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Mon, 4 May 2026 00:40:38 -0700 From: Vishwaroop A To: , CC: , , , , , Subject: [PATCH v3 2/2] docs: spi: add documentation for userspace device instantiation Date: Mon, 4 May 2026 07:40:37 +0000 Message-ID: <20260504074037.704833-3-va@nvidia.com> X-Mailer: git-send-email 2.17.1 In-Reply-To: <20260504074037.704833-1-va@nvidia.com> References: <20260504074037.704833-1-va@nvidia.com> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain X-NV-OnPremToCloud: ExternallySecured X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SN1PEPF00036F3E:EE_|CH3PR12MB8185:EE_ X-MS-Office365-Filtering-Correlation-Id: e48382d1-7ce3-4ada-7720-08dea9b0771f X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|82310400026|1800799024|36860700016|376014|30052699003|56012099003|22082099003|18002099003; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:216.228.118.233;CTRY:US;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:mail.nvidia.com;PTR:dc7edge2.nvidia.com;CAT:NONE;SFS:(13230040)(82310400026)(1800799024)(36860700016)(376014)(30052699003)(56012099003)(22082099003)(18002099003);DIR:OUT;SFP:1101; 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Signed-off-by: Vishwaroop A --- .../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: + + [ []] + + 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``:: + + [ []] + +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