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

[Bartosz Golaszewski]

On Fri, Feb 2, 2024 at 1:03 AM Bjorn Andersson <andersson at kernel.org> wrote:
>

[snip]

> > >
> > > 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.
>
> Are you referring to the ath11k SNOC (firmware running on co-processor
> in the SoC) variant?
>
> Afaict the PCIe-attached ath11k is not represented as a platform_device
> in DeviceTree.
>

My bad. In RB5 it isn't (yet - I want to add it in the power
sequencing series). It is in X13s though[1].

> Said platform_device is also not a child under the PCIe bus, so this
> would be a different platform_device...
>

It's the child of the PCIe port node but there's a reason for it to
have the `compatible` property. It's because it's an entity of whose
existence we are aware before the system boots.

> > 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".
> >
>
> This is a perfectly reasonable desire. Look at our PMICs, they are full
> of platform_devices. But through the years it's been said many times,
> that this is not a valid or good reason for using platform_devices, and
> as a result we have e.g. auxiliary bus.
>

Ok, so I cannot find this information anywhere (nor any example). Do
you happen to know if the auxiliary bus offers any software node
integration so that the `compatible` property from DT can get
seamlessly mapped to auxiliary device IDs?

> Anyway, (please) don't claim that "we need to", when it actually is "we
> want to use platform_device because that's more convenient"!

Bart

[snip]

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/boot/dts/qcom/sc8280xp-lenovo-thinkpad-x13s.dts#n744