On Thu, Apr 20, 2006 at 08:57:46PM +0200, Gerhard Pircher wrote: > Hi, > > I try to implement not coherent cache/DMA support for G3/G4 processors, by > reserving some physical memory for DMA operations. The memory used for > consistent allocations (removed from the top of the physical memory below > 896MB) is excluded from the BAT mapping and the pages are marked as > reserved. This seems to work just fine, although I still have to mark the > pages as cache inhibited. > > Whilst working on this workaround for the AmigaOne and reading some articles > about the Linux kernel page tables and memory management, I came to the > conclusion that there may be some problems with this approach for not > coherent DMA: > > 1. The AmigaOne is similar to the PREP platform, i.e. DMA can only be > performed in the first 16MB for ISA devices (there's only a VIA southbridge, > no other SuperI/O IC with 32bit capable DMA controller). I guess the first > 16MB cannot be reserved for not coherent DMA operation, because this memory > area is occupied by kernel data? (not to talk about the performance loss, if > the kernel data area would be excluded from the BAT mapping). > > 2. I'm not sure how to allocate memory for DMA operation. I think > alloc_pages() will not do the job for me, as the page tables for not > coherent DMA are reserved (SetPageReserved()) and removed from the available > lowmem. Also memory fragmentation may be a problem, if a lot DMA operations > with different buffer sizes are performed. Therefore a system could quickly > run out of memory for not coherent DMA operation, right? > Is there a way to minimize fragmentation? > > 3. How are DMA buffers used outside the kernel? Do user programs get a > pointer to the DMA buffer (in theory) from the device driver or is the data > copied to another buffer allocated by an user program?
There are already non-coherent cache PPC archs (8xx, 4xx) just look how all this implemented there, don't reinvent the wheel. Also, read Documentation/DMA-API.txt and DMA-mapping.txt -- Eugene -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]
