Re: [PATCH v7 0/8] Add support for handling PCIe M.2 Key E connectors in devicetree

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Wed, Apr 15, 2026 at 04:31:24PM +0800, Chen-Yu Tsai wrote:
> On Tue, Apr 14, 2026 at 8:03 PM Andy Shevchenko
> <andriy.shevchenko@linux.intel.com> wrote:
> > On Tue, Apr 14, 2026 at 06:29:02PM +0800, Chen-Yu Tsai wrote:
> > > On Tue, Apr 14, 2026 at 4:28 PM Andy Shevchenko
> > > <andriy.shevchenko@linux.intel.com> wrote:
> > > > On Tue, Apr 14, 2026 at 01:03:19PM +0800, Chen-Yu Tsai wrote:
> > > > > On Tue, Apr 14, 2026 at 12:08 AM Manivannan Sadhasivam <mani@kernel.org> wrote:
> > > > > > On Mon, Apr 13, 2026 at 07:33:12PM +0530, Manivannan Sadhasivam wrote:
> > > > > > > On Mon, Apr 13, 2026 at 03:54:59PM +0800, Chen-Yu Tsai wrote:
> > > > > > > > On Thu, Mar 26, 2026 at 01:36:28PM +0530, Manivannan Sadhasivam wrote:

...

> > > > > > > > - Given that this connector actually represents two devices, how do I
> > > > > > > >   say I want the BT part to be a wakeup source, but not the WiFi part?
> > > > > > > >   Does wakeup-source even work at this point?
> > > > > > >
> > > > > > > You can't use the DT property since the devices are not described in DT
> > > > > > > statically. But you can still use the per-device 'wakeup' sysfs knob to enable
> > > > > > > wakeup.
> > > > >
> > > > > I see. I think not being able to specify generic properties for the devices
> > > > > on the connector is going to be a bit problematic.
> > > >
> > > > This is nature of the open-connectors, especially on the busses that are
> > > > hotpluggable, like PCIe. We never know what is connected there _ahead_.
> > >
> > > I believe what you mean by "hotpluggable" is "user replaceable".
> >
> > From the OS perspective it's the same. From platform perspective
> > there is a difference, granted.
> 
> Yes. I just wanted to clarify.
> 
> > > > In other words you can't describe in DT something that may not exist.
> > >
> > > But this is actually doable with the PCIe slot representation. The
> > > properties are put in the device node for the slot. If no card is
> > > actually inserted in the slot, then no device is created, and the
> > > device node is left as not associated with anything.
> >
> > But you need to list all devices in the world if you want to support this
> 
> Why would I need to? The PCIe slot representation just describes a
> PCIe bridge. Granted this might not be entirely correct, but it's
> what we currently have.
> 
> And even then, there are properties like memory-region or wakeup-source
> that are generic and aren't tied to specific devices.

Yes,  see below what I replied...

> > somehow. Yes, probably many of them (or majority) will be enumerated as is,

^^^ "the majority" will work without any assistance.

> > but some may need an assistance via (dynamic) properties or similar mechanisms.

> Even if we wanted to add dynamic properties, there is currently no proper
> device node to attach them to.

Isn't that's node created dynamically as well and attached to the PCI bus?

> > > It's just that for this new M.2 E-key connector, there aren't separate
> > > nodes for each interface. And the system doesn't associate the device
> > > node with the device, because it's no longer a child node of the
> > > controller or hierarchy, but connected over the OF graph.
> > >
> > > Moving over to the E-key connector representation seems like one step
> > > forward and one step backward in descriptive ability. We gain proper
> > > power sequencing, but lose generic properties.
> >
> > The "key" is property of the connector. Hence if you have an idea what can be
> > common for ALL "key":s, that's probably can be abstracted. Note, I'm not
> > familiar with the connector framework in the Linux kernel, perhaps it's already
> > that kind of abstraction.
> 
> I'm not arguing for a even more generic "M.2" connector. The "key" is
> already described in the compatible. I'm saying we should have some way
> of describing the individual interfaces (PCIe, SDIO, USB, UART, I2S, I2C)
> on the connector so further nodes or properties can be attached to them,
> either with overlays or dynamically within the kernel. Right now the
> are only described as individual ports, but we can't actually tie a
> device to a OF graph port.

Shouldn't it be described as a DT subtree? Sorry, I am not familiar with DT
enough to understand the issue you have.

> But maybe I'm overthinking the representation part. AFAICT for Qualcomm's
> UART-based BT bit part, Mani just had the driver create a device node
> under the UART (by traversing the OF graph to find the UART). If that's
> the desired way then the connector binding should mention it. And that
> works for me. But I think it's messier and also we're missing an
> opportunity to make the M.2 connector a standardized attachment point
> for overlays.

Okay, now it might get clearer to me, but still, I am not an expert.

> Mani, could you also chime in a bit on what you envisioned?

+1, please elaborate to me as well.

> (Added Luca from Bootlin to CC, as I think there are parallels to the
>  "Hotplug of Non-discoverable Hardware" work)
> 
> > > The latter part is solvable, but we likely need child nodes under the
> > > connector for the different interfaces. Properties that make sense for
> > > one type might not make sense for another.
> > >
> > > P.S. We could also just add child device nodes under the controller to
> > > put the generic properties, but that's splitting the description into
> > > multiple parts. Let's not go there if at all possible.

-- 
With Best Regards,
Andy Shevchenko