On Thu, Nov 05, 2020 at 02:27:58PM +0000, Jack Winch wrote: > Hi all, > > Over the last couple of months, I've been reading the hardware > documentation and Linux device driver source code for a range of > different PCI and PCIe devices. Those examined range from > multi-function data acquisition cards through to avionics bus > interface devices. In doing so, I have referenced numerous resources > (including the Third Edition of LDD - what a great book - and the > documentation available for the Linux PCI Bus Subsystem on > kernel.org). > > One thing I'm still a little unclear on is why vendors might opt to > map PCI / PCIe device memory into the system memory map as either > Memory-Mapped I/O (MMIO) or Port-Mapped I/O (PMIO). That is, for what > reasons would a device manufacturer choose to make use of one address > space over the other for regions of a PCI / PCIe device's memory? > Some of the general reasons are alluded to by the aforementioned > resources (e.g., more instruction cycles are required to access data > via PMIO, MMIO can be marked as prefetchable and handled as such, > etc). I think you should be talking to hardware people about this, as this is almost always due to hardware limitations/issues/design decisions. The PCI-SIG should have a bunch of resources about this, have you looked into that? good luck! greg k-h
