Add GPDI pins to ulx3s.py

[GuzTech]

This adds the GPDI pins to the ULX3S platform. The constraints were taken from https://github.com/emard/ulx3s-misc/blob/master/constraints/ulx3s_v20.lpf

[whitequark]

What is "GPDI"?

[GuzTech]

It is the HDMI port (but it's called the General Purpose Differential Interface for licensing reasons).

[awygle]

It would be nice to be consistent with other HDMI resources, even if the name is different. That would mean collecting the signals into a single Resource with Subsignals. I think the Arty Z7 offers the most complete example:

        Resource("hdmi_rx", 0,                                  # J10
            Subsignal("cec", Pins("H17", dir="io")),
            Subsignal("clk", DiffPairs("N18", "P19", dir="i"),
                Attrs(IOSTANDARD="TMDS_33")),
            Subsignal("d",   DiffPairs("V20 T20 N20", "W20 U20 P20", dir="i"),
                Attrs(IOSTANDARD="TMDS_33")),
            Subsignal("hpd", Pins("T19", dir="o")),
            Subsignal("scl", Pins("U14", dir="io")),
            Subsignal("sda", Pins("U15", dir="io")),
            Attrs(IOSTANDARD="LVCMOS33")),

Can you please make this resource follow this style?

[GuzTech]

Since the GPDI HDMI port is directly connected to the FPGA and can therefore be used as a source as well as a sink, do you think it would be better to define a single resource as bidirectional, or as two separate resources (one for rx, one for tx)?

[awygle]

The convention (derived from the atlys.py and supercon19badge.py files) appears to be a single resource. Compare/contrast:

        Resource("hdmi", 2, # J3, input only due to on board buffer, HDMI A connector
            Subsignal("scl",     Pins("M16"), Attrs(IOSTANDARD="I2C")),
            Subsignal("sda",     Pins("M18"), Attrs(IOSTANDARD="I2C")),
            Subsignal("clk",     DiffPairs("H17", "H18", dir="i")),
            Subsignal("d",       DiffPairs("K17 L17 J16", "K18 L18 J18", dir="i")),
            Attrs(IOSTANDARD="TMDS_33"),
        ),
        Resource("hdmi", 3, # JA, input/output as it is unbuffered, HDMI D connector
            Subsignal("scl",     Pins("C13"), Attrs(IOSTANDARD="I2C")),
            Subsignal("sda",     Pins("A13"), Attrs(IOSTANDARD="I2C")),
            Subsignal("clk",     DiffPairs("T9", "V9")),
            Subsignal("d",       DiffPairs("R3 T4 N5", "T3 V4 P6")),
            Attrs(IOSTANDARD="TMDS_33"),
        ),

[GuzTech]

All right, one resource it is. In your first example, the directions are explicit, but in the second one they are implicit for "hmdi, 3". Which one is preferred?

[awygle]

I'd recommend being explicit, I think it causes less confusion down the line.

[awygle]

Thanks a lot! This looks good to me now, when whitequark gets back from vacation we'll see what she says :)

[whitequark]

Thank you!