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From: Geert Uytterhoeven <geert@linux-m68k.org>
Date: Tue, 9 Jan 2024 10:29:04 +0100
Message-ID: <CAMuHMdV7YJZTPFL+ECLh3f9jAuaxqnuooggjHp-QLLxsrMu1Mw@mail.gmail.com>
Subject: Re: [RFC 0/9] PCI: introduce the concept of power sequencing of PCIe devices
To: Chen-Yu Tsai <wenst@chromium.org>
Cc: Florian Fainelli <f.fainelli@gmail.com>, Bartosz Golaszewski <brgl@bgdev.pl>, Kalle Valo <kvalo@kernel.org>, 
	"David S . Miller" <davem@davemloft.net>, Eric Dumazet <edumazet@google.com>, 
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>, Rob Herring <robh+dt@kernel.org>, 
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	Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>, 
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	Jernej Skrabec <jernej.skrabec@gmail.com>, Chris Morgan <macromorgan@hotmail.com>, 
	Linus Walleij <linus.walleij@linaro.org>, Geert Uytterhoeven <geert+renesas@glider.be>, 
	Arnd Bergmann <arnd@arndb.de>, Neil Armstrong <neil.armstrong@linaro.org>, 
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	Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Jim Quinlan <jim2101024@gmail.com>, 
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	linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, 
	linux-arm-kernel@lists.infradead.org, linux-pci@vger.kernel.org, 
	Bartosz Golaszewski <bartosz.golaszewski@linaro.org>, 
	Wolfram Sang <wsa+renesas@sang-engineering.com>, 
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>
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Hi ChenYu,

CC wsa + renesas-soc

On Tue, Jan 9, 2024 at 8:08=E2=80=AFAM Chen-Yu Tsai <wenst@chromium.org> wr=
ote:
> On Tue, Jan 9, 2024 at 12:09=E2=80=AFPM Florian Fainelli <f.fainelli@gmai=
l.com> wrote:
> > On 1/4/2024 5:01 AM, Bartosz Golaszewski wrote:
> > > From: Bartosz Golaszewski <bartosz.golaszewski@linaro.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 o=
n
> > > 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 realizat=
ion
> > > 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 o=
f
> > > 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 becom=
e
> > > 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 reve=
rted:
> >
> > 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 losin=
g
> > 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 servic=
e
> > 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

--=20
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k=
.org

In personal conversations with technical people, I call myself a hacker. Bu=
t
when I'm talking to journalists I just say "programmer" or something like t=
hat.
                                -- Linus Torvalds