Hello,
On 1/4/2024 5:01 AM, Bartosz Golaszewski wrote:
From: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
During last year's Linux Plumbers we had several discussions centered
around the need to power-on PCI devices before they can be detected on
the bus.
The consensus during the conference was that we need to introduce a
class of "PCI slot drivers" that would handle the power-sequencing.
After some additional brain-storming with Manivannan and the realization
that the DT maintainers won't like adding any "fake" nodes not
representing actual devices, we decided to reuse the existing
infrastructure provided by the PCIe port drivers.
The general idea is to instantiate platform devices for child nodes of
the PCIe port DT node. For those nodes for which a power-sequencing
driver exists, we bind it and let it probe. The driver then triggers a
rescan of the PCI bus with the aim of detecting the now powered-on
device. The device will consume the same DT node as the platform,
power-sequencing device. We use device links to make the latter become
the parent of the former.
The main advantage of this approach is not modifying the existing DT in
any way and especially not adding any "fake" platform devices.
There is prior work in that area which was applied, but eventually reverted:
https://www.spinics.net/lists/linux-pci/msg119136.html
and finally re-applied albeit in a different shape:
https://lore.kernel.org/all/20220716222454.29914-1-jim2101024@xxxxxxxxx/
so we might want to think about how to have pcie-brcmstb.c converted
over your proposed approach. AFAIR there is also pcie-rockchip.c which
has some rudimentary support for voltage regulators of PCIe end-points.
What does not yet appear in this RFC is support for suspend/resume,
especially for power states where both the RC and the EP might be losing
power. There also needs to be some thoughts given to wake-up enabled
PCIe devices like Wi-Fi which might need to remain powered on to service
Wake-on-WLAN frames if nothing else.
I sense a potential for a lot of custom power sequencing drivers being
added and ultimately leading to the decision to create a "generic" one
which is entirely driven by Device Tree properties...
Thanks for doing this!
--
Florian
