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Re: Unmapping pages from the linear addressing without HIGHMEM support

> --- Ursprüngliche Nachricht ---
> Von: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> An: Gerhard Pircher <gerhard_pircher@gmx.net>
> Kopie: linuxppc-dev@ozlabs.org, debian-powerpc@lists.debian.org
> Betreff: Re: Unmapping pages from the linear addressing without
> HIGHMEM support
> Datum: Fri, 10 Mar 2006 09:11:41 +1100
> 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.
That would mean I cannot reuse the code in dma-mapping.c, right? Killing the
BAT mappings or limiting the memory size covered by the BATs seems to be
fairly easy, but I guess I have to setup my own page table for the reserved
DMA memory area and implement my own alloc_pages() function!?



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