A very specific and technical topic!
Chapter 4. Circuit diagram * A. B. C. D. A. B. C. D. Date: KiCad E.D.A. kicad (5.99.0-6751-g76ac8b5acf) Rev: 1. Size: A4. Id: 1/7. Raspberry Pi Raspberry Pi Compute Module 4 IO Board - Farnell cm4 94v0 boardview exclusive
The PCIe differential pairs on the CM4 are tightly controlled. By inspecting the boardview, you can see how the designers avoided vias to minimize impedance mismatch. Tip for designers: A very specific and technical topic