Date: Tue, 9 Jan 2024 10:29:04 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Chen-Yu Tsai <wenst@...omium.org>
Cc: Florian Fainelli <f.fainelli@...il.com>, 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>, 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>, 
	Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>, 
	Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>, 
	Huacai Chen <chenhuacai@...nel.org>, Alex Elder <elder@...aro.org>, 
	Srini Kandagatla <srinivas.kandagatla@...aro.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jim Quinlan <jim2101024@...il.com>, 
	james.quinlan@...adcom.com, 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>, 
	Wolfram Sang <wsa+renesas@...g-engineering.com>, 
	Linux-Renesas <linux-renesas-soc@...r.kernel.org>
Subject: Re: [RFC 0/9] PCI: introduce the concept of power sequencing of PCIe devices

Hi ChenYu,

CC wsa + renesas-soc

On Tue, Jan 9, 2024 at 8:08 AM Chen-Yu Tsai <wenst@...omium.org> wrote:
> On Tue, Jan 9, 2024 at 12:09 PM Florian Fainelli <f.fainelli@...il.com> wrote:
> > 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.
>
> I think the current in-tree approaches mostly target either PCIe slots,
> whether full size or mini-PCIe or M.2, or soldered-on components that
> either only have a single power rail, have internal regulators, or have
> surrounding circuitry that would be incorporated on a PCIe card.
>
> These all have standardized power rails (+12V, +3.3V, +3.3V aux, etc.).

Indeed. E.g. R-Car PCIe just got support for that in commit
6797e4da2dd1e2c8 ("PCI: rcar-host: Add support for optional regulators")
in pci/next.

> > 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...
>
> We can have one "generic" slot power sequencing driver, which just
> enables all the power rails together. I would very much like to see that.
>
> I believe the power sequencing in this series is currently targeting more
> tightly coupled designs that use power rails directly from the PMIC, and
> thus require more explicit power sequencing.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

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