On Sun, Sep 22, 2013 at 11:30:59PM +0100, Maciej W. Rozycki wrote: > There is no reliable way to tell R4000/R4400 SC and MC variations apart, > however simple heuristic should give good results. Only the MC version > supports coherent caching so we can rely on such a mode having been set > for KSEG0 by the power-on firmware to reliably indicate an MC processor. > SC processors reportedly hang on coherent cached memory accesses and Linux > is linked to a cached load address so the firmware has to use the correct > caching mode to download the kernel image in a cached mode successfully. > > OTOH if the firmware chooses to use either the non-coherent cached or the > uncached mode for KSEG0 on an MC processor, then the SC variant will be > reported, just as we currently do, so no regression here. > > Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxxxxxxxx> > --- > Ralf, > > I believe we discussed this once long ago and you had some concerns about > such an approach although I don't recall exactly what they were. I > maintain that this heuristic is reasonable, has no drawbacks and has a > potential to make some optimisations or errata workarounds easier. Also > we can collect data about systems affected to see what their firmware does > -- R4000SC/R4400SC DECstations definitely get CP0.Config.K0 right. I'm fairly sure it gets k0 right - otherwise it'd likely not work at all! My reservations may have been about userland reading /proc/cpuinfo and looking at the CPU type. Some software may know how to handle the PC/SC variants but not the MC versions. But this seems to be a fairly weak concern - and I trust you checked gcc's parsing of /proc/cpuinfo. Ralf
