[myhdl-list] Keeping hierarchy for conversion

[Oscar D. <osc...@gm...>]

Hi

I want to start a discussion about keeping the hierarchy on conversion
and how to implement. Although flattening feature of current converter
is a nice "killer feature", I can provide couple examples which it's
needed to keep hierarchy inside a design:

1. Special instances on an FPGA: suppose you need an special component
(my experience is limited to Xilinx, I guess Altera and other vendors
had similar features), for instance a block RAM. Sometimes I need to
explicitly use a block RAM in a particular configuration, I know
exactly how to instantiate it in VHDL, and I can write a MyHDL
equivalent model for simulation. However, when converting, it is
difficult to write code that synthesizer use to infer a block RAM. And
for other special components it can be worse (ICAP, PLL, DCM, and
others).

2. Partial re-configuration [1]: design tools can work only with
structural descriptions because it needs complete information about
boundary signals between sub-modules. A converted MyHDL design only
has a top module with all the sub-modules flattened in multiple
process (or always blocks in Verilog).

It's worth mention that, for the previous examples, it's explicit
stated the need to keep hierarchy, and those components will be
instantiated in the nearest-top component (I mean, in VHDL it will be
some component declarations, instantiations and port mappings).

Now the question is: how to implement it? I've been thinking and
working on it for a long time, and those are some of my thoughts
(disclaimer: since I'm more accustomed to VHDL to Verilog, I'll give
only examples with VHDL, Verilog should be somehow alike):

* Current converter rely on flattening the structure before do actual
conversion. Perhaps a different function is convenient/needed here?
something like "toVHDL_keep_hierarchy()" can run through the
hierarchical structure and call toVHDL() for its internal components.
This is "relatively" easy to implement in the case of simple
hierarchies (for instance, a top module with sub-components), but
mixed "keep-hierarchy" components with "normal" components could be
tricky without deep changes to converter code (something probably not
desirable right now).

* Internally, converter needs to generate a separate output file for
that component and also need component declarations to put in that
file. Perhaps a function that infer the external interface of a
component is needed, something that toVHDL() already do internally to
generate the entity declaration, and would be nice to move to another
function for this purpose.

* user-defined code, if used, must declare almost the entire code
(interface declaration is inferred from the call signature). Other
option is point to an external file, and in this case, its interface
must match the call signature.

Since I was in a hurry to have "keep hierarchy" feature in my
projects, I wrote a custom converter, but it's tightly coupled to my
specific designs (for instance, I use objects instead of functions to
declare components, and my converter looks through those specific
objects and its attributes. Also, there's a separate mechanism for
interface declaration). However, I want to decouple that code for
general use, and I want to hear your opinions.

Best regards, Oscar

PD: <spam warning> Part of the code I mentioned is part of a
open-source project I've been working in, and I think now I can
present it to the list. Its called NoCModel, and it's available on
github (https://github.com/dargor0/nocmodel), also I keep a copy on
opencores. </spam>

[1] http://en.wikipedia.org/wiki/Partial_re-configuration

-- 
Oscar Díaz
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