From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: From: Hua Ji To: jerry , Dan Malek Cc: linuxppc-embedded@lists.linuxppc.org Subject: RE: how to get the physival address Date: Thu, 5 Apr 2001 09:58:46 -0700 MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Sender: owner-linuxppc-embedded@lists.linuxppc.org List-Id: >Yes, if I use kmalloc is OK, either __pa or virt_to_phys will return the >physical address. >But it does not work for defined variable (it was defined in driver so it >should be in kernel space, isn't it?). I wonder which memory space, this >variable belong to? My 2 cents: Given that ONLY 2 alternatives you can use for memory allocation when working with kernel, which are kmalloc and vmalloc, you are facing different memory usages: Before talking further, you may have realized: >>From 0xC0000000 to 0XC0000000 + physical_memory_size is 1:1 mapped to 0x000000~physical memory size-1. Surely, kernel also manages virtual memory from 0XC0000000 + physical_memory_size + 8M hole to 4G Therefore, if you used kmalloc to allocate memory, surely, you will easily find: virtual address-3G is the physical address. However, for vmalloc, the case is otherwise. The reason now is clear: That virtual address is very probably from 0XC0000000 + physical_memory_size + 8M hole to 4G. This is REALLY dynamic. Kernel's vmalloc, just like libc malloc, will go through the free_area to find a first-bit(libc malloc is not first fit) block for you. Anyway, back to your question. Your variable is in a memory space after 3G. But it is byond the 3G+physical_memory_size + 8M. For example, your machine have a 256M memory, so your variable virtual address is possible: 3G+256M+8M+SOME_VALUE. Wish helpful, Mike ** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/