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Think of the BAR as the interface needed by the host (the RC). The only way the host can talk to the PCIe EP is via the BAR.
Not quite, you need to be clear in your descriptions, so its clear that you understand what is going on.
The DMA controller must be a PCIe bus master and so it generates its own 64-bit addresses. If the RC needs to program the DMA controller, then the DMA control registers might live in BAR2.
The RC needs to program the EP DMA controller registers.
For a simple DMA controller, controller registers have a source address, a destination address, the data length to transfer, a control register (with a "go!" bit) and a status register (with a "done!" bit).
"Real" DMA controllers are more complicated than that. They have scatter-gather buffers, which are basically linked lists of data transfers to perform. The DMA controller will consume the linked list based on other register settings, eg., "do this every time you get an interrupt". This relieves the processor of doing anything ... other than the original setup of the scatter-gather lists. The host (RC) can optionally be interrupted as DMA events occur.
No, if you make your BAR 20G, you will not be able to boot your PC. The BIOS will choke. If you read the notes I linked to in the other thread, I could not boot my EliteBook laptop if I made the BAR too big.
If you want the RC to get data from the 20G drive, then it has to program the DMA controller with source addresses that correspond to the 20G drive (or memory buffers that drive creates), and then transfer those buffers using DMA to the host.
If you dig into the filesystem for your OS, you'll find that it works in pages/sectors, i.e., blocks of bytes. Your DMA controller needs to move a request page of bytes from the PCIe EP over the PCIe bus to the host memory (where it is possibly copied into a page that the filesystem driver gave you).
Something like that anyway ...
Cheers,
Dave
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Really appreciated, I was hoping to avoid using DMA and just make a simple read and write function.
I guess I still have lots of readings to do....T_T.....
So many stuff....
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