On Wed, 2007-03-21 at 09:37 +0530, Daniel Rodrick wrote: > Hi list, > > I'm a newbie, and I just read about the mmap system call. I have a > very basic confusion regarding why would a driver ever want to > implement mmap. > > 1) The purpose of mmap is to map the device memory to user space > programs, right? Isn't that alreay available via /dev/mem? no /dev/mem doesn't allow convenient access to scatter-gather memory etc while your own mmap can make the memory look linear > 2) Is it that mmapping /dev/mem works only for memory (RAM)? Are all > physical addresses not available via /dev/mem? /dev/mem is a HORRIBLE interface, and it won't work in the future or even today on some machines. The reason for this is that this interface doesn't tell the kernel what device you want to talk to, so there is no way the kernel can set up the IOMMU correctly to allow access to the memory. Today only high end machines have an IOMMU, but I suspect that within a few years all PCs will have one as well.... -- if you want to mail me at work (you don't), use arjan (at) linux.intel.com Test the interaction between Linux and your BIOS via http://www.linuxfirmwarekit.org - To unsubscribe from this list: send the line "unsubscribe linux-newbie" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.linux-learn.org/faqs
