PYNQ开发板使用-使用DMA进行数据搬移（Simple DMA transfer 模式）

       该篇是学习使用PYNQ开发板，实际上是对ZYNQ PL端AXI_CDMA 核的应用。实验步骤参照官网的教程，一步一步地做，但是由于在硬件资源布置方面与官方教程稍有出入，所以在SDK的源码里也进行了修改。

AXI_CDMA特性：

       如果是使用ZYNQ 7系列芯片（可能其他Xilinx也是通用的），xilinx的AXI_CDMA 核有两种传输模式：轮询（poll）和中断（intr）；这两种传输模式指的是arm应如何获取DMA的传输状态，如果是轮询模式，就需要arm查询DMA是否bussy；如果是中断模式，那么在DMA传输完成时会触发外部中断，此时需要相应的中断处理函数来填写DMA传输状态。

       axi_cdma还有两种工作状态：Simple DMA transfer和Scatter gather (SG) DMA transfer，简单传输适合任务压力不大的情况；分散收集传输，实际上是AXI_CDMA的并发特性，可以并行传输多包数据，此时要求传输的数据必须是一种特定的数据类型下的实体对象。

      综合上述情况，AXI_CDMA至少就有四种应用场景分别为：Simple DMA transfer_poll、Simple DMA transfer_intr、SG DMA transfer_poll和SG DMA transfer_intr。

以下是axi_CDMA的文档，如果安装了SDK就可以直接在板级支持包（BSP）的system.mss文件中找到链接：

axicdma_v4_5 Documentation
This is the driver API for the AXI CDMA engine. For a full description of the features of the AXI CDMA engine, please refer to the hardware specification. This driver supports the following features:
Simple DMA transfer
Scatter gather (SG) DMA transfer
Interrupt for error or completion of transfers
For SG DMA transfer:
Programmable interrupt coalescing
Programmable delay timer counter
Managing the buffer descriptors (BDs)
Two Hardware Building Modes
The hardware can be built in two modes:
Simple only mode, in this mode, only simple transfers are supported by the hardware. The functionality is similar to the XPS Central DMA, however, the driver API to do the transfer is slightly different.
Hybrid mode, in this mode, the hardware supports both the simple transfer and the SG transfer. However, only one kind of transfer can be active at a time. If an SG transfer is ongoing in the hardware, a submission of a simple transfer fails. If a simple transfer is ongoing in the hardware, a submission of an SG transfer is successful, however the SG transfer will not start until the simple transfer is done.
Transactions
The hardware supports two types of transfers, the simple DMA transfer and the scatter gather (SG) DMA transfer.
A simple DMA transfer only needs source buffer address, destination buffer address and transfer length to do a DMA transfer. Only one transfer can be submitted to the hardware at a time.
A SG DMA transfer requires setting up a buffer descriptor (BD), which keeps the transfer information, including source buffer address, destination buffer address, and transfer length. The hardware updates the BD for the completion status of the transfer. BDs that are connected to each other can be submitted to the hardware at once, therefore, the SG DMA transfer has better performance when the application is doing multiple transfers each time.
Callback Function
Each transfer, for which the application cares about its completion, should provide with the driver its callback function. The signature of the callback function is as the following:
void XAxiCdma_CallBackFn(void *CallBackRef, u32 IrqMask, int *NumPtr);
Where the CallBackRef is a reference pointer that the application passes to the driver along with the callback function. The driver passes IrqMask to the application when it calls this callback. The NumPtr is only used in SG mode to track how many BDs still left for this callback function.
The callback function is set upon transfer submission:
Simple transfer callback function setup:
Only set the callback function if in interrupt mode.
For simple transfers, the callback function along with the callback reference pointer is passed to the driver through the submission of the simple transfer: 
 XAxiCdma_SimpleTransfer(...)
SG transfer callback function setup: For SG transfers, the callback function and the callback reference pointer are set through the transfer submission call: 
 XAxiCdma_BdRingToHw(...)
Simple Transfers
For an application that only does one DMA transfer at a time, and the DMA engine is exclusively used by this application, simple DMA transfer is sufficient.
Using the simple DMA transfer has the advantage of ease of use comparing to SG DMA transfer. For an individual DMA transfer, simple DMA transfer is also faster because of simplicity in software and hardware.
Scatter Gather (SG) Transfers
For an applicatio