linux-kernel - Re: Re: [PATCH 4/9] PCI: create platform devices for child OF nodes of the port node

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Date: Wed, 31 Jan 2024 12:04:14 +0100
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Bjorn Andersson <andersson@...nel.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 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>, 
	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>, 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: Re: [PATCH 4/9] PCI: create platform devices for child OF nodes
 of the port node

On Tue, Jan 30, 2024 at 10:54 PM Bjorn Andersson <andersson@...nel.org> wrote:
>
> On Thu, Jan 18, 2024 at 12:15:27PM +0100, Greg Kroah-Hartman wrote:
> > On Thu, Jan 18, 2024 at 11:58:50AM +0100, Bartosz Golaszewski wrote:
> > > On Wed, Jan 17, 2024 at 5:45 PM Greg Kroah-Hartman
> > > <gregkh@...uxfoundation.org> wrote:
> > > >
> > > > On Wed, Jan 17, 2024 at 05:07:43PM +0100, Bartosz Golaszewski wrote:
> > > > > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> > > > >
> > > > > In order to introduce PCI power-sequencing, we need to create platform
> > > > > devices for child nodes of the port node.
> > > >
> > > > Ick, why a platform device?  What is the parent of this device, a PCI
> > > > device?  If so, then this can't be a platform device, as that's not what
> > > > it is, it's something else so make it a device of that type,.
> > > >
> > >
> > > Greg,
> > >
> > > This is literally what we agreed on at LPC. In fact: during one of the
> > > hall track discussions I said that you typically NAK any attempts at
> > > using the platform bus for "fake" devices but you responded that this
> > > is what the USB on-board HUB does and while it's not pretty, this is
> > > what we need to do.
> >
> > Ah, you need to remind me of these things, this changelog was pretty
> > sparse :)
> >
>
> I believe I missed this part of the discussion, why does this need to be
> a platform_device? What does the platform_bus bring that can't be
> provided by some other bus?
>

Does it need to be a platform_device? No, of course not. Does it make
sense for it to be one? Yes, for two reasons:

1. The ATH11K WLAN module is represented on the device tree like a
platform device, we know it's always there and it consumes regulators
from another platform device. The fact it uses PCIe doesn't change the
fact that it is logically a platform device.
2. The platform bus already provides us with the entire infrastructure
that we'd now need to duplicate (possibly adding bugs) in order to
introduce a "power sequencing bus".

Bart

> (I'm not questioning the need for having a bus, creating devices, and
> matching/binding them to a set of drivers)
>
> Regards,
> Bjorn
>

[snip]