"Maciej W. Rozycki" wrote: > > On Fri, 9 Feb 2001, Jun Sun wrote: > > > Do you like run-time detection better because it allows a kernel to run on > > CPUs both with a FPU and without a FPU? Or there is something else to it? > > Nope. There are certain explicit actions that are to be performed by the > kernel if a real FPU is present, such as saving and restoring its > registers or setting the control register. Therefore the kernel has to > know if a real part is present and act accordingly. Maintaining a table > of all CPU ids ever manufactured and manually setting the FPU presence bit > is unreliable, especially as there are chips which cannot be classified > this way, e.g. knowing your CPU is an R3000A you don't know if an R3010A > FPU is soldered as well or not. > Apparently you did not read my first email on this thread. :-) I agree "Maintaining a table of all CPU ids ever manufactured and manually setting the FPU presence bit is unreliable ...". I was debating about how we let kernel know if there is a real FPU: a) an explicit config option, CONFIG_HAS_FPU, (which is not associated with PrID), plus "#ifdef CONFIG_HAS_FPU ..." code. Or b) have run-time detection and many "if .. then .." code. I listed some pro's and con's for both of them in my first email. Right now, I found myself not having a strong preference but still biased towards config option approach ( - as if that really matters. :-0) > > Another question. I know with mips32 and mips64 we can do run-time detection > > reliably. What about other existing processors? > > I've sent a quote from an IDT manual recently. It recommended to use the > FPU implementation ID to check if an FP hw is present. I believe it > should work for any sane implementation of a MIPS CPU. See the mail for > details. > I actually don't understand your IDT quote. It requires one to call mfc1 to get FCR0. On many CPUs without a FPU, this will generate an exception. Are you suggesting that we should catch the exception and from that we conclude there is no FPU present? Jun
