Re: [coreboot] [PATCH] AMD MMCONF Support

[Arne Georg Gleditsch]

Myles Watson  writes:
> Rev 5810.

Thanks!
> Arne:
> Could you help me understand MMCONF_SUPPORT and
> MMCONF_SUPPORT_DEFAULT?  It looks like the area gets reserved for
> MMCONF_SUPPORT, even if MMCONF_SUPPORT_DEFAULT isn't selected.

My understanding of the code as it was before I started messing with it,
is that MMCONF_SUPPORT is intended to indicate whether the facility is
available at all, and MMCONF_SUPPORT_DEFAULT toggles the actual use of
it (by Coreboot).  Both src/northbridge/amd/amdfam10/northbridge.c and
src/northbridge/intel/i945/northbridge.c contains code like this:

#if CONFIG_MMCONF_SUPPORT_DEFAULT
          .ops_pci_bus      = &pci_ops_mmconf,
  #else
          .ops_pci_bus      = &pci_cf8_conf1,
  #endif

which, alongside romcc_io.h's

#if CONFIG_MMCONF_SUPPORT_DEFAULT
          return pci_mmio_read_config32(dev, where);
  #else
          return pci_io_read_config32(dev, where);
  #endif

is more or less the only use for MMCONF_SUPPORT_DEFAULT as far as I can
see.  I'll leave it to others to determine if this actually makes sense.

(Having SUPPORT without SUPPORT_DEFAULT gives you an MCFG area that you
can communicate to the OS (at least potentially), as well as the option
of explicitly doing individual config accesses via mmio instead of
ports.  I don't think anyone is doing either of these today.)