linux-kernel - Re: [RFC 0/9] PCI: introduce the concept of power sequencing of PCIe devices

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Message-ID: <a85dbfc3-e327-442a-9aab-5115f86944f7@gmail.com>
Date: Mon, 8 Jan 2024 20:08:33 -0800
From: Florian Fainelli <f.fainelli@...il.com>
To: Bartosz Golaszewski <brgl@...ev.pl>, Kalle Valo <kvalo@...nel.org>,
 "David S . Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Rob Herring <robh+dt@...nel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Conor Dooley <conor+dt@...nel.org>, Bjorn Andersson <andersson@...nel.org>,
 Konrad Dybcio <konrad.dybcio@...aro.org>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Bjorn Helgaas <bhelgaas@...gle.com>, Heiko Stuebner <heiko@...ech.de>,
 Jernej Skrabec <jernej.skrabec@...il.com>,
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Cc: linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-arm-msm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-pci@...r.kernel.org,
 Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [RFC 0/9] PCI: introduce the concept of power sequencing of PCIe
 devices

Hello,

On 1/4/2024 5:01 AM, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> 
> 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@gmail.com/

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