Support a streaming table

[coretl]

We have a requirement for 50kHz table based position compare. The SEQ block has 4096 rows, and though it is double buffered this would be 11 table writes a second, which gives very little margin for error. Long table support has much bigger buffers, but appending while reading is not currently supported. After discussing the possibilities with @EmilioPeJu and @Araneidae, we think:

• The FPGA side of the long table support (reading from a sequence of DMA blocks) seems suitable for this application, but may be lacking some flow control to say when it is ready for the next block
• There is a need to run PGEN in a stream mode too, but need to retain the current static mode
• If we can make long table support both stream and static mode, then maybe we don't need a short table mode

A possible long term plan is:

• Remove short table support and convert SEQ to use long table mode
• Remove table append support
• Table support (formerly long table support) gains a new MODE attribute that can be fixed or fifo
• In fixed mode it does what it currently does
• In fifo mode then the value is always empty, clearing of the fifo is done from the FPGA side, and writing to the table will append to the fifo

A plan of how to get there:

• Test how fast PGEN can consume the table
• Add the MODE attribute to table support on the table, and make writes append to it if in fifo mode
• Switch the mode, and run the PGEN test again but make sure the buffers are filled in time
• Then add the flow control and clearing to the server and PGEN
• Convert SEQ to use long tables
• Remove short table and append support from server

[coretl]

@tomtrafford