How is data transferred from memory to PCIe cards?

[shaiko]

My question is about data transfer between a PCIe peripherial and system memory. Example: Suppose we need to send a big chunk of data ( stored in system memory ) over an Ethernet network.

How will it usually be done ? Will the Ethernet card's controller have to request the data via "Bus Mastering" ( after being programmed by the CPU to do so ) ? Or will a centralized DMA controller near the CPU just write the card's buffers ( after being programmed by the CPU to do so ) ?

In other words: Does the card have to ask for the data ? Or can it simply be written to it via the CPU's DMA ?

 

[summitville]

Exactly how any one specific NIC operates, is Driver dependent.

Dolphin Driver uses both PIO & RDMA ...
https://www.dolphinics.com/products/fast_TCP_UDP_IP_network_driver.html

Some Drivers may turn Interrupts ON and OFF, as needed, to optimize throughput.

In a server, a NIC has a dedicated PCIe direct connection to the Motherboard Chipset, a NIC in a desktop does not.

Typically, the NIC DRIVER programs the PCIe NIC hardware to become a DMA Master.
The NIC Driver allocates the RAM space and also manages the Message Buffer pointers.

 

[shaiko]

Typically, the NIC DRIVER programs the PCIe NIC hardware to become a DMA Master.
The NIC Driver allocates the RAM space and also manages the Message Buffer pointers.

So if we examine the TLP layer messages running over the PCIe link - we'll usually see the NIC's hardware asking for data from the South Bridge and the South Bridge responding to it ?
As opposed to having the South Bridge sending TLP write requests to the NIC...