Re: [PATCH v6 2/3] xen/arm: Enable the existing x86 virtual PCI support for ARM.

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.10.2021 14:13, Bertrand Marquis wrote:
> Hi Roger,
> 
>> On 15 Oct 2021, at 12:35, Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
>>
>> On Fri, Oct 15, 2021 at 12:18:59PM +0200, Jan Beulich wrote:
>>> On 15.10.2021 12:14, Ian Jackson wrote:
>>>> Bertrand Marquis writes ("Re: [PATCH v6 2/3] xen/arm: Enable the existing 
>>>> x86 virtual PCI support for ARM."):
>>>>>> On 15 Oct 2021, at 09:00, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>>>> The latter is fine to be put here (i.e. FTAOD I'm fine with it
>>>>>> staying here). For the former I even question its original placement
>>>>>> in asm-x86/pci.h: It's not generally correct as per the PCI spec, as
>>>>>> the bus portion of the address can be anywhere from 1 to 8 bits. And
>>>>>> in fact there is a reason why this macro was/is used in only a
>>>>>> single place, but not e.g. in x86'es handling of physical MCFG. It
>>>>>> is merely an implementation choice in vPCI that the entire segment 0
>>>>>> has a linear address range covering all 256 buses. Hence I think
>>>>>> this wants to move to xen/vpci.h and then perhaps also be named
>>>>>> VPCI_ECAM_BDF().
>>>>>
>>>>> On previous version it was request to renamed this to ECAM and agreed
>>>>> to put is here. Now you want me to rename it to VPCI and move it again.
>>>>> I would like to have a confirmation that this is ok and the final move if 
>>>>> possible.
>>>>>
>>>>> @Roger can you confirm this is what is wanted ?
>>>>
>>>> I think Roger is not available today I'm afraid.
>>>>
>>>> Bertrand, can you give me a link to the comment from Roger ?
>>>> Assuming that it says what I think it will say:
>>>>
>>>> I think the best thing to do will be to leave the name as it was in
>>>> the most recent version of your series.  I don't think it makes sense
>>>> to block this patch over a naming disagreement.  And it would be best
>>>> to minimise unnecessary churn.
>>>>
>>>> I would be happy to release-ack a name change (perhaps proposed bo Jan
>>>> or Roger) supposing that that is the ultimate maintainer consensus.
>>>>
>>>> Jan, would that approach be OK with you ?
>>>
>>> Well, yes, if a subsequent name change is okay, then I could live with
>>> that. I'd still find it odd to rename a function immediately after it
>>> already got renamed. As expressed elsewhere, I suspect in his request
>>> Roger did not pay attention to a use of the function in non-ECAM code.
>>
>> Using MMCFG_BDF was original requested by Julien, not myself I think:
>>
>> https://lore.kernel.org/xen-devel/a868e1e7-8400-45df-6eaa-69f1e2c99383@xxxxxxx/
>>
>> I'm slightly loss in so many messages. On x86 we subtract the MCFG
>> start address from the passed one before getting the BDF, and then we
>> add the startting bus address passed in the ACPI table. This is so far
>> not need on Arm AFAICT because of the fixed nature of the selected
>> virtual ECAM region.
> 
> At the end my patch will add in xen/pci.h:
> #define ECAM_BDF(addr)         (((addr) & 0x0ffff000) >> 12)

Since you still make this proposal, once again: I'm not going to
accept such a macro in this header, whatever the name. Its prior
placement was wrong as well. Only ...

> #define ECAM_REG_OFFSET(addr)  ((addr) & 0x00000fff)

... this one is fine to live here (and presumably it could gain uses
elsewhere).

Jan

> Now seeing the comment the question is should those be renamed with a VPCI
> prefix and be moved to xen/vpci.h.
> 
> So far ECAM_BDF is only used in vpci_mmcfg_decode_addr which is only called
> before calling vpci_ecam_{read/write}.
> 
> ECAM_REG_OFFSET is only used in arm vpci code.
> 
> Do you think the current state is ok of the renaming + moving should be done ?
> 
> Cheers
> Bertrand
>