Interposer tag #3266

AmirhosseinPoolad · 2025-09-08T18:57:09Z

This PR contains an initial proposal for a new interposer tag. To define an interposer cut, you would add an <interposer_cut dim=x|y loc=val> tag under a <layout> tag that defines the location of the cut and the axis of the cut. This tag would then have multiple <interdie_wire> children specifying the connections between the two sides of the cut-line.

<interdie_wire sg_name="interposer_conn_sg" sg_link="X4" offset_start="0" offset_end="2" offset_increment="1" num=50/>

sg_name: name of the scatter-gather pattern to be used for the connections
sg_link: name of the sg_link to be used for the connections
offset_start,end,increment: Connections can be laid out in various offsets from the cut-line, these options
num: number of scatter-gather patterns to be instantiated at each switchblock location

Example:

Note that in this example we have 3 different switchblock locations and 3*50=150 sg patterns would be instantiated.

vaughnbetz

Suggest doxygen comments right away ...

libs/libarchfpga/src/interposer_types.h

vaughnbetz · 2025-09-08T20:37:09Z

Syntax of the interposer tag looks good. You'll need to add documentation for it. Make sure you explain that the offset etc. defines the start points of the sg_links (they always go towards the interposer, even if bidirectional).
Once this is done, we should make a new section in the architecture tutorial showing how to model a sample interposer device.

AmirhosseinPoolad · 2025-09-15T14:19:14Z

I think the code is ready for review now! @vaughnbetz @soheilshahrouz

vaughnbetz · 2025-09-15T16:37:35Z

doc/src/arch/reference.rst

+.. arch:tag:: <interposer_cut dim=x|y loc="int"/>
+
+    :req_param dim: Dimension or axis of the cut. 'X' or 'x' means a horizontal cut while 'Y' or 'y' means a vertical cut.
+    :req_param loc: Location of the cut. Currently only absolute positions are supported.


Can this be an equation, or does it have to be a number? Should say ...

Or maybe that's what absolute positions means?

Yes, it was absolute versus relative [to device height/width] position, as in numbers only. Changed the comment to say "integer values" instead.

vaughnbetz · 2025-09-15T16:40:36Z

doc/src/arch/reference.rst

+    .. figure:: scatter_gather_images/interposer_diagram.png
+        
+        An example of how specifying interposers in VTR works. Connections between the two sides of a cut are first severed. The two sides are then reconnected using scatter_gather patterns. Note that there are 'num' of each pattern at each switchblock locations.
+


I think the figure should show the die-crossing wires at each x location, since that's what would happen.

On the figure or in the text, say the length of the sg_link wire in this case is 3.

I assume the interposer cuts all layers? Should say that too. Or say it only supports 1 layer architectures for now.

I'm not entirely sure about what happens with 3D architectures and interposers. The idea I had was that each layer could have it's own independent cut lines to keep everything flexible. This might make physical sense for a 2 layer face-to-face stacked chip, but I'm not sure about that either.

Right now the code does independent cut lines for each layer, but I could change it if you think it wouldn't make physical sense.

I'm fine with that. I'd mention that somewhere in the docs, unless it is super-obvious from its position in the various tags.

vaughnbetz · 2025-09-15T16:43:27Z

libs/libarchfpga/src/interposer_types.h

+ */
+struct t_interposer_cut_inf {
+    e_interposer_cut_dim dim;                        ///< Dimension or axis of interposer cut
+    int loc;                                         ///< Location of the cut on the grid


Give an example for this one. If the interposer cut is at y = 13, is the cut above or below the tiles with coordinate y = 13?

vaughnbetz · 2025-09-15T16:45:51Z

libs/libarchfpga/src/interposer_types.h

+struct t_interdie_wire_inf {
+    std::string sg_name;  ///< Name of the scatter-gather pattern to be used for the interdie connection
+    std::string sg_link;  ///< Name of the scatter-gather link to be used for the interdie connection
+    int offset_start;     ///< Starting point of scatter-gather instantiations


I think you can beef up these comments a bit. If the interposer cut is at y = 13, is an offset of 0 for an upward going sg_link a switch block / wire start point at y = 13? For a downward going wire with a cut at y = 13, is an offset of 0 a sg_link wire start point at y = 14?
Are the start and end points inclusive (I think they are, but you should say that).

libs/libarchfpga/src/read_xml_arch_file_interposer.cpp

This commit adds parsing logic for defining interposer-based architectures.

vaughnbetz

I suggest a few more commenting updates, and it would be good to get the grid_def data structure in the online documentation (maybe as another item in the grid tab).

Or, maybe we should put the architecture file capturing structs (grid_def and interposer) in a new Doxygen tab for architecture constructs?

vaughnbetz · 2025-09-18T18:23:39Z

libs/libarchfpga/src/grid_types.h

+ *                            <-------------->
+ *                                 repeatx
+ *
+ * startx/endx and endx/endy define a rectangular region instances dimensions.


instances -> instance's

vaughnbetz · 2025-09-18T18:27:18Z

libs/libarchfpga/src/grid_types.h

+
+    std::string block_type; //The block type name
+
+    int priority = 0; //Priority of the specification.


