On Tuesday 09 August 2011, David Howells wrote: > Arnd Bergmann <arnd@xxxxxxxx> wrote: > > > > +#define IMCR_BASE 0x01840000 > > > > Please don't hardcode MMIO regions like this. You should have the base > > address in the device tree and use of_iomap() like you do in some other > > cases. If you need this really early, you might need to > > What about for memory-mapped CPU registers that you might need to access in > head.S? > > MN10300, for example, has a bunch of these - such the atomic operation > parameter and control registers. Getting some of these through the device > tree adds potentially quite a lot of overhead. > It really depends on the specific problem. The registers in this file (cache.c) do not seem to fall into that category, since they are ioremapped much later. For any addresses that you don't get from a device tree but assume to be constant, I would recommend: * documenting them well, with the specific reasons why you know they are safe to hardcode and why it would be a pain not to. * putting them in one architecture-wide header file that contains all of these constants, rather than putting them into a local C file. On ARM, we have a case where we want to access MMIO registers extremely early, for printing debugging output on the serial port. Since the specific addresses are different from one board to another, there is no generic solution, but it's possible to have a compile-time option to enable that code conditionally. In some other cases, where you have to do some very early setup of registers, a new architecture port can require the boot loader to set the registers up to a sensible value before entering the kernel. Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
