Re: [PATCH v2 00/10] Add support for handling PCIe M.2 Key E connectors in devicetree

[Stephan Gerhold <stephan.gerhold@linaro.org>]

On Tue, Nov 25, 2025 at 08:15:04PM +0530, Manivannan Sadhasivam via B4 Relay wrote:
> This series is the continuation of the series [1] that added the initial support
> for the PCIe M.2 connectors. This series extends it by adding support for Key E
> connectors. These connectors are used to connect the Wireless Connectivity
> devices such as WiFi, BT, NFC and GNSS devices to the host machine over
> interfaces such as PCIe/SDIO, USB/UART and NFC. This series adds support for
> connectors that expose PCIe interface for WiFi and UART interface for BT. Other
> interfaces are left for future improvements.
> 
> Serdev device support for BT
> ============================
> 
> Adding support for the PCIe interface was mostly straightforward and a lot
> similar to the previous Key M connector. But adding UART interface has proved to
> be tricky. This is mostly because of the fact UART is a non-discoverable bus,
> unlike PCIe which is discoverable. So this series relied on the PCI notifier to
> create the serdev device for UART/BT. This means the PCIe interface will be
> brought up first and after the PCIe device enumeration, the serdev device will
> be created by the pwrseq driver. This logic is necessary since the connector
> driver and DT node don't describe the device, but just the connector. So to make
> the connector interface Plug and Play, the connector driver uses the PCIe device
> ID to identify the card and creates the serdev device. This logic could be
> extended in the future to support more M.2 cards. Even if the M.2 card uses SDIO
> interface for connecting WLAN, a SDIO notifier could be added to create the
> serdev device.
> 
> Open questions
> ==============
> 
> Though this series adds the relevant functionality for handling the M.2 Key M
> connectors, there are still a few open questions exists on the design. 
> 
> 1. I've used the M.2 card model name as the serdev device name. This is found
> out by comparing the PCIe VID:PID in the notifier. Is this approach acceptable?
> I did not use the PID as the serdev name since it will vary if the SDIO
> interface is used in the future.
> 
> 2. PCIe client drivers of some M.2 WLAN cards like the Qcom QCA6390, rely on
> the PCIe device DT node to extract properties such as
> 'qcom,calibration-variant', 'firmware-name', etc... For those drivers, should we
> add the PCIe DT node in the Root Port in conjunction with the Port node as
> below?
> 
> pcie@0 {
> 	wifi@0 {
> 		compatible = "pci17cb,1103";
> 		...
> 		qcom,calibration-variant = "LE_X13S";
> 	};
> 
> 	port {
> 		pcie4_port0_ep: endpoint {
> 			remote-endpoint = <&m2_e_pcie_ep>;
> 		};
> 	};
> };
> 
> This will also require marking the PMU supplies optional in the relevant ath
> bindings for M.2 cards.
> 
> 3. Some M.2 cards require specific power up sequence like delays between
> regulator/GPIO and such. For instance, the WCN7850 card supported in this series
> requires 50ms delay between powering up an interface and driving it. I've just
> hardcoded the delay in the driver, but it is a pure hack. Since the pwrseq
> driver doesn't know anything about the device it is dealing with before powering
> it ON, how should it handle the device specific power requirements? Should we
> hardcode the device specific property in the connector node? But then, it will
> no longer become a generic M.2 connector and sort of defeats the purpose of the
> connector binding.
> 
> I hope to address these questions with the help of the relevant subsystem
> maintainers and the community. 
> 
> Testing
> =======
> 
> This series, together with the devicetree changes [2] was tested on the
> Qualcomm X1e based Lenovo Thinkpad T14s Laptop which has the WCN7850 WLAN/BT M.2
> card connected over PCIe and UART.
> 
> [2] https://github.com/Mani-Sadhasivam/linux/commit/acbee74a5c90fc8839bb7b6f326c677ee1c0d89c

Thanks for working on describing the M.2 connectors properly in the
device tree!

I haven't had time to look into this in detail yet, but a quick look at
the dt-bindings and examples looks good to me! Thanks for keeping the
bindings as generic as possible.

I have a small nitpick for the specific example you have here: The
Lenovo ThinkPad T14s does not actually have a "M.2 Mechanical Key E
connector". If you look at a picture of the mainboard [1], the WLAN/BT
module is "soldered-down" (look on the right, on the right side next to
the large copper bracket). In the M.2 specification, "soldered-down"
modules do not have a "key", they have a specific pinout that is
followed (see section 5.4). The power sequencing etc and the set of pins
is quite similar/the same though.

My notes (from a few months ago) suggest the T14s probably uses a
non-standard M.2 Type 1620 LGA pinout. I don't remember the exact chain
of thought behind that, but you can find similarly looking modules with
this type, e.g. https://www.sparklan.com/product/wnsq-290be/. There is a
1620 *BGA* pinout in the M.2 specification, but a 1620 *LGA* pinout does
not exist there. Interestingly, in the block diagram of the module in
the link above this type is called *Q*M.2 1620 LGA 168 pin, as if this
is some Qualcomm-specific form factor.

A real mechanical key E connector can be found e.g. in the X1E CRD, X1E
Devkit, or I think some of the X1E-based HP laptops (would need to check
which one exactly).

I'm not sure if it's really appropriate modeling the "soldered-down"
variant as "Mechanical Key E connector" in the DT. We might need
a separate compatible for this. Do you have any thoughts about that?

Thanks,
Stephan

[1]: https://www.notebookcheck.com/fileadmin/_processed_/d/c/csm_DSC_0003_aadae1ddd2.jpg