of drivers that need "physical" addresses for DMA, and 
>   that use virt_to_phys, but they should use the DMA interfaces 
>   that do this right, and even for legacy things that don't use the proper 
>   DMA allocator things virt_to_phys is wrong, because it's about _bus_ 
>   addresses, not CPU physical addresses. Only architecture code can know 
>   when the two actually mean the same thing ]
> 
> Quite frankly, I think it's totally wrong to use kernel-virtual addresses 
> in those interfaces in first place. Either you use "struct page *" or you 
> use a pfn number. Nothing else is simply valid.

Although they were already using kernel-virtual addresses before I got
there, we want to remove the requirement to have a struct page, and
there are no good accessors to kmap a pfn (AFAIK) otherwise we could
indeed just use a pfn.

We'll scrap the virt_to_phys idea and make the interface return both
the kaddr and the pfn, I think.