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

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Date: Thu, 18 Jan 2024 08:29:01 -0600
From: Rob Herring <robh+dt@...nel.org>
To: Bartosz Golaszewski <brgl@...ev.pl>
Cc: 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>, 
	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>, Chris Morgan <macromorgan@...mail.com>, 
	Linus Walleij <linus.walleij@...aro.org>, Geert Uytterhoeven <geert+renesas@...der.be>, 
	Arnd Bergmann <arnd@...db.de>, Neil Armstrong <neil.armstrong@...aro.org>, 
	Nícolas F . R . A . Prado <nfraprado@...labora.com>, 
	Marek Szyprowski <m.szyprowski@...sung.com>, Peng Fan <peng.fan@....com>, 
	Robert Richter <rrichter@....com>, Dan Williams <dan.j.williams@...el.com>, 
	Jonathan Cameron <Jonathan.Cameron@...wei.com>, Terry Bowman <terry.bowman@....com>, 
	Lukas Wunner <lukas@...ner.de>, Huacai Chen <chenhuacai@...nel.org>, Alex Elder <elder@...aro.org>, 
	Srini Kandagatla <srinivas.kandagatla@...aro.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Abel Vesa <abel.vesa@...aro.org>, 
	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: [PATCH 0/9] PCI: introduce the concept of power sequencing of
 PCIe devices

On Wed, Jan 17, 2024 at 10:08 AM Bartosz Golaszewski <brgl@...ev.pl> wrote:
>
> From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
>
> The responses to the RFC were rather positive so here's a proper series.

Thanks for tackling this.

> 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 DT maintainers won't like adding any "fake" nodes not representing
> actual devices, we decided to reuse existing PCI infrastructure.

Thank you. :)

> 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.

Suspend/resume has been brought up already, but I disagree we can
worry about that later unless there is and always will be no power
sequencing during suspend/resume for all devices ever. Given the
supplies aren't standard, it wouldn't surprise me if standard PCI
power management isn't either. The primary issue I see with this
design is we will end up with 2 drivers doing the same power
sequencing: the platform driver for initial power on and the device's
PCI driver for suspend/resume.

Rob