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Subject [GitHub] [incubator-tvm-vta] remotego commented on pull request #7: [pynq_driver] fix device early return
Date Thu, 30 Apr 2020 02:52:21 GMT

remotego commented on pull request #7:
URL: https://github.com/apache/incubator-tvm-vta/pull/7#issuecomment-621581807


   Thanks @huajsj @tmoreau89 for pointing out the potential performance issue for this fix.
   
   The root cause of the issue is because on line 124, we instruct the VTA to start calculation
by writing to the VTA_START bit of control register. The FPGA will then respond by changing
its VTA_COMPUTE_DONE bit of control register from '1' to '0', indicating a "busy" state of
the VTA hardware. And VTA will change it back to '1' after the completion of the calculation.
   
   The original code created a racing condition here between the processor code and the FPGA
hardware, by attempting to check the VTA_COMPUTE_DONE bit immediately after the issuing of
"start" command. If we got a more powerful processor, the checking of VTA_COMPUTE_DONE bit
will happen before FPGA could ever change it to '0'.
   
   As @tmoreau89 mentioned, we are using a pooling approach here. And in my opinion, a short
sleep before checking is quite common in the pooling world. Here we proposed a 1us sleeping,
which translates to around ~250 clock cycles on FPGA, it would give FPGA sufficient time to
change VTA_COMPUTE_DONE from '1' to '0'. As 250 cycles are way shorter than any meaningful
calculations, I believe there should not be any performance impact.
   
   I agree with @tmoreau89 that interrupt is a more robust solution compare to pooling, but
it is quite a huge task involving changes from both HW and driver archs. 


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