[doc] add crr doc

Author: amin1377

doc/src/vpr/custom_rr_graph.rst

@@ -0,0 +1,102 @@
+.. _custom_rr_graph:
+
+Custom RR Graph
+===============
+
+For users who want more control over how connections are made in the routing resource graph, VPR provides a way to describe them through a Custom RR Graph (CRR) generator.
+
+Currently, the CRR Generator is only based on the tileable RR Graph. Support for the default VPR RR Graph Generator is in progress. To generate a CRR, the following files are required:
+
+* Switch block map file
+* Switch block template files
+
+Switch Block Map File
+~~~~~~~~~~~~~~~~~~~~~~
+
+This is a YAML file that specifies which switch block template should be used for each tile. The mapping format supports the following patterns:
+
+* ``SB_1__1_: [sb_template.csv]`` - Tile at location (1, 1) uses switch block template ``sb_template.csv``
+* ``SB_1__*_: [sb_template.csv]`` - All tiles with x coordinate 1 use switch block template ``sb_template.csv``
+* ``SB_[7,20]__[2:32:3]_: [sb_template.csv]`` - Tiles with x equal to 7 or 20, and y coordinates from 2 to 32 (inclusive) with step 3 use switch block template ``sb_template.csv``
+
+**Important:** The order in which patterns are defined matters, as the first matching pattern is used.
+
+Switch Block Template Files
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Terminology
+-----------
+
+Before describing the template files, let's define some key terminology:
+
+**Lane:** A group of wires with the same length. The starting points of consecutive wires in a lane are one switch block apart.
+
+**Tap:** A switch block location where a wire passes through and can have fan-out connections.
+
+See the figure below for an illustration:
+
+.. figure:: images/lane_and_tap.png
+   :alt: Lane and Tap
+   :width: 50%
+   :align: center
+
+   In the figure above, the taps for the red wire are shown, and the lanes are separated by dotted lines. Note that the figure is simplified for illustration purposes.
+
+In the actual RR Graph, when a wire passes through a switch block, its track number changes. Below is a more realistic example:
+
+.. figure:: images/lane_and_tap_realistic.png
+   :alt: Lane and Tap Realistic
+   :width: 50%
+   :align: center
+
+   A more realistic example of a lane and a tap. Each box contains a lane.
+
+Template File Format
+--------------------
+
+There should be a directory containing the pattern files specified in the switch block maps file. Each template is a CSV file with the following format:
+
+* **Rows** represent source nodes
+* **Columns** represent sink nodes
+* An **'x' mark** at an intersection indicates that the source and sink nodes are connected
+* A **number** at an intersection indicates that the nodes are connected with the switch delay specified by that number
+
+**Note:** The pattern currently only supports uni-directional segments. Therefore, wires can only be driven from their starting point.
+
+Row Headers (Source Nodes)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Each row has four header columns describing the source node:
+
+1. **Column 1 - Direction:** The side from which the source node is entering the switch block (e.g., 'left', 'right', 'top', 'bottom')
+2. **Column 2 - Segment Type:** The segment length to which the source node belongs
+3. **Column 3 - Lane Number:** The lane to which the source node belongs (see terminology above)
+4. **Column 4 - Tap Number:** Which tap of the source node is at this switch block
+
+Column Headers (Sink Nodes)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Each column has header rows describing the sink node:
+
+1. **Row 1 - Direction:** The side from which the sink node is exiting the switch block
+2. **Row 2 - Segment Type:** The segment length to which the sink node belongs
+3. **Row 3 - Fan-in:** The fan-in of the sink node (optional)
+4. **Row 4 - Lane Number:** The lane to which the sink node belongs
+
+Example
+^^^^^^^
+
+Let’s consider an architecture with a channel width of 16 that contains only wire segments of length 4.
+
+The number of rows should be calculated as:
+
+- ``4`` (number of sides) × ``160/2`` (number of tracks in one direction) = ``320`` rows.
+
+On each side, there should be:
+
+- ``20`` lanes (``80 / 4``), and
+- each lane requires ``4`` rows (because length-4 wires require 4 tap positions).
+
+For the columns, the count should be:
+
+- ``4`` (number of sides) × ``160/2`` (number of tracks in one direction) ÷ ``4`` (number of starting tracks per lane) = ``80`` columns.