On Mon, 11 Dec 2000, Jun Sun wrote: > I am surprised. I thought /dev/mem is for accessing SYSTEM RAM. (do a 'man' You access the memory space. Whatever is found at the address you specify, be it RAM, an MMIO area or unoccupied space. You may receive a bus error in the latter case (depending on a system configuration). > on /dev/mem) It is also confirmed by the code in drivers/char/mem.c. If you > want to access anything beyond 'high_memory", nothing is read. Yep, you may only use read()/write() for system RAM. For other areas you have to mmap() the interesting part of /dev/mem and then access it directly (which is easier and better anyway, as you may control the width of bus transfers -- not all MMIO devices support all widths). > Note that drivers/char/mem.c is cross-platform code. I am not sure how X > would access video memory through /dev/mem on either MIPS or other platforms. It mmap()s the areas it's interested in. Read the code! > That reason I want to fix /dev/kmem is that in some cases before a driver is > written people want to play with the hardware directly from the userland > (especially for demo purpose. :-0) Very useful for embedded systems. I'm not sure how to use /dev/kmem for this purpose -- it's kernel's *virtual* memory... > Potentially fixing /dev/mem can do the same job. However /dev/mem cannot > differentiate cached or uncached accesses. With /dev/kmem, we just specify > 0x8.. or 0xa.... Yep, /dev/mem for non system RAM areas is always uncached which is what is almost always desired (for system RAM areas you may request uncacheability by passing O_SYNC when opening the file). Anyway, /dev/mem works great -- I've successfully used it to access APICs (at 0xfec00000 and 0xfee00000) in my i386 system and various stuff (an NVRAM at 0x1c000000, a graphics accelerator at 0x1e000000, a FDDI network controller at 0x1f000000, onboard I/O stuff at 0x1f800000 and a system ROM at 0x1fc00000) in my DECstation. -- + Maciej W. Rozycki, Technical University of Gdansk, Poland + +--------------------------------------------------------------+ + e-mail: macro@ds2.pg.gda.pl, PGP key available +
