PCIe coherency in spec (was: [RFC PATCH 2/2] drm/ttm: downgrade cached to write_combined when snooping not available)

[Jiaxun Yang]

在2024年7月2日七月 下午6:03，Jiaxun Yang写道：
> 在2024年7月2日七月 下午5:27，Christian König写道：
>> Am 02.07.24 um 11:06 schrieb Icenowy Zheng:
>>> [SNIP] However I don't think the definition of the AGP spec could apply on all
>>> PCI(e) implementations. The AGP spec itself don't apply on
>>> implementations that do not implement AGP (which is the most PCI(e)
>>> implementations today), and it's not in the reference list of the PCIe
>>> spec, so it does no help on this context. 
>> No, exactly that is not correct.
>>
>> See as I explained the No-Snoop extension to PCIe was created to help 
>> with AGP support and later merged into the base PCIe specification.
>>
>> So the AGP spec is now part of the PCIe spec.

Hi Bjorn & linux-pci folks,

It seems like we have some disputes on interpretation pf PCIe specification.

We are seeking your expertise on the question: Does PCIe specification mandate Cache
coherency via snoop?

There are some further context in this thread [1].

[1]:  https://lore.kernel.org/all/0db974d40cd8c5dcc723d43c328bac923e0fe33a.camel@icenowy.me/
Thanks
- Jiaxun

>
> We don't really buy this theory.
>
> Keyword "AGP" doesn't appear in "PCI Express Base 4.0 Base Specification" even
> once.
>
> If PCIe is a predecessor of AGP, where does AGP specific software interface like
>  AGP aperture goes? PCIe GPUs are only borrowing software concepts from AGP,
> but they didn't inherit any hardware properties.
>
> [...]
>> We seem to have a misunderstanding here, this is not a software issue. 
>> The hardware platform is considered broken by the hardware vendor!
>
> It's up to the specification text to define compliance means. So far as 
> per analysis
> from Icenowy of PCIe specification text itself it's not prohibited.
>
>>
>> In other words people have stitched together hardware in a way which is 
>> not supported by the creator of that hardware.
>>
>> So as long as you can't convince anybody from ARM or the RISC-V team or 
>> whoever created that hardware to confirm that the hardware actually 
>> works you won't get any support for that.
>
> Well we are trying to support them on our own in mainline, we are not asking
> for any support.
>
> Thanks
> - Jiaxun
>>
>> Regards,
>> Christian.
>
> -- 
> - Jiaxun

-- 
- Jiaxun