README: Flow
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README
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README
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@ -389,10 +389,47 @@ If the device can write, the field object provides the following signals:
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- dev_w, which provides the value to be written into the field.
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- dev_we, which strobes the value into the field.
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Migen Flow
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Migen Flow (TODO)
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==========
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Many hardware acceleration problems can be expressed in the dataflow
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paradigm, that is, using a directed graph representing the flow of data
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between actors.
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(TODO: there is no implementation of Migen Flow in the repository yet)
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Actors in Migen are written directly in FHDL. This maximizes the
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flexibility: for example, an actor can implement a DMA master to read
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data from system memory. It is conceivable that a CAL [7] to FHDL
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compiler be implemented at some point, to support higher level
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descriptions of some actors and reuse of third-party RVC-CAL
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applications. [8] [9] [10]
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Actors communicate by exchanging tokens, whose flow is typically
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controlled using handshake signals (strobe/ack).
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Each actor has a "scheduling model". It can be:
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- N-sequential: the actor fires when tokens are available at all its
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inputs, and it produces one output token after N cycles. It cannot
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accept new input tokens until it has produced its output. A
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multicycle integer divider would use this model.
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- N-pipelined: similar to the sequential model, but the actor can
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always accept new input tokens. It produces an output token N cycles
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of latency after accepting input tokens. A pipelined multiplier would
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use this model.
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- Dynamic: the general case, when no simple hypothesis can be made on
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the token flow behaviour of the actor. An actor accessing system
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memory on a shared bus would use this model.
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Migen Flow automatically generates handshake logic for the first two
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scheduling models. In the third case, the FHDL descriptions for the logic
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driving the handshake signals must be provided by the actor.
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If sequential or pipelined actors are connected together, Migen Flow will
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attempt to find a static schedule, remove the handshake signals, optimize
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away the control logic in each actor and replace it with a centralized
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FSM implementing the static schedule.
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An actor can be a composition of other actors.
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Actor graphs are managed using the NetworkX [11] library.
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References:
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@ -402,6 +439,12 @@ References:
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[ 4] http://www.xilinx.com/publications/archives/xcell/Xcell77.pdf p30-35
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[ 5] http://cdn.opencores.org/downloads/wbspec_b4.pdf
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[ 6] http://www.ohwr.org/projects/wishbone-gen
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[ 7] http://opendf.svn.sourceforge.net/viewvc/opendf/trunk/doc/
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GentleIntro/GentleIntro.pdf
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[ 8] http://orcc.sourceforge.net/
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[ 9] http://orc-apps.sourceforge.net/
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[10] http://opendf.sourceforge.net/
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[11] http://networkx.lanl.gov/
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Practical information
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=====================
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