RFC: Deprecating io_block_mapping

[Dan Malek]

On May 25, 2005, at 1:20 AM, Benjamin Herrenschmidt wrote:

> Ok, that was my point... since init page tables can be loaded by it, 
> why
> not make ioremap work that early and do the same ?

Because you lose the efficiency of mapping with BATs or CAMs.

> ... The problem is of
> course allocating the pte pages but how does io_block_mapping() do on
> CPUS without BAT/CAMs/whatever ?

It just loads init's page tables directly.

> We have the pgdir, but not the PTE pages...

The PTE pages are allocated as they are needed.  The PTE pages
are there :-)

> No, we have a trick with ioremap_bot, we don't need to get vmalloc 
> space
> for ioremap to work early.

That's only if you have already done something to already allocate BATs
or CAMs.  On processors that don't  have these, ioremap() would fail 
because
it would think it has to allocate VM space.

>  ....  In fact, it would be nice to just have
> io_block_mapping be able to "dynamically" allocate virtual space using
> the same mecanism instead of beeing passed a virtual address. That 
> would
> fix most of the problems with hard coded 1:1 mappings.

I think we should make ioremap() smarter and have some board 
initialization
that helps it by setting up BATs, CAMs, or unique page table mappings.
There is also an interdependence between ioremap() and other IO
initialization.  In the past some of the fixed addressing was necessary 
due
to assumptions built into IO setup, mapping functions, or macros.  I 
don't
know how much of this is still present.

> Well, my problem is with hard-coded v:p mappings... If we can simply
> have io_block_mapping take, for example, 0 for v (or -1) and use the
> ioremap_bot trick to "allocate" virtual space, that would make me happy
> (it needs to return the allocated address too).

Somewhere, at some point, prior to VM setup, we need to forcibly map
virtual to physical addresses.  These are going to be "hard coded"
mappings, that's exactly how ioremap_bot is set.  This is why
io_block_mapping was created in the first place.  Somehow you have
to specify this mapping before you have a VM allocator to give it to 
you. :-)
Even if you don't call it io_block_mapping(), you are going to need
a function that does exactly this.

Thanks.

	-- Dan