Accessing External I/O via Direct Hardware Connections

Whereas the previous example demonstrated how applications can access system I/O through memory buffers, a platform can also provide direct hardware connectivity to hardware accelerators within an application generated in the SDSoC Environment. The following figure shows how a functions2mm_data_copycommunicates to the platform via an AXI4-Stream channel, and writes to DDR memory using the zero_copy datamover (implemented as an AXI4 master interface). This design template is calledUnpacketized AXI4-Stream to DDRdesign example in thesamples/platforms/zc702_axis_ioplatform (a similar design for the Zybo board is insamples/xc7z010/zybo_axis_io).

In this example, thezc702_axis_ioplatform proxies actual I/O by providing a free-running binary counter (labeled Platform IP in the diagram) running at 50 MHz, connected to an AXI4-Stream Data FIFO IP block that exports an AXI4-Stream master interface to the platform clocked at the data motion clock (which might differ from the 50 MHz input clock).

The direct I/O software interface can be found in thezc702_axis_io.hheader file located in the SDSoC install directory undersamples/platforms/zc702/aarch32-linux/include.

#pragma SDS data access_pattern(rbuf:SEQUENTIAL) void pf_read_stream(unsigned *rbuf);

In the code snippet below, the application defines a direct signal path from the platform input to a hardware function before transferring the output to memory.

// This function's data flow defines the accelerator network void s2mm_data_copy_wrapper(unsigned *buf) { unsigned rbuf0[1]; pf_read_stream(rbuf0); s2mm_data_copy(rbuf0,buf); }

The platform library provides thepf_read_streamfunction that the sds++ linker maps onto the hardware stream port. Because the only use of therbuf0output is the input to thes2mm_data_copyfunction, the linker creates a direct hardware connection over an AXI4-Stream channel. Because thes2mm_data_copyfunction transfersbufusing the zero_copy data mover, the buffer must be allocated in physically contiguous memory usingsds_alloc, and released usingsds_free.

int main() { unsigned *bufs[NUM_BUFFERS]; for(int i=0; i

A tutorial of how to use this example design is provided inSDSoC Environment Tutorial: Introduction(UG1028).

A detailed tutorial on creating a platform using AXI4-Stream to write memory directly can be found inIntroduction.