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>

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