The Realtek PHY support is now visible in the 7.3 release-candidate source. That is a concrete upstream milestone, but not certification of every router using the chip.
What the source actually handles
The 7.3-rc2 Realtek driver identifies RTL8261C and RTL8261D through sub-model values. Its C branch selects rtl_nic/rtl8261c.bin; the D branch does not select that firmware. The implementation handles capability discovery, negotiation, status and interrupts. “Both chips supported” therefore does not mean identical initialization requirements.
The firmware patch explains the parameter-loading path and its validation. A kernel package and a firmware package are distinct pieces of a usable installation.
Two connections between software and the wire
An Ethernet MAC processes frames; the PHY handles the physical link. The frame data path between them must be distinguished from MDIO management, which reads and writes PHY registers. The original article incorrectly compressed those roles into a single management-bus description.
Our diagram separates the data path from the control path. A driver can correctly recognize a PHY even while the board’s MAC interface, firmware availability or cable link prevents the intended speed. Conversely, a link reporting 10 Gb/s does not show that the CPU and forwarding software can sustain that rate.
A useful validation sequence
Record the board revision, running kernel and loaded driver. Check initialization errors and any requested firmware, then inspect the negotiated link with the intended peer and cable. Only after that should you measure application throughput, packet loss and behaviour after suspend or a power cycle.
Do not replace a vendor module simply because the new upstream name exists: first establish that your distribution has the relevant code, configuration and board support. The upstream integration reduces a maintenance burden; it does not remove the need to validate the complete device. No hardware throughput or power measurement was performed for this article.
September 8: verify RTL8261C/D in 7.3-rc2, distinguish C firmware selection, and correct the conflation of Ethernet data with MDIO management.