PCIS DMA BAR4 Disable Prefetch

Question

I see that BAR4 (used by PCIS) is prefetchable per this document:  
https://github.com/aws/aws-fpga/blob/master/hdk/docs/AWS_Fpga_Pcie_Memory_Map.md  
  
How can I disable prefetching for that BAR, similar to BAR0 and BAR1? I know prefetch is a FPGA DMA core setting - but I think it can be disabled in software?  
  
I am looking into if/how it could be done in XDMA. I am about to try setting MAX_EXTRA_ADJ=0  (no extra adjacent descriptors to prefetch). https://github.com/aws/aws-fpga/blob/7f1e76765766f579d3a767bedf669019d50342f3/sdk/linux_kernel_drivers/xdma/libxdma.h  
  
Any information would be helpful.  
  
**Update** Apparently for 64b address space BARs (like BAR4) you cannot disable prefetching? Only for 32b address spaces?  
  
Thanks  
  
Edited by: absurdfatalism on Jul 5, 2020 7:26 PM

absurdfatalism · Answer

Posted follow up thread.

awssunil · Answer

Hi absurdfatalism,  
  
The Prefetchable attribute of any of the PCI BAR in AWS F1 cannot be modified by the software.  
  
AWS F1 support the BARs and their attributes listed in the link below  
  
https://github.com/aws/aws-fpga/blob/master/hdk/docs/AWS_Fpga_Pcie_Memory_Map.md  
  
The setting MAX_EXTRA_ADJ under sdk/linux_kernel_drivers/xdma/libxdma.h is unrelared to the BAR prefetchable attribute.  
  
Thanks,  
Sunil.

PCIS DMA BAR4 Disable Prefetch

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