spi: add new_device/delete_device sysfs interface

Development boards such as the Jetson AGX Orin expose SPI buses
on expansion headers (e.g. the 40-pin header) so that users can
connect and interact with SPI peripherals from userspace. The
standard way to get /dev/spidevB.C character device nodes for
this purpose is to register spi_device instances backed by the
spidev driver.

Today there is no viable way to do this on upstream kernels:

  - The spidev driver rejects the bare "spidev" compatible
    string in DT, since spidev is a Linux software interface
    and not a description of real hardware.
  - Vendor-specific compatible strings (e.g. "nvidia,tegra-spidev")
    have been rejected by DT maintainers for the same reason.

The I2C subsystem solved an analogous problem by exposing
new_device/delete_device sysfs attributes on each adapter. Add
the same interface to SPI host controllers, so that userspace
(e.g. a systemd unit at boot) can instantiate SPI devices at
runtime without needing anything in device-tree.

The new_device file accepts:

  <modalias> <chip_select> [<max_speed_hz> [<mode>]]

where chip_select is required, while max_speed_hz and mode are
optional and default to 0 if omitted. max_speed_hz == 0 is
clamped to the controller's maximum by spi_setup(); mode == 0
selects SPI mode 0 (CPOL=0, CPHA=0).

The modalias is used both as the device identifier and as a
driver_override, so that the device binds to the named driver
directly. This is necessary because some drivers like spidev
deliberately exclude generic names from their id_table.

Devices created this way are limited compared to those declared
via DT or board files:

  - No IRQ is assigned (the device gets IRQ 0 / no interrupt).
  - No platform_data or device properties are attached.
  - No OF node is associated with the device.

These limitations are acceptable for spidev, which only needs a
registered spi_device to expose a character device to userspace.

Only devices created via new_device can be removed through
delete_device; DT and platform devices are unaffected.

The sysfs attributes are gated behind CONFIG_SPI_DYNAMIC since
this feature adds a new way of dynamically instantiating and
removing SPI devices, and the add_lock locking in
spi_unregister_controller() is already conditional on
CONFIG_SPI_DYNAMIC.

The userspace sysfs group is created manually as the last step
of spi_register_controller() and removed as the first step of
spi_unregister_controller().  Removing the group before taking
add_lock means kernfs_drain() completes any in-flight
new_device_store()/delete_device_store() calls before
add_lock is acquired, so unregister never blocks on a store
that is itself waiting for add_lock and no store can be
touching an spi_device that is about to be torn down.

Non-sysfs callers of __spi_add_device() (DT/ACPI dynamic add,
ancillary registration) continue to be protected by the
pre-existing !device_is_registered(&ctlr->dev) check added in
commit ddf75be47ca7 ("spi: Prevent adding devices below an
unregistering controller"): device_del(&ctlr->dev) runs inside
add_lock in spi_unregister_controller() so state_in_sysfs flips
to 0 before add_lock is released.

Link: https://lore.kernel.org/linux-tegra/909f0c92-d110-4253-903e-5c81e21e12c9@nvidia.com/

Signed-off-by: Vishwaroop A <va@nvidia.com>
Link: https://patch.msgid.link/20260803104614.2548375-2-va@nvidia.com
Signed-off-by: Mark Brown <broonie@kernel.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 4c285d3ede1d..88d17fce02dc 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -179,6 +179,8 @@ extern void spi_transfer_cs_change_delay_exec(struct spi_message *msg,
  * @num_tx_lanes: Number of transmit lanes wired up.
  * @rx_lane_map: Map of peripheral lanes (index) to controller lanes (value).
  * @num_rx_lanes: Number of receive lanes wired up.
+ * @userspace_node: entry on the parent controller's userspace_clients list
+ *	when this device was instantiated via the sysfs new_device interface
  *
  * A @spi_device is used to interchange data between an SPI target device
  * (usually a discrete chip) and CPU memory.
@@ -252,6 +254,10 @@ struct spi_device {
 	u8			rx_lane_map[SPI_DEVICE_DATA_LANE_CNT_MAX];
 	u8			num_rx_lanes;
 
+#if IS_ENABLED(CONFIG_SPI_DYNAMIC)
+	struct list_head	userspace_node;
+#endif
+
 	/*
 	 * Likely need more hooks for more protocol options affecting how
 	 * the controller talks to each chip, like:
@@ -555,6 +561,11 @@ extern struct spi_device *devm_spi_new_ancillary_device(struct spi_device *spi,
  * @defer_optimize_message: set to true if controller cannot pre-optimize messages
  *	and needs to defer the optimization step until the message is actually
  *	being transferred
+ * @userspace_clients: list of SPI devices instantiated from userspace via
+ *	the sysfs new_device interface; protected by @add_lock
+ * @userspace_registered: true once the new_device/delete_device sysfs
+ *	group has been added by spi_register_controller(); used by
+ *	spi_unregister_controller() to know whether to remove it
  *
  * Each SPI controller can communicate with one or more @spi_device
  * children.  These make a small bus, sharing MOSI, MISO and SCK signals
@@ -807,6 +818,13 @@ struct spi_controller {
 	bool			queue_empty;
 	bool			must_async;
 	bool			defer_optimize_message;
+
+#if IS_ENABLED(CONFIG_SPI_DYNAMIC)
+	/* List of userspace-instantiated devices; protected by @add_lock */
+	struct list_head	userspace_clients;
+	/* True after new_device/delete_device sysfs group is created */
+	bool			userspace_registered;
+#endif
 };
 
 static inline void *spi_controller_get_devdata(struct spi_controller *ctlr)