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

[Christoph Hellwig]

On Thu, Jul 04, 2024 at 10:00:52AM +0800, Icenowy Zheng wrote:
> So I here want to ask a question as an individual hacker: what's the
> policy of linux-pci towards these non-coherent PCIe implementations?
> 
> If the sentences of Christian is right, these implementations are just
> out-of-spec, should them get purged out of the kernel, or at least
> raising a warning that some HW won't work because of inconformant
> implementation?

Nothing in the PCIe specifications that mandates a programming model.
Non-coherent DMA is extremely common in lower end devices, and despite
all the issues that it causes well supported in Linux.

What are you trying to solve?