Documentation: Add documentation for seqcount.

This commit added ducumentation for seqcount.

Signed-off-by: ouyangxiangzhen <ouyangxiangzhen@xiaomi.com>

Author: Fix-Point

Documentation/components/concurrency/index.rst

@@ -0,0 +1,17 @@
+===============
+Concurrency
+===============
+
+This page discusses the concurrency support for NuttX.
+
+Related Subdirectories
+======================
+
+* ``seqcount`` - Contains the sequential count based read-write lock
+                 implementation.
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Contents:
+   
+   seqcount/index.rst

Documentation/components/concurrency/seqcount/index.rst

@@ -0,0 +1,120 @@
+===============
+Seqcount
+===============
+
+Summary
+=======
+
+This an efficient ``Seqlock`` (sequential lock) mechanism suitable for
+concurrent scenarios with frequent reads and rare writes.
+``Seqlock`` enables lock-free reading while ensuring data consistency
+through sequence counting.
+
+Core Features
+-------------
+
+1. Lock-Free Reading
+~~~~~~~~~~~~~~~~~~~~
+
+- Readers access shared data without acquiring locks, eliminating lock
+  contention
+- Sequence number tracking detects data modifications during read operations
+- Retry mechanism guarantees read consistency
+
+2. Write Protection
+~~~~~~~~~~~~~~~~~~~
+
+- Writers utilize atomic operations and interrupt protection for exclusive
+  access
+- Sequence number parity indicates write state (even: readable, odd: writing)
+- SMP environments employ memory barriers for operation ordering
+
+Main Interfaces
+---------------
+
+Initialization
+~~~~~~~~~~~~~~
+
+.. code-block:: c
+
+   void seqlock_init(seqcount_t *s);
+
+Read Operations
+~~~~~~~~~~~~~~~
+
+.. code-block:: c
+
+   uint32_t read_seqbegin(const seqcount_t *s);
+   uint32_t read_seqretry(const seqcount_t *s, uint32_t start);
+
+Reader usage pattern:
+
+.. code-block:: c
+
+   uint32_t seq;
+   do {
+       seq = read_seqbegin(&seqlock);
+       // Read shared data
+   } while (read_seqretry(&seqlock, seq));
+
+Write Operations
+~~~~~~~~~~~~~~~~
+
+.. code-block:: c
+
+   irqstate_t write_seqlock_irqsave(seqcount_t *s);
+   void write_sequnlock_irqrestore(seqcount_t *s, irqstate_t flags);
+
+Writer usage pattern:
+
+.. code-block:: c
+
+   irqstate_t flags = write_seqlock_irqsave(&seqlock);
+   // Modify shared data
+   write_sequnlock_irqrestore(&seqlock, flags);
+
+Technical Details
+-----------------
+
+1. Sequence Number Mechanism
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+- Sequence counter initializes to 0 (even)
+- Write start increments by 1 (becomes odd)
+- Write completion increments by 1 (returns to even)
+
+2. Memory Barriers
+~~~~~~~~~~~~~~~~~~
+
+- SMP systems utilize appropriate read/write memory barriers
+- ``SMP_WMB()``: Write memory barrier
+- ``SMP_RMB()``: Read memory barrier
+- Ensures operation ordering and memory visibility
+
+3. Atomic Operations
+~~~~~~~~~~~~~~~~~~~~
+
+- SMP environments employ atomic read/write and CAS operations
+- Prevents data races and maintains consistency
+
+4. Interrupt Protection
+~~~~~~~~~~~~~~~~~~~~~~~
+
+- Write operations disable interrupts
+- Prevents interrupt handlers from interfering with critical sections
+
+Applicable Scenarios
+--------------------
+
+- Read operations significantly outnumber write operations
+- Readers can tolerate temporary data inconsistency
+- High-performance read operations are required
+
+Performance Advantages
+----------------------
+
+- Read operations completely lock-free with minimal overhead
+- Significant performance improvement for read-intensive applications
+
+This implementation accounts for differences between SMP and uniprocessor
+environments, ensuring correct operation across various configurations.

Documentation/components/index.rst

@@ -12,6 +12,7 @@ case, you can head to the :doc:`reference <../reference/index>`.
    :caption: Contents:
 
    binfmt.rst
+   concurrency/index.rst
    drivers/index.rst
    nxflat.rst
    nxgraphics/index.rst