Re: [PATCH] PCI: drop pci_segments_init()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.02.2025 20:57, Stewart Hildebrand wrote:
> 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.

Right, that was the vague understanding I had.

>> --- 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()

Is there? I can't see one, so maybe you're working from a tree with extra
patches applied?

Jan