Critical Issue
The DisplayPort specification requires video data to be sent in a transfer unit (TU), which can be 32 to 64 link symbols long. To reduce complexity, the DisplayPort TX uses fixed 64-symbol TU. To avoid the oversubscription of the link bandwidth, the packed data rate must be equal or lower than the link symbol rate. When the packed data rate is lower than the link symbol rate, the DisplayPort TX must perform symbol stuffing which is framed with control symbols FS and FE within TU while all the symbols in the TU before FS must be valid data symbols. The specification also requires that the video data be evenly distributed within the TUs composing a full active video line. If the calculated valid data symbols within the TU is greater than 62.75, the DisplayPort TX indicates the link is oversubscribed and no image output is expected.
Consider a transfer unit example:
Resolution = 1856x1392@75 Hz
Color depth = 18 bpp
Stream clock = 288 MHz
Lane count = 4
Link rate = RBR (1.62Gbps)
TU size = 64
The number of valid symbols within the transfer unit is calculated:
Stream = 18 bpp, 288MHz > Packed data rate over 4 lanes = 18 x 288 / 4 = 1296 Mbit/second/lane
Link bandwidth = 1620 x 0.8 = 1296 Mbit/second/lane
Average valid symbols per TU = 1296 / 1296 x 64 = 64
The packed data rate is equal to the link symbol rate, and the average valid symbols per TU is 64, which reaches the TU size. In this example, the TX mistakenly considerrs the link as oversubscribed and does not produce any image.
To work around this issue, change the link rate and lane count configurations so that the average valid symbols per TU is less than 62.75. For example, to transmit 1856x1392@75 Hz at 18 bpp, you can use HBR (2.7Gbps) with 4 lanes:
Link bandwidth - 2700 x 0.8 = 2160 Mbit/second/lane
Average valid symbols per TU = 1296 / 2160 x 64 = 38.4
This issue is fixed in version 16.0 of the DisplayPort IP core.