[PATCH v2 2/2] docs: spi: add documentation for userspace device instantiation

Vishwaroop A
2026-04-25T17:34:36Z

Document the new_device and delete_device sysfs attributes on SPI
controllers:

- Documentation/spi/instantiating-devices.rst: describes when and
why this interface is needed, accepted parameters, usage examples,
and limitations.
- Documentation/ABI/testing/sysfs-class-spi-master: formal ABI
entry for both attributes.

Signed-off-by: Vishwaroop A <va@nvidia.com>
---
.../ABI/testing/sysfs-class-spi-master | 34 +++++++
Documentation/spi/index.rst | 1 +
Documentation/spi/instantiating-devices.rst | 88 +++++++++++++++++++
3 files changed, 123 insertions(+)
create mode 100644 Documentation/ABI/testing/sysfs-class-spi-master
create mode 100644 Documentation/spi/instantiating-devices.rst

diff --git a/Documentation/ABI/testing/sysfs-class-spi-master b/Documentation/ABI/testing/sysfs-class-spi-master
new file mode 100644
index 000000000000..b498be128bad
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-spi-master
@@ -0,0 +1,34 @@
+What: /sys/class/spi_master/spiB/new_device
+Date: April 2026
+KernelVersion: 7.2
+Contact: linux-spi@vger.kernel.org
+Description: (WO) Instantiate a new SPI device on bus B, where B
+ is the bus number (0, 1, 2, ...). Takes parameters
+ in the format:
+
+ <modalias> <chip_select> [<max_speed_hz> [<mode>]]
+
+ where modalias is the driver name, chip_select is the
+ CS line number, and max_speed_hz and mode are optional.
+
+ The device can later be removed with delete_device.
+
+ Only devices created via this interface can be removed
+ with delete_device; platform and DT devices are not
+ affected.
+
+ Example:
+ # echo spidev 0 > /sys/class/spi_master/spi0/new_device
+ # echo spidev 0 10000000 > /sys/class/spi_master/spi0/new_device
+ # echo spidev 0 10000000 3 > /sys/class/spi_master/spi0/new_device
+
+What: /sys/class/spi_master/spiB/delete_device
+Date: April 2026
+KernelVersion: 7.2
+Contact: linux-spi@vger.kernel.org
+Description: (WO) Remove a SPI device previously created via
+ new_device. Takes a single parameter: the chip select
+ number of the device to remove.
+
+ Example:
+ # echo 0 > /sys/class/spi_master/spi0/delete_device
diff --git a/Documentation/spi/index.rst b/Documentation/spi/index.rst
index ac0c2233ce48..3f723e2c07da 100644
--- a/Documentation/spi/index.rst
+++ b/Documentation/spi/index.rst
@@ -8,6 +8,7 @@ Serial Peripheral Interface (SPI)
:maxdepth: 1

spi-summary
+ instantiating-devices
spidev
multiple-data-lanes
butterfly
diff --git a/Documentation/spi/instantiating-devices.rst b/Documentation/spi/instantiating-devices.rst
new file mode 100644
index 000000000000..9ed08d94ae01
--- /dev/null
+++ b/Documentation/spi/instantiating-devices.rst
@@ -0,0 +1,88 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==============================
+How to instantiate SPI devices
+==============================
+
+SPI devices are normally declared statically via device-tree, ACPI, or
+board files. When the SPI controller is registered, these devices are
+instantiated automatically by the SPI core. This is the preferred method
+for any device with a proper kernel driver.
+
+Instantiate from user-space
+---------------------------
+
+In certain cases a SPI device cannot be declared statically:
+
+* The ``spidev`` driver, which provides raw userspace access to SPI
+ buses, explicitly rejects the bare ``"spidev"`` compatible string in
+ device-tree because spidev is a Linux implementation detail, not a
+ hardware description. Vendor-specific compatible strings for spidev
+ (e.g. ``"vendor,board-spidev"``) are also generally not accepted
+ upstream. Device-tree overlays do not help here either, since the
+ spidev driver performs the same compatible check regardless of how
+ the DT node was loaded.
+
+* You are developing or testing a SPI device on a development board
+ where the SPI bus is exposed on expansion headers, and the connected
+ device may change frequently.
+
+For these cases, a sysfs interface is provided on each SPI controller
+(similar to the I2C ``new_device``/``delete_device`` interface described
+in Documentation/i2c/instantiating-devices.rst). Two write-only
+attribute files are created in every SPI controller directory:
+``new_device`` and ``delete_device``.
+
+File ``new_device`` takes 2 to 4 parameters: the name of the SPI
+device (a string), the chip select number, and optionally
+``max_speed_hz`` and ``mode``::
+
+ <modalias> <chip_select> [<max_speed_hz> [<mode>]]
+
+The modalias is set both as the device's ``modalias`` field and as its
+``driver_override``. This ensures that the device binds to the named
+driver directly, bypassing the normal bus matching logic (OF, ACPI,
+and ``id_table``). This is necessary because drivers like ``spidev``
+deliberately exclude generic names from their ``id_table``.
+
+If ``max_speed_hz`` is omitted or 0, ``spi_setup()`` clamps it to
+the controller's maximum speed. If ``mode`` is omitted, SPI mode 0
+(CPOL=0, CPHA=0) is used.
+
+File ``delete_device`` takes a single parameter: the chip select
+number. As no two devices can share a chip select on a given SPI bus,
+the chip select is sufficient to uniquely identify the device.
+
+Examples::
+
+ # Create a spidev device on SPI bus 0, chip select 0
+ echo spidev 0 > /sys/class/spi_master/spi0/new_device
+
+ # Create with explicit clock rate and SPI mode
+ echo spidev 0 10000000 3 > /sys/class/spi_master/spi0/new_device
+
+ # Remove the device
+ echo 0 > /sys/class/spi_master/spi0/delete_device
+
+On systems that need spidev access at boot, a systemd service or
+udev rule can write to ``new_device`` after the SPI controller is
+available.
+
+Limitations
+^^^^^^^^^^^
+
+Devices created through this interface have the following limitations
+compared to devices declared via device-tree:
+
+* No interrupt (IRQ) support.
+* No additional properties such as ``spi-max-frequency`` DT bindings
+ or controller-specific configuration.
+* No platform data or software nodes.
+
+For ``spidev`` usage these limitations are not relevant, since spidev
+provides a raw byte-level interface that does not require any of these
+features.
+
+Only devices created via ``new_device`` can be removed through
+``delete_device``. Devices declared via device-tree, ACPI, or board
+files are not affected by this interface.
--
2.17.1