From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Stephen C. Tweedie" MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Message-ID: <14283.46406.589808.626933@dukat.scot.redhat.com> Date: Tue, 31 Aug 1999 11:58:14 +0100 (BST) Subject: Re: question on remap_page_range() In-Reply-To: References: <14281.20264.576540.243956@dukat.scot.redhat.com> Sender: owner-linux-mm@kvack.org Return-Path: To: Gilles Pokam Cc: "Stephen C. Tweedie" , linux-mm@kvack.org List-ID: Hi, On Tue, 31 Aug 1999 10:23:37 +0200 (MET DST), Gilles Pokam said: >> No. Either Rubini is wrong or you have misinterpreted. A physical >> address is just that --- the physical address of the memory as it >> appears on the cpu bus when the cpu goes to read from ram. It is >> completely untranslated. The first physical address in the system is >> usually zero, not PAGE_OFFSET. > Sorry, i forget to said "from the kernel point of vue" : > Rubini's book, page 274 about PAGE_OFFSET: > " (...) PAGE_OFFSET must be considered whenever "physical" addresses are > used. What the kernel considers to be a physical address is actually a > virtual address, offset by PAGE_OFFSET from the real physical > address.(..)" That is wrong. A physical address is a physical address is a physical address. Even from the kernel's point of view. We need to know physical addresses when we are setting up page tables, for example, and these do NOT have a PAGE_OFFSET applied. Ever. However, the kernel cannot directly access the contents of a physical address in memory: all memory accesses, without exception, go through virtual address translation, and _that_ is where PAGE_OFFSET comes in. Physical addresses do not, ever, include the PAGE_OFFSET bias. It would be true to say that "What the kernel uses to *access* a physicall address is actually a virtual address, offset by PAGE_OFFSET from the real physical addres." However, the kernel still calls that latter, offset address a virtual address, not a physical address. It's a kernel virtual address as opposed to a user virtual address, but it is still virtual, not physical. > About remap_page_range Rubini said: (page 280-281) > " remap_page_range(unsigned long virt_addr,unsigned long phys_add, > unsigned log size,pgprot_t prot); > unsigned long phys_add: > The phyical address to which the virtual address should be mapped. The > address is physical in the sense outline above" (in PAGE_OFFSET) That is nonsense, the physaddr in remap_page_range does not include PAGE_OFFSET bias. It makes no sense at all for it to do so. > To map to user space a region of memory beginning at physical address > simple_region_start with size = simple_region_size he used the following > example: > unsigned long physical = simple_region_start + off + PAGE_OFFSET That will work on 2.0, but only because PAGE_OFFSET is zero. It won't work on 2.2. --Stephen -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://humbolt.geo.uu.nl/Linux-MM/