Re: DMA Transfers to PCIe Bar Memory

[Stephen Hemminger <stephen@networkplumber.org>]

On Thu, 7 Nov 2024 13:09:41 +0000
"Wieckowski, Jacob" <Jacob.Wieckowski@vector.com> wrote:

> Hi Jacob,
> 
> 2024-11-07 11:50 (UTC+0000), Wieckowski, Jacob:
> > Hi Dimitry,
> > 
> > I am sorry, I was a bit too far ahead with my thoughts.
> > 
> > We have started a DPDK evaluation project to build knowledge and understanding of the DPDK framework. 
> > We used an E1000 driver with a workaround to gain access to the PCIe 5.0 R-Tile on the Intel Agilex7 FPGA under Windows. 
> > 
> > Accesses to the PCIe BAR work in principle, but we can currently only carry out 64-bit accesses to the BAR memory.  
> > In the PCIe Config Space in the Capabilities Register, a maximum payload size of 512 bytes is configured. 
> > The Intel Core in the FPGA and the Root Complex also support TLPs of this length. 
> > 
> > We use the rte_read32 and rte_write32 functions to access the bar memory, which obviously executes accesses with max 2 DWs. 
> > We were able to trigger this in the FPGA because only TLP with length 2 arrived on the RX Interface. 
> > 
> > How can a block transfer be initiated in DPDK so that TLPs with a length of 128 DWs are generated on the PCIe Bus?  
> 
> Thanks, now I understand what you need.
> Unfortunately DPDK has no relevant API; rte_read**() is for small registers.
> Maybe there's some useful code in "drivers/raw", but this area is completely unknown to me, sorry.
> Try replying with what you wrote above to the thread in users@dpdk.org and Cc: Intel people from MAINTAINERS file responsible for "drivers/raw".

In general, direct access to PCI by applications is discouraged.
All PCI access should be in driver code. What you describe is a bug in the E1000
driver, and should fixed there.