Unmapping pages from the linear addressing without HIGHMEM support

[Benjamin Herrenschmidt]

On Thu, 2006-03-09 at 14:51 +0100, Gerhard Pircher wrote:
> Hi,
> 
> I'm trying to implement non-coherent DMA for PPC desktop systems (like the
> AmigaOne with G3/G4 CPU). For this I want to use the code in
> arch/ppc/kernel/dma-mapping.c. The DMA memory allocation function
> implemented in this file allocates pages with alloc_pages() and maps them to
> its own linear address space, but without unmapping the allocated pages from
> the kernel linear addressing. Due to this the pages are mapped twice, which
> results in a conflict between the different WIMG settings of the pages.
> 
> Is there an API that can be used to unmap the allocated pages from the
> kernel linear addressing? I thought about using kunmap() and
> flush_all_zero_pkmaps(), but I'm not sure if this is the right approach and
> HIGHMEM doesn't work on the AmigaOne too (the highmem base is occupied by
> the PCI/ISA I/O space!). Wouldn't it be possible to just clear the valid (V)
> bit of the PTE and do a TLB cache flush?

The main problem is that the mappings may be covered by a BAT and thus
pages may not be unmappable individually...

What I would suggest is that your dig in the low level RAM mapping code
and limit it at boot to RAM minus a pool of the size you want.

Ben.