Make these member comments Doxygen format

vaughnbetz · 2025-09-18T18:28:48Z

libs/libarchfpga/src/grid_types.h

+ * and 'H' (device height). Formulas can be evaluated using parse_formula()
+ * from expr_eval.h.
+ */
+struct t_grid_loc_spec {


I think we should build these into the grid part of the VPR API documentation on the web. You'd need to edit some doxyfile for that, and add an overview at the top of this struct saying this is used to capture the user's specification of the architecture will eventually be turned into a flattened grid.

vaughnbetz · 2025-09-18T18:28:58Z

libs/libarchfpga/src/grid_types.h

+                                     // that come from a common definition.
+};
+
+enum class GridDefType {


Add Doxygen comment.

vaughnbetz · 2025-09-18T18:30:44Z

libs/libarchfpga/src/physical_types.h


 struct t_layer_def {
-    std::vector<t_grid_loc_def> loc_defs; //The list of block location definitions for this layer specification
+    std::vector<t_grid_loc_def> loc_defs;              ///< List of block location definitions for this layer specification


It would be good to make an overall Doxygen comment for what this struct is intended for.

AmirhosseinPoolad · 2025-09-18T21:59:37Z

Added the requested changes. For now, I added the doxygen documentation for the grid structs in the grid tab of VPR, but it might be useful to have a LibArchFPGA specific tab. Unfortunately much of LibArchFPGA is not Doxygen documented so it won't be much of a tab.

soheilshahrouz · 2025-09-18T22:29:42Z

Is there a test for parsing this new tag?

vaughnbetz · 2025-09-18T23:10:31Z

I like the idea of a libarchfpga tab, even if it is currently sparse. Could you add that?

@soheilshahrouz is also right that you should add a parsing test.

AmirhosseinPoolad · 2025-09-19T20:06:21Z

Added a test and also a libarchfpga tab to docs. The added tab is pretty sparse at the moment, but I think fixing that would go beyond the scope of this PR.

vaughnbetz

Looks good, but I have two minor nits about the high-level doxygen comments. If you fix those, it's good to merge.

vaughnbetz · 2025-09-19T21:32:13Z

doc/src/api/libarchfpga/physical_types.rst

+Physical Types
+========
+
+These types are used to capture user's intended architecture specification and will be later turned into a flattened device grid according to the device's size.


This comment isn't right ; this goes with the grid defs, but not the physical types.

I used the grid defs page as a template for the other pages and forgot to change this one's description. Fixed it now.

vaughnbetz · 2025-09-19T21:32:41Z

doc/src/api/libarchfpga/scatter_gather_types.rst

+Scatter-Gather Types
+========
+
+These types store information about about scatter-gather patterns


about scatter-gather routing patterns (I'd add the word routing).

github-actions bot added libarchfpga Library for handling FPGA Architecture descriptions lang-cpp C/C++ code labels Sep 8, 2025

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github-actions bot added the docs Documentation label Sep 14, 2025

AmirhosseinPoolad force-pushed the interposer_parsing branch from 5920582 to 5b4046f Compare September 14, 2025 21:53

AmirhosseinPoolad changed the title ~~[RFC] [WIP] Interposer tag~~ Interposer tag Sep 14, 2025

AmirhosseinPoolad force-pushed the interposer_parsing branch from 5b4046f to 47ec730 Compare September 14, 2025 22:02

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AmirhosseinPoolad added 5 commits September 15, 2025 18:53

Add interposer tag parsing

4ea84e9

This commit adds parsing logic for defining interposer-based architectures.

Change interposer scatter-gather offset to use expressions

bd53a1b

Improve comments

3459bbe

Enhance interposer diagram

18193e9

Fix circular dependency between physical_types and interposer_types

1230745

AmirhosseinPoolad force-pushed the interposer_parsing branch from 4527776 to 1230745 Compare September 15, 2025 22:53

Fix compile error for t_grid_loc_spec constructor

6911986

vaughnbetz approved these changes Sep 18, 2025

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AmirhosseinPoolad added 2 commits September 18, 2025 16:21

Add comments and rename GridDefType

7b887c3

Add grid specification types to documents

6d3a21d

github-actions bot added the VPR VPR FPGA Placement & Routing Tool label Sep 18, 2025

Add LibArchFPGA tab to doxygen generated docs

0298a69

AmirhosseinPoolad force-pushed the interposer_parsing branch from ccf22b6 to 7f108d2 Compare September 19, 2025 20:06

AmirhosseinPoolad force-pushed the interposer_parsing branch from 7f108d2 to d8dce13 Compare September 19, 2025 20:07

Add interposer regression test

b257a62

AmirhosseinPoolad force-pushed the interposer_parsing branch from d8dce13 to b257a62 Compare September 19, 2025 20:07

Merge branch 'master' into interposer_parsing

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vaughnbetz approved these changes Sep 19, 2025

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Improve LibArchFPGA doxygen documentation

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vaughnbetz merged commit 35542cd into master Sep 20, 2025
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vaughnbetz deleted the interposer_parsing branch September 20, 2025 01:51

		.. figure:: scatter_gather_images/interposer_diagram.png

		An example of how specifying interposers in VTR works. Connections between the two sides of a cut are first severed. The two sides are then reconnected using scatter_gather patterns. Note that there are 'num' of each pattern at each switchblock locations.


		std::string block_type; //The block type name

		int priority = 0; //Priority of the specification.

Interposer tag #3266

Interposer tag #3266

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AmirhosseinPoolad commented Sep 8, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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vaughnbetz commented Sep 8, 2025

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AmirhosseinPoolad commented Sep 18, 2025

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AmirhosseinPoolad commented Sep 8, 2025 •

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