Re: Query Regarding PCI Configuration Space Mapping and BAR Programming

[Hans Zhang <18255117159@163.com>]

On 2025/3/20 03:15, Bjorn Helgaas wrote:
> EXTERNAL EMAIL
> 
> On Wed, Mar 19, 2025 at 11:27:07PM +0530, Muni Sekhar wrote:
>> Dear Linux-PCI Maintainers,
>>
>> I have a few questions regarding PCI configuration space and Base
>> Address Register (BAR) programming:
>>
>> PCI Configuration Space Mapping:
>> PCI devices have standard registers (Device ID, Vendor ID, Status,
>> Command, etc.) in their configuration space.
>> These registers are mapped to memory locations.
> 
> In general, config registers are not mapped to memory locations.
> 

Yes, this is just a segment of the address visible to the CPU, not the 
actual memory.

Best regards,
Hans

> Some systems use the ECAM mechanism for config access, which is
> basically memory-mapped I/O that converts CPU memory accesses to PCI
> config accesses.  This mechanism is hidden inside the Linux config
> accessors (pci_read_config_word(), etc), and drivers should not use
> ECAM directly.
> 
>> How can we determine
>> the exact memory location where these configuration space registers
>> are mapped?
> 
> Drivers use pci_read_config_word() and similar interfaces to access
> config registers.  Userspace can use the setpci utility.
> 
>> Base Address Registers (BARs) Programming:
>> How are BAR registers programmed, and what ensures they do not
>> conflict with other devices mapped memory in the system?
>> If a BAR mapping clashes with another device’s memory range, how does
>> the system handle this?
>>
>> Does the BIOS/firmware allocate BAR addresses, or does the OS have a
>> role in reconfiguring them during device initialization?
> 
> On x86 systems, the BIOS generally assigns BAR addresses.  Linux
> doesn't change these unless they are invalid.  Some firmware does not
> assign BARs, and generally firmware does not assign BARs for hot-added
> devices.  In those cases, Linux assigns them.
> 
> If Linux notices a conflict, it tries to reassign BARs to avoid the
> conflict.
> 
>> If you could provide any source code references from the Linux kernel
>> that handle these aspects, it would be greatly appreciated.
> 
> Here's the function that reads BARs when Linux enumerates a device:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/probe.c?id=v6.13#n176
>