Re: [PATCH v5 06/13] PCI/P2PDMA: Add P2P DMA driver writer's documentation
On 31/08/18 02:08 AM, Christian König wrote:
> +One of the biggest issues is that PCI doesn't require
forwarding
> +transactions between hierarchy domains, and in PCIe, each Root Port
> +defines a separate hierarchy domain. To make things worse, there is no
> +simple way to determine if a given Root Complex supports this or not.
> +(See PCIe r4.0, sec 1.3.1). Therefore, as of this writing, the kernel
> +only supports doing P2P when the endpoints involved are all behind the
> +same PCI bridge, as such devices are all in the same PCI hierarchy
> +domain, and the spec guarantees that all transacations within the
> +hierarchy will be routable, but it does not require routing
> +between hierarchies.
Can we add a kernel command line switch and a whitelist to enable P2P
between separate hierarchies?
In future work, yes. But not for this patchset. This is definitely the
way I see things going, but we've chosen to start with what we've presented.
Logan