Re: [PATCH 06/19] xen: Clean up asm-generic/device.h

[Alejandro Vallejo <agarciav@xxxxxxx>]

On Thu Jun 5, 2025 at 4:20 PM CEST, Jan Beulich wrote:
> On 05.06.2025 16:15, Alejandro Vallejo wrote:
>> On Mon Jun 2, 2025 at 4:24 PM CEST, Jan Beulich wrote:
>>> On 02.06.2025 16:19, Alejandro Vallejo wrote:
>>>> On Mon Jun 2, 2025 at 9:51 AM CEST, Jan Beulich wrote:
>>>>> On 30.05.2025 14:02, Alejandro Vallejo wrote:
>>>>>> --- a/xen/include/asm-generic/device.h
>>>>>> +++ b/xen/include/asm-generic/device.h
>>>>>> @@ -6,9 +6,7 @@
>>>>>>  
>>>>>>  enum device_type
>>>>>>  {
>>>>>> -#ifdef CONFIG_HAS_DEVICE_TREE
>>>>>>      DEV_DT,
>>>>>> -#endif
>>>>>
>>>>> Why would this enumerator need exposing on a non-DT arch? In fact I would 
>>>>> have
>>>>> hoped for ...
>>>>
>>>> A non-DT arch would not include this. x86 doesn't.
>>>
>>> Both here and ...
>>>
>>>>>>      DEV_PCI
>>>>>
>>>>> ... this to be hidden for arch-es not supporting PCI.
>>>>>
>>>>> Similar concerns elsewhere in this change.
>>>>
>>>> This file is exclusively used by arches supporting DT to abstract away 
>>>> where
>>>> the device came from. x86 does not use it at all, and while it wouldn't be
>>>> impossible to compile-out DEV_PCI, it would needlessly pollute the 
>>>> codebase with
>>>> no measurable gain, because the abstractions still need to stay.
>>>
>>> ... here: In "xen/include/asm-generic/device.h" there's nothing at all 
>>> saying
>>> that this file is a DT-only one. Instead there is something in there saying
>>> that it's suitable to use in the entirely "generic" case.
>>>
>>> Jan
>> 
>> Try to use it from x86 and observe the build system catch fire. It could be 
>> made
>> to not go on fire, but it implies heavy refactoring in x86 (particularly 
>> IOMMU
>> code) for no good reason because there's no devices in a DTB to disambiguate.
>> 
>> How about adding this to the top of the header?
>> 
>> ```
>>  /*
>>   * This header helps DTB-based architectures abstract away where a 
>> particular
>>   * device comes from; be it the DTB itself or enumerated on a PCI bus. 
>>   */
>> 
>>   [snip]
>> 
>>  #ifndef CONFIG_HAS_DEVICE_TREE
>>  #error "Header meant to be used exclusively by DTB-base architectures."
>>  #endif
>> ```
>
> Might be fine, together with giving the file a name somewhat referring to DT.
>
> Jan

That would bring it out of sync with x86's asm/device.h. Both of them define
device_t and doing so in differently named headers would just be confusing for
everyone.

Cheers,
Alejandro