[PATCH v4 0/9] Add support for handling PCIe M.2 Key E connectors in devicetree

Manivannan Sadhasivam via B4 Relay
2026-01-12T16:28:33Z

Hi,

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. Created a dynamic 'bluetooth' node with the compatible property matching the
WCN7850 device and attached it to the serdev device. This allowed reusing the
existing OF based BT driver without much modifications.

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
1620 LGA card connected over PCIe and UART.

Dependency
==========

This series is dependent on the M.2 Key M series [1] on top of v6.19-rc1.

[1] https://lore.kernel.org/linux-pci/20260107-pci-m2-v5-0-8173d8a72641@oss.qualcomm.com
[2] https://github.com/Mani-Sadhasivam/linux/commit/753033861360171f2af1fdd56e8985ff916e1ac2

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
---
Changes in v4:
- Switched to dynamic OF node for serdev instead of swnode and dropped all
swnode related patches
- Link to v3: https://lore.kernel.org/r/20260110-pci-m2-e-v3-0-4faee7d0d5ae@oss.qualcomm.com

Changes in v3:
- Switched to swnode for the serdev device and dropped the custom
serdev_device_id related patches
- Added new swnode APIs to match the swnode with existing of_device_id
- Incorporated comments in the bindings patch
- Dropped the UIM interface from binding since it is not clear how it should get
wired
- Incorporated comments in the pwrseq driver patch
- Splitted the pwrseq patch into two
- Added the 1620 LGA compatible with Key E fallback based on Stephan's finding
- Link to v2: https://lore.kernel.org/r/20251125-pci-m2-e-v2-0-32826de07cc5@oss.qualcomm.com

Changes in v2:
- Used '-' for GPIO names in the binding and removed led*-gpios properties
- Described the endpoint nodes for port@0 and port@1 nodes
- Added the OF graph port to the serial binding
- Fixed the hci_qca driver to return err if devm_pwrseq_get() fails
- Incorporated various review comments in pwrseq driver
- Collected Ack
- Link to v1: https://lore.kernel.org/r/20251112-pci-m2-e-v1-0-97413d6bf824@oss.qualcomm.com

---
Manivannan Sadhasivam (9):
serdev: Convert to_serdev_*() helpers to macros and use container_of_const()
serdev: Add an API to find the serdev controller associated with the devicetree node
serdev: Do not return -ENODEV from of_serdev_register_devices() if external connector is used
dt-bindings: serial: Document the graph port
dt-bindings: connector: Add PCIe M.2 Mechanical Key E connector
dt-bindings: connector: m2: Add M.2 1620 LGA soldered down connector
Bluetooth: hci_qca: Add M.2 Bluetooth device support using pwrseq
power: sequencing: pcie-m2: Add support for PCIe M.2 Key E connectors
power: sequencing: pcie-m2: Create serdev device for WCN7850 bluetooth

.../bindings/connector/pcie-m2-e-connector.yaml | 161 ++++++++++++
.../devicetree/bindings/serial/serial.yaml | 3 +
MAINTAINERS | 1 +
drivers/bluetooth/hci_qca.c | 9 +
drivers/power/sequencing/Kconfig | 1 +
drivers/power/sequencing/pwrseq-pcie-m2.c | 278 ++++++++++++++++++++-
drivers/tty/serdev/core.c | 25 +-
include/linux/serdev.h | 24 +-
8 files changed, 482 insertions(+), 20 deletions(-)
---
base-commit: cb6649f6217c0331b885cf787f1d175963e2a1d2
change-id: 20251112-pci-m2-e-94695ac9d657
prerequisite-message-id: 20251125-pci-m2-v3-0-c528042aea47@oss.qualcomm.com
prerequisite-patch-id: 58778d8eb97ab86008cd48fb5d28ed6cc0bbbc1b
prerequisite-patch-id: 2dd7d793a67f59ef6e6b5137e69436896198b965
prerequisite-patch-id: 8ccaa5fdd95e64e69cd942f93c26e89b827d0453
prerequisite-patch-id: 3d3e1bb7959ab1e140c5024acdd8655e7a7e99ef

Best regards,
--
Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>