[PATCH net-next V5 01/15] documentation: networking: add shared devlink documentation

[Tariq Toukan <tariqt@nvidia.com>]

From: Jiri Pirko <jiri@nvidia.com>

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

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
---
 .../networking/devlink/devlink-shared.rst     | 95 +++++++++++++++++++
 Documentation/networking/devlink/index.rst    |  1 +
 2 files changed, 96 insertions(+)
 create mode 100644 Documentation/networking/devlink/devlink-shared.rst

diff --git a/Documentation/networking/devlink/devlink-shared.rst b/Documentation/networking/devlink/devlink-shared.rst
new file mode 100644
index 000000000000..74655dc671bc
--- /dev/null
+++ b/Documentation/networking/devlink/devlink-shared.rst
@@ -0,0 +1,95 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+============================
+Devlink Shared Instances
+============================
+
+Overview
+========
+
+Shared devlink instances allow multiple physical functions (PFs) on the same
+chip to share an additional devlink instance for chip-wide operations. This
+is implemented within individual drivers alongside the individual PF devlink
+instances, not replacing them.
+
+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.
+
+The shared devlink instance is backed by a faux device and provides a common
+interface for operations that affect the entire chip rather than individual PFs.
+A faux device is used as a backing device for the 'entire chip' since there's no
+additional real device instantiated by hardware besides the PF devices.
+
+Implementation
+==============
+
+Architecture
+------------
+
+The implementation uses:
+
+* **Faux device**: Virtual device backing the shared devlink instance
+* **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:
+     - Creates faux device with chip identifier as name
+     - 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 (device list becomes empty)
+
+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.
diff --git a/Documentation/networking/devlink/index.rst b/Documentation/networking/devlink/index.rst
index 35b12a2bfeba..f7ba7dcf477d 100644
--- a/Documentation/networking/devlink/index.rst
+++ b/Documentation/networking/devlink/index.rst
@@ -68,6 +68,7 @@ general.
    devlink-resource
    devlink-selftests
    devlink-trap
+   devlink-shared
 
 Driver-specific documentation
 -----------------------------
-- 
2.44.0