> -----Original Message----- > From: StephanT [mailto:stman937-linewbie@xxxxxxxxx] > Sent: Tuesday, October 18, 2011 5:29 PM > To: Jeff Haran; kernelnewbies > Subject: Re: How can I test if a physical address is already mapped or not. > > > >> > >> > unsigned long * const pReg = (unsigned long*) 0XC0000000 ; > >> > >> > > >> > printk( KERN_INFO"** read (%p) %08lX.\n", pReg, *pReg) ; > >> > > > > I am assuming the processor you are running this on has an MMU (Memory > > Management Unit). This statement: > > > > unsigned long * const pReg = (unsigned long*) 0XC0000000 ; > > > > will cause the variable pReg to contain the constant 0xC0000000. > > > > When *pReg is evaluated in your printk(), the MMU will load the unsigned > long > > located at virtual address 0xC0000000 into some processor register. How > the > > mapping of the virtual address to physical address occurs is processor > specific, > > but in Linux ultimately the physical address to get the data from will be > > derived from a page table that provides a mapping between the physical > and > > virtual addresses of 4KB (usually 4KB) pages . > > > > > Jeff, > > Thanks for your kind explanation. I understand now your comment. > > Yes you are right when you are in user space. However in a kernel > module this would be physical address. Nope. Unless you are using some really strange processor that I am not familiar with, a memory read is a memory read. *pReg is going to read what's at virtual address 0xc0000000 regardless of whether you are executing in user or kernel context. I know it's going to work this way on a PC with an Intel x86 processor. > I test on an embedded > Linux board but I translated the problem to a PC which everyone > can test. > > The 0xC0000000 falls under PCI address range. I guess on a PC Linux > just doesn't map this address range or it maps it one2one (phys==virt) > I can dereference this address and I get something plausible. On most 32 bit Intel processors running "standard" Linux 0xC0000000 is the first address of kernel VM. This is a function of kernel configuration, but it's this way by default. It's still a virtual address. So yes, on your PC virtual address 0xC0000000 is mapped to physical RAM. > The > physical 0x00100000 is the Linux load address - see /proc/iomap. It was > previously discussed on this group: > (http://lists.kernelnewbies.org/pipermail/kernelnewbies/2011- > October/003483.html) > When I try to dereference it (physical level) I get the error listed in my first > post and Linux kills my module (or if it is installed as a driver kills the > application > which uses the driver). I guess it is normal because this address range is > already > mapped by Linux - in virtual address Linux starts at 0xC0000000. By using > ioremap > I can ask for a second map of this physical address and I can dereference it > without > getting killed. > > To get back to my initial question I'd like to be able to check this status before > dereferencing the physical address and avoid to be killed if the address is > already mapped :) Are there any kernel calls allowing me to do so ? Not to my knowledge. Doesn't mean they don't exist, I just don't know of any such thing. Jeff Haran _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
