Support IOB tiles with only one site (IOB_SING)

[mcmasterg]

Fixes #341

These were held off because they generate some weird special cases I didn't want to think about at the time. So here's a proposed solution.

First, the problem: in a7, IOBs are typically in LIOB33 tiles. These have two IOB sites, just like a SLICE has several CLBs. However, unlike CLBs, these IOBs are sometimes split into LIOB33_SING tiles, which only have a single IOB33. The problem is that in some cases the single instance is the Y=1 case, which basically starts at word 2 in our LIOB33_SING tile (if you follow the pattern in LIOB33 or want a Y=0 and a Y=1 LIOB33_SING to have a consistent address space). The issue is that this actually occurs at FDRI word 0, meaning we essentially have a negative tile address (-2) if we want the reference bits, starting at address 2, to be in frame 0.

Sample tilegrid output from running this fuzzer:

//Special bottom entry
//its missing the Y=0 site
"LIOB33_SING_X0Y100": {
    "bits": {
        "CLB_IO_CLK": {
            "baseaddr": "0x00020000",
            "frames": 42,
            "height": 4,
            "offset": -2,
            "words": 4
        }
    },
//A typical entry
"LIOB33_X0Y101": {
    "bits": {
        "CLB_IO_CLK": {
            "baseaddr": "0x00020000",
            "frames": 42,
            "height": 4,
            "offset": 2,
            "words": 4
        }
    },
...
//Special bottom entry
//its missing the Y=1 site
"LIOB33_SING_X0Y149": {
    "bits": {
        "CLB_IO_CLK": {
            "baseaddr": "0x00020000",
            "frames": 42,
            "height": 2,
            "offset": 99,
            "words": 2
        }
    },

I ran segprint and the 030-iob fuzzer. Both seemed happy with this, so I'm inclined to commit this as is and move forward.

[litghost]

What does the segbits file look like for a negative base word offset?
Do the negative base word offset round trip (bits -> fasm -> bit -> fasm)?

[mcmasterg]

No, there is not a negative segbits file. Only the upper half of the file is used

[litghost]

No, there is not a negative segbits file. Only the upper half of the file is used

Not sure I understand what is being suggested. Are we saying that LIOB33_SING_X0Y100 and LIOB33_SING_X0Y149 will use different database entries? LIOB33_SING_X0Y100 will use Y1 entries and LIOB33_SING_X0Y149 will use Y0 entries?

[mcmasterg]

the latter

[mithro]

IOB33S was actually the main? I would have guess the S and M stood for Master, Slave or Main, Secondary.

[mithro]

If I understand correctly, what you are saying is that;

• Like CLBs, an IOBs has two sites, Y0 and Y1.
• However, LIOB33_SING is a special case of an LIOB33 were either the Y0 or the Y1 site is missing.

How do we deal with this in CLBL verse CLBM. In one case we have two SLICEL and the other case we have SLICEL and a SLICEM? IE The table below;

Tile	Site Y0	Site Y1
CLBL	SLICEL	SLICEL
CLBM	SLICEL	SLICEM

Isn't the IOB just three cases like below?

Tile	Site Y0	Site Y1
IOB33	IOB	IOB
IOB33S_TOP	<empty>	IOB
IOB33S_BOT	IOB	<empty>

Is it really hard to make the tools able to handle the "<empty>" in the first site of a tile?

[JohnDMcMaster]

Tim: is that question for me or @litghost ?

Maybe to answer: the problem is that the tile_type for "IOB33S_TOP" and "IOB33S_BOT" is the same

[litghost]

Tim: is that question for me or @litghost ?

Maybe to answer: the problem is that the tile_type for "IOB33S_TOP" and "IOB33S_BOT" is the same

Exactly, IOB33S_TOP and IOB33S_BOT are both just IOB33_SING. To make matters worse, the existing convention for tile type definition of IOB33_SING doesn't even defined a site Y0 and site Y1, only a site Y0. The current way tile types are generated has no ability to detect whether the _SING tile is a top or bottom and generate a site Y0 only for the top and site Y1 only for the bottom (or vise-versa).

The ideal case would've been for these tiles to shares the same segbits. I assume we've confirmed this is not the case?

[mithro]

Can we just special case IOBS and generate the IOBS_TOP / IOBS_BOT ourselves?

[litghost]

Can we just special case IOBS and generate the IOBS_TOP / IOBS_BOT ourselves?

Sure, we just need to agree on an approach. If we go with iob_top and iob_bot, it changes how the fuzzers will output database data. The original approach was to have both _top and _bot in the same segbit. It's not obvious which requires less rework overall, but I'd say that iob_top and iob_bot is probably most consistent with how the other tiles work.