Re: [PATCH] PCI: drop pci_segments_init()

[Stewart Hildebrand <stewart.hildebrand@xxxxxxx>]

On 2/26/25 06:38, Jan Beulich wrote:
> Have callers invoke pci_add_segment() directly instead: With radix tree
> initialization moved out of the function, its name isn't quite
> describing anymore what it actually does.
> 
> On x86 move the logic into __start_xen() itself, to reduce the risk of
> re-introducing ordering issues like the one which was addressed by
> 26fe09e34566 ("radix-tree: introduce RADIX_TREE{,_INIT}()").
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> This is entirely optional and up for discussion. There certainly also is
> an argument towards keeping the function. Otoh on Arm there is the still
> open question whether segment 0 really is kind of special there (as it
> is on x86, largely for historical reasons), or whether the code can be
> dropped there altogether.

Segment 0 is not special on Arm as far as I'm aware. You can have a
perfectly functioning system with only, say, segment 1, for example:

(XEN) ==== PCI devices ====
(XEN) ==== segment 0001 ====
(XEN) 0001:00:01.0 - d0 - node -1
(XEN) 0001:00:00.0 - d0 - node -1

Segment numbers can be arbitrarily chosen by specifying the
linux,pci-domain device tree property.

> ---
> v4: Move x86 logic into __start_xen() itself.
> v3: Adjust description to account for and re-base over dropped earlier
>     patch.
> v2: New.
> 
> --- a/xen/arch/arm/pci/pci.c
> +++ b/xen/arch/arm/pci/pci.c
> @@ -88,7 +88,8 @@ static int __init pci_init(void)
>      if ( !pci_passthrough_enabled )
>          return 0;
>  
> -    pci_segments_init();
> +    if ( pci_add_segment(0) )
> +        panic("Could not initialize PCI segment 0\n");

IMO it's okay to remove the call here since there is already a call to
pci_add_segment() in
xen/arch/arm/pci/pci-host-common.c:pci_host_common_probe()

If there happens to be an Arm SoC with segment number quirks, that
could be worked out in a SoC-specific xen/arch/arm/pci/pci-host-*.c.