Clear, concise and comprehensive as always ! :) Thanks a ton again Bjorn ! On Tue, Nov 14, 2023 at 2:45 AM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote: > > On Sun, Nov 12, 2023 at 09:36:23AM +0530, Ajay Garg wrote: > > Hi everyone. > > > > I had written a (hello-world) PCI-driver couple of years back, and was > > able to read/write from/to BARs. > > I might have a requirement of writing more drivers, so was revisiting > > the concepts. > > > > I have some queries (we can assume no IOMMU is in the picture) : > > > > 0. > > Assertion : During PCI-enumeration, the physical-memory assigned to a > > BAR is always contiguous. (Yes/No)? > > Yes, a BAR defines only a base address and a size, so the region is > always contiguous in the PCI bus address space. If there's no IOMMU, > it is also contiguous in the CPU physical address space. > > If there *is* an IOMMU, it is still be contiguous in the PCI bus > address space but could be discontiguous in CPU physical address > space. > > > 1. > > During ioremap() of BAR-physical memory, does the (mapped) > > kernel-virtual-memory correspond to > > > > * one used in vmalloc(), wherein the > > kernel-virtual-address => physical-address > > goes through page-tables? > > * one used in kmalloc(), wherein the > > kernel-virtual-address => physical-address > > *in all likelihood* requires only adding an offset? > > ioremap(), kmalloc(), and vmalloc() all return kernel virtual > addresses, and the regions are all virtually contiguous. Space from > kmalloc() will be physically contiguous. Space from vmalloc() may not > be physically contiguous, and I don't think you can assume ioremap() > space is physically contiguous either, although it will be contiguous > in the *bus* address space. > > > 2. > > On a related front, how does ioremap() differ from kmap()? > > kmap() is for highmem (see Documentation/mm/highmem.rst) and has > nothing to do with I/O mappings like ioremap(). Very few drivers use > it, and I doubt you should use it either. > > Bjorn
