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Visitor arminh
Visitor
289 Views
Registered: ‎03-25-2019

XDMA RX Transfers

Hello,

I just wanted to ask if that is correct:
The XDMA in streaming mode does need the correct recv descriptor lenght?
E.g Just as an example:
I have the Axi Ethernet more or less directly connected to the XDMA IP i'm using two channels e.g ctlr and data channel from the axi ethernet and that all through 2 DMA Engines. So now i just wanted to know do i need the correct size e.g get the rx_status from the AxiEthernet first which is fixed size and therefor can be just gathered from the dma and then i need to set the lenght on the dma descriptor for the data and start the engine.

Or does the XDMA IP also allow for usage of bigger sized buffers and just finishes the descriptor even if it didn't reach the full size of it?
Example:
I have a buffer with 1522 bytes and i recv just 80 bytes would this still count as completed or would the dma get stuck?

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3 Replies
Visitor yuripoddobny
Visitor
267 Views
Registered: ‎08-28-2019

Re: XDMA RX Transfers

Hi!

I think in Rx direction DMA will stop getting on AXI4 tlast signal. In this case it may just partially fill the buffer and will set in buffer descriptor EOP bit. You will get interrupt (if enabled) and then to know how many bytes data is exactly in the buffer you are supposed to check C2H writeback location (if WB is enabled). 

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Visitor arminh
Visitor
192 Views
Registered: ‎03-25-2019

Re: XDMA RX Transfers

So what you say is that bytes/lenght in the descritpor is just a max size?
(I mean that's how we expected it to be but some other guy's told us that is not the case and it wait's for ever)

Not very clear by that in the documentation..

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Visitor yuripoddobny
Visitor
139 Views
Registered: ‎08-28-2019

Re: XDMA RX Transfers

For H2C transfers Length field of BD is actual lenth of data to transmit. For C2H transfers Length is length of a buffer associated with a BD, but actual received data can be found in writeback location. Writeback location is defined in Src_adr of BD.  

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