Re: [RFC PATCH 2/2] drm/ttm: downgrade cached to write_combined when snooping not available

["Jiaxun Yang" <jiaxun.yang@flygoat.com>]

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.

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