Using-PCIe-Write-Combining example returns unexpected results

[itamarblum]

Hi,

I'm an F1 AWS user, and I'm trying to run the PCIE Write Combine example on a cl_dram_dma example design:

https://github.com/awslabs/aws-fpga-app-notes/tree/master/Using-PCIe-Write-Combining

In this example, I'm trying to experiment with the wc_perf.c example to write a single line (16 DWORDS) from the host to the DDRA. According to the above note, running the example with the below command:

**$ sudo ./wc_perf **Followed by:

**$ sudo fpga-describe-local-image -S 0 -C ** outputs the below:

DDR0
** write-count=16
** read-count=0

While running with "-w" option, **$ sudo ./wc_perf -w **Followed by:

**$ sudo fpga-describe-local-image -S 0 -C ** outputs the below:

DDR0
** write-count=1
** read-count=0

meaning, that the write_count when we use the -w (Write Combine option), should have a write count that is reduced from 16 to 1. Quoting from the notes: "The -w option tells wc_perf to use WC, and the number of write data beats was reduced from 16 to 1. This is the reason why writing a WC region with small operations is faster, because they are accumulated into larger chunks using a 64 byte buffer located in the CPU core bus interface (BIU). This is also the reason why it cannot be used for all accesses."

However, when I run the experiment I get different results than the ones described in the notes:

When I either run "sudo ./wc_perf -w" or "sudo ./wc_perf" followed by

"sudo fpga-describe-local-image -S 0 -C"

I get the same write_count of 1600:

DDR0
write-count=1600
read-count=0
DDR1
While I understand why the count is 1600 and not 16 (num_of_passes is 100 in wc_perf.c), the count expect to be when using "-w" option is 100 and not 1600, which means 16 times less than without "-w". Am I doing anything wrong in the experiment?

[czfpga]

Hi @itamarblum,

Thank you for contacting us on this issue. I'll try to see if I can reproduce this on my end. Just curious, which version of the developer kit are you using for testing the WC feature? And which branch are you using?

Thanks,

Chen

[itamarblum]

Hi,
Thank you so much for replying.
HDK shell version shell_v04261818.
The branch we're using is aws-fpga master, but not up to date. the commit is 90bf058
release 1.4.21

[czfpga]

Hi @itamarblum,

Thanks for providing the information. I was able to reproduce the issue on my end. I'll need to investigate this further and keep you posted.

Thanks,

Chen