On 10 December 2013 21:42, Michael Roth <mdr...@linux.vnet.ibm.com> wrote: > Quoting Peter Maydell (2013-12-05 17:33:48) >> And presumably whoever put that specific check for 0 into >> QEMU had a reason for it. >> >> On the other hand I can't now find whatever document it was >> that I was reading that claimed 0 wasn't valid :-( > > Can't seem to find anything either, checked the 2.3 spec as well. I tried to > look up the git history for the new_addr == 0 check but unfortunately it > seemed > to be part of the initial check-in. > > The only clue I've found regarding special-casing for a 0-bar is this: > > "Power-up software can determine how much address space the device requires by > writing a value of all 1's to the register and then reading the value back. > The > device will return 0's in all don't-care address bits, effectively specifying > the address space required. Unimplemented Base Address registers are hardwired > to zero." - PCI 3.0, 6.2.5.1
Googling again brought up this mailing list thread: http://www.pcisig.com/reflector/msg00459.html which includes what is supposedly a quote from the PCI 2.1 spec: # "Note: A Base Address register does not contain a valid # address when it is equal to "0"" (I don't have access to the 2.1 version to check.) This text seems to have been removed from the 2.2 spec. thanks -- PMM
