Week 9: Weekly Report

Continuing with my regular weekly report, I am now in 9th week of Google Summer of Code. My regular college semester has started from this week and I am required to go to lab for my thesis work, so there haven’t been a lot of updates here. Hoping to complete a lot of things during the upcoming weekend. This report pertains to work done from 16th to 22nd July.

First half of the week was spent fixing errors and issues in previous work. One issue I was facing since a long time was the regarding testing and porting to litex. Like always the errors were some silly mistake on my part, and took longer than required to debug.

Testing of float16 using CSR:

The idea was to use the pipelined hardware block to compute floating point multiplication of two numbers in hardware. The numbers were given through firmware by using CSR registers defined in hardware. After correctly initialising floatmult datapath in target/opsis_base.py, and adding the relevant csr functions in ci.c, I expected the output to be correct but somehow input was not transferring across. Which I thought was because incorrect implementation of CSR registers as wires, but it turned out way of connecting at input side was incorrect.

   # Incorrect way
        self.comb += [
            self._float_in2.storage.eq(sink.in1),
            self._float_in2.storage.eq(sink.in2),
            self._float_out.status.eq(source.out)
        ]

   # Correct way
        self.comb += [
            sink.in1.eq(self._float_in1.storage),
            sink.in2.eq(self._float_in2.storage),
            self._float_out.status.eq(source.out)
        ]

Porting migen designs to litex for review

I was facing an error while porting my floating point arithmetic migen code to litex for review. Command line output of that error, link. Since this was coming up only while converting to litex, I thought there was some error with litex functionality. Later found out that this was because int variables being generated using numpy functions and hence the int variables were of numpy.uint type which was not correctly recognise by Signal object.

Although I had started working on run length encoding things after this, my mentor mithro suggested that I should first look at implementing mixer equation using a static mask for now.

Video Pipeline:

The first step was to add all the submodules to video pipeline to check that they follow the timing constraints. I found that the existing color space conversion was done in gateware/hdmi_in/analysis.py in the FrameExtraction class. For testing purpose I added the rgbtofloat16 module and reverse module to this pipeline, connecting output of one to input to another in the hope of getting the exact same input. This also meant that the float16 color space conversion modules were tested in the hardware for the first time. The next step was to subsequently test floatmult and floatadd units by adding them in pipeline.

There were some issues in uploading the bit stream, which I eventually circumvented and here is a link to picture output when all the things are added in video pipeline. The picture is not too sharp, and I feel that there is a missing color in the gradient. I am gonna check this for static image in simulation.

Issue: The generated bitstream (which took close to 20 minutes each time), once uploaded didn’t work and the USB connection was even lost after this. My mentor mithro suggested that I should use the lightweight videomixer target, ($echo TARGET=video), and the problem might be because of FPGA resetting FX2 IC after it boots. This was basically bypassing the problem.

Task for Upcoming Week:

Right now I am trying to figure out where exactly to add video mixer equation in the bunch of gateware python files. I think I should add this at output side, because in that way I can operate on pattern input as well. The idea is that I will setup the standard video pipeline with one input for floatadd left open, for hdmi_out as well. And in the target/opsis_video.py file I will connect the unconnected input of floatadd of hdmi_out0 to output of floatmult of hdmi_out1 and vice versa. Once this is figured out, I will add CSR register to set the multiplier value from firmware.

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