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

[Bartosz Golaszewski <brgl@bgdev.pl>]

On Fri, Feb 2, 2024 at 11:02 AM Bartosz Golaszewski <brgl@bgdev.pl> wrote:
>
> On Fri, Feb 2, 2024 at 1:03 AM Bjorn Andersson <andersson@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?
>

So I was just trying to port this to using the auxiliary bus, only to
find myself literally reimplementing functions from
drivers/of/device.c. I have a feeling that this is simply wrong. If
we're instantiating devices well defined on the device-tree then IMO
we *should* make them platform devices. Anything else and we'll be
reimplementing drivers/of/ because we will need to parse the device
nodes, check the compatible, match it against drivers etc. Things that
are already implemented for the platform bus and of_* APIs.

Greg: Could you chime in and confirm that it's alright to use the
platform bus here? Or maybe there is some infrastructure to create
auxiliary devices from software nodes?

Bartosz

> > 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