On Thu, Jun 24, 2004 at 03:46:43PM +0300, Yaron Presente wrote: > I'm running montavista linux (2.4.18_mvl30-malta-mips_fp_le) on a board > that has 2 memory banks of physical memory. > 1. 32MB from physical address 0x00000000 > 2. 16MB from physical address 0x20000000 > > Currently I can only access the first bank (by add_memory_region(0, 32 > << 20, BOOT_MEM_RAM) in prom_init() ). > I tried the obvious solution of adding another region at 0x20000000 > (add_memory_region(0x20000000, 16 << 20, BOOT_MEM_RAM)) > but that didn't seem to work. I've also tried to add a BOOT_MEM_RESERVED > region in between the regions in order to create a seemingly contiguous > memory, with no success. > My questions are: > Is it possible to access the second bank as well under MIPS ? > Is there a way to define a "hole" in the physical memory? > Do I have to use CONFIG_DISCONTIGMEM ? is it fully supported ? > Thanks for your help, Your initial approach was nearly right - you can solve the problem of holes in the memory map as long as they're small enough by only adding the available regions with add_memory_region(). Typically uses for this are small holes due to memory in use by firmware, for example. Now, in your case the whole isn't small. In fact, with 480MB it's big ;-) What Linux will try to do is to allocate the mem_map array for the entire memory range from 0x0 - 0x21000000, that's 528MB. mem_map contains one page per 4k page; each entry is 64 bytes in size for 32-bit kernels so that makes a total size for mem_map[] of 8.25MB of which just 768kB are actually being used. Just to make life a little bit more misserable memory 32-bit kernels can only use memory above the 512MB boundary as highmem. CONFIG_DISCONTIGMEM can solve this problem - but Linux/MIPS really doesn't much an attempt to make that easy to use. Right now only a single MIPS system is using CONFIG_DISCONTIGMEM and that system is using it in combination with CONFIG_NUMA which is quite an additional complication. With CONFIG_DISCONTIGMEM there will be no more mem_map[] array. Instead there will be one such array for each memory region which means you'll loose a bit of performance due to additional complexity but you'll save all the wasted memory. Ralf
