On Tue, 2008-11-04 at 18:30 +1100, Nigel Cunningham wrote: > One other question, if I may. Would you please explain (or point me to > an explanation) of PHYS_PFN_OFFSET/ARCH_PFN_OFFSET? I've been dealing > occasionally with people wanting to have hibernation on arm, and I don't > really get the concept or the implementation (particularly when it comes > to trying to do the sort of iterating over zones and pfns that was being > discussed in previous messages in this thread. First of all, I think PHYS_PFN_OFFSET is truly an arch-dependent construct. It only appears in arm an avr32. I'll tell you only how ARCH_PFN_OFFSET looks to me. My guess is that those two arches need to reconcile themselves and start using ARCH_PFN_OFFSET instead. In the old days, we only had memory that started at physical address 0x0 and went up to some larger address. We allocated a mem_map[] of 'struct pages' in one big chunk, one for each address. mem_map[0] was for physical address 0x0 and mem_map[1] was for 0x1000, mem_map[2] was for 0x2000 and so on... If a machine didn't have a physical address 0x0, we allocated mem_map[] for it anyway and just wasted that entry. What ARCH_PFN_OFFSET does is let us bias the mem_map[] structure so that mem_map[0] does not represent 0x0. If ARCH_PFN_OFFSET is 1, then mem_map[0] actually represents the physical address 0x1000. If it is 2, then mem_map[0] represents physical addr 0x2000. ARCH_PFN_OFFSET means that the first physical address on the machine is at ARCH_PFN_OFFSET*PAGE_SIZE. We bias all lookups into the mem_map[] so that we don't waste space in it. There will never be a zone_start_pfn lower than ARCH_PFN_OFFSET, for instance. What does that mean for walking zones? Nothing. It only has meaning for how we allocate and do lookups into the mem_map[]. But, since everyone uses pfn_to_page() and friends, you don't ever see this. I'm curious why you think you need to be concerned with it. -- Dave _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
