documentation: networking: add shared devlink documentation

Document shared devlink instances for multiple PFs on the same chip.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Link: https://patch.msgid.link/20260312100407.551173-13-jiri@resnulli.us
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/devlink/devlink-shared.rst

@@ -0,0 +1,97 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+========================
+Devlink Shared Instances
+========================
+
+Overview
+========
+
+Shared devlink instances allow multiple physical functions (PFs) on the same
+chip to share a devlink instance for chip-wide operations.
+
+Multiple PFs may reside on the same physical chip, running a single firmware.
+Some of the resources and configurations may be shared among these PFs. The
+shared devlink instance provides an object to pin configuration knobs on.
+
+There are two possible usage models:
+
+1. The shared devlink instance is used alongside individual PF devlink
+   instances, providing chip-wide configuration in addition to per-PF
+   configuration.
+2. The shared devlink instance is the only devlink instance, without
+   per-PF instances.
+
+It is up to the driver to decide which usage model to use.
+
+The shared devlink instance is not backed by any struct *device*.
+
+Implementation
+==============
+
+Architecture
+------------
+
+The implementation uses:
+
+* **Chip identification**: PFs are grouped by chip using a driver-specific identifier
+* **Shared instance management**: Global list of shared instances with reference counting
+
+API Functions
+-------------
+
+The following functions are provided for managing shared devlink instances:
+
+* ``devlink_shd_get()``: Get or create a shared devlink instance identified by a string ID
+* ``devlink_shd_put()``: Release a reference on a shared devlink instance
+* ``devlink_shd_get_priv()``: Get private data from shared devlink instance
+
+Initialization Flow
+-------------------
+
+1. **PF calls shared devlink init** during driver probe
+2. **Chip identification** using driver-specific method to determine device identity
+3. **Get or create shared instance** using ``devlink_shd_get()``:
+
+   * The function looks up existing instance by identifier
+   * If none exists, creates new instance:
+     - Allocates and registers devlink instance
+     - Adds to global shared instances list
+     - Increments reference count
+
+4. **Set nested devlink instance** for the PF devlink instance using
+   ``devl_nested_devlink_set()`` before registering the PF devlink instance
+
+Cleanup Flow
+------------
+
+1. **Cleanup** when PF is removed
+2. **Call** ``devlink_shd_put()`` to release reference (decrements reference count)
+3. **Shared instance is automatically destroyed** when the last PF removes (reference count reaches zero)
+
+Chip Identification
+-------------------
+
+PFs belonging to the same chip are identified using a driver-specific method.
+The driver is free to choose any identifier that is suitable for determining
+whether two PFs are part of the same device. Examples include:
+
+* **PCI VPD serial numbers**: Extract from PCI VPD
+* **Device tree properties**: Read chip identifier from device tree
+* **Other hardware-specific identifiers**: Any unique identifier that groups PFs by chip
+
+Locking
+-------
+
+A global mutex (``shd_mutex``) protects the shared instances list during registration/deregistration.
+
+Similarly to other nested devlink instance relationships, devlink lock of
+the shared instance should be always taken after the devlink lock of PF.
+
+Reference Counting
+------------------
+
+Each shared devlink instance maintains a reference count (``refcount_t refcount``).
+The reference count is incremented when ``devlink_shd_get()`` is called and decremented
+when ``devlink_shd_put()`` is called. When the reference count reaches zero, the shared
+instance is automatically destroyed.

Documentation/networking/devlink/index.rst

@@ -68,6 +68,7 @@ general.
    devlink-resource
    devlink-selftests
    devlink-trap
+   devlink-shared
 
 Driver-specific documentation
 -----------------------------