Apply the same release/acquire fence pattern as e1000d and
rtl8168d:
- acquire fence before reading the RX status / length / data
bytes, so the compiler does not reorder the buffer reads
past the rxsts observation on weakly-ordered architectures.
- release fence before the CAPR write (RX doorbell) so the
device sees the buffer reads