linux-os (Dick Johnson) wrote: > On Mon, 27 Nov 2006, Jon Ringle wrote: > > > Robert Hancock wrote: > >> Jon Ringle wrote: > >>> Hi, > >>> > >>> I need to reserve a page of memory at a specific area of RAM that > >>> will be used as a "shared memory" with another processor > over PCI. > >>> How can I ensure that the this area of RAM gets reseved > so that the > >>> Linux's memory management (kmalloc() and friends) don't use it? > >>> > >>> Some things that I've considered are iotable_init() and ioremap(). > >>> However, I've seen these used for memory mapped IO > devices which are > >>> outside of the RAM memory. Can I use them for reseving RAM too? > >>> > >>> I appreciate any advice in this regard. > >> > >> Sounds to me like dma_alloc_coherent is what you want.. > >> > > It looks promising, however, I need to reserve a physical > address area > > that is well known (so that the code running on the other processor > > knows where in PCI memory to write to). It appears that > > dma_alloc_coherent returns the address that it allocated. Instead I > > need something where I can tell it what physical address > and range I want to use. > > > > Jon > > First, "PCI memory" is some memory inside a board that is > addressed through the PCI bus. This address is allocated upon > machine start and is available though the PCI interface > (check any of the PCI card drivers). If you want to access > this memory, you need to follow the same procedures that > other boards use.
In my hardware setup, Linux is running in PCI option mode on a IXP455 processor and it exposes a segment of the IXP455's memory so that it is available on the PCI bus. The other processor (a pentium M running Windows OS) sees this exposed IXP455 memory as PCI memory from it's perspective. > > However, perhaps you don't mean "PCI memory". Perhaps you > mean real memory in the address-space, and you need to > reserve it and give its physical address to something inside > a PCI-bus card, perhaps for DMA. In that case, you can either > memory-map some physical memory (get_dma_pages()) or you can > tell the kernel you have less memory than you really have, > and use the memory the kernel doesn't know about for your own > private purposes. To access this private memory you use > ioremap() and friends. This can be memory-mapped to > user-space as well so you can perform DMA directly to > user-space buffers. You can find the highest address that the > kernel uses by reading kernel variable num_physpages. This > tells the number of pages the kernel uses. The kernel does > write to the next one so you need to start using pages that > are two higher than num_physpages. I'll take a look telling the kernel it has less memory that there physically exists and use ioremap() to map it in. Thanks, Jon - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
