Hi Chan,
Saad
The O/S builds out multiple memory nodes using the addresses in the 'ranges' property. 'Each node is divided up into a number of blocks called zones which represent ranges within memory...' from (https://www.kernel.org/doc/gorman/html/understand/understand005.html)
Also, please read this:
"A memory device node is required for all device trees and describes the physical memory layout for the system. If a system has multiple ranges of memory, multiple memory nodes can be created, or the ranges can be specified in the reg property of a single memory node."
Thanks & regards,
Saad
On Mon, Nov 21, 2022 at 8:16 AM Chan Kim <ckim@xxxxxxxxxx> wrote:
Hello linux experts,
I read https://elinux.org/Device_Tree_Usage, and it says about the "ranges"
property in the PCIe Root Complex node,
"The above ranges define how the CPU see the PCI memory, and helps the CPU
to set up the right memory windows and write the right parameters into
various PCI device registers. This is sometimes referred to as outbound
memory."
I know the 'ranges' property shows how the PCI devices's addresses are
mapped to host CPU's addresses. The PCI devices are identified by the
bus/device/function pair and the address is indicated the
config/IO/memory32/memory64 types and start and length in the PCI bus.
I understand that the BIOS or bootloader initializes PCI RC and does the
enumeration and assigns the BAR registers so that the device's resources are
mapped in host CPU's address space.
And I guess the bootloader fixes the device tree (or ACPI table) by filling
in the 'ranges' property. Is it correct?
My another question is, what does the OS do with this 'ranges' information
in the device tree? Of course the PCI device's driver uses BAR to get the
physical address (seen from the host CPU) and by ioremap, it can access the
devices memory and registers using virtual address. In the above link it
says "The above ranges define how the CPU see the PCI memory, and helps the
CPU to set up the right memory windows and write the right parameters into
various PCI device registers. This is sometimes referred to as outbound
memory." What does it mean? Does the OS do something with this 'ranges'
value?
I'll appreciated if someone could explain it to me.
Thank you!
Chan Kim
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