AVTP: avtpsink synchronization mechanism
The avtpsink element is expected to transmit AVTPDUs at specific times, according to GstBuffer timestamps. Currently, the transmission time is controlled in software via the rendering synchronization mechanism provided by GstBaseSink class. However, that mechanism may not cope with some AVB use-cases such as Class A streams, where AVTPDUs are expected to be transmitted at every 125 us. Thus, this merge request introduces avtpsink’s own mechanism to improve the transmission time accuracy.
The mechanism implemented in this merge request leverages the socket transmission scheduling infrastructure introduced in Linux kernel 4.19. When supported by the NIC, the transmission scheduling is offloaded to the hardware, improving transmission time accuracy considerably.
To illustrate that, a before-after experiment was carried out. The experimental setup consisted in 2 PCs with Intel i210 card connected back-to-back running an up-to-date Archlinux with kernel 5.3.1. In one host, gst-launch-1.0 was used to generate a 2-minute Class A stream while the other host captured the packets. The metric under evaluation is the transmission interval and it is measured by checking the 'time_delta' information from ethernet frames captured at the receiving side.
The table below shows the outcome for a 48 kHz, 16-bit sample, stereo audio stream. The unit is nanoseconds.
| Mean | Stdev | Min | Max | Range | -------+--------+---------+---------+---------+---------+ Before | 125000 │ 2401 │ 110056 │ 288432 │ 178376 | After | 125000 │ 18 │ 124943 │ 125055 │ 112 |
Before this merge request, the transmission interval mean is equal to the optimal value (Class A stream -> 125 us interval), and it is kept the same after the patch. The dispersion measurements, however, had improved considerably, meaning the system is now consistently transmitting AVTPDUs at the correct time.
Quick note to help the review process: the initial patches do some code refactoring while the main functionality is implemented in the last patch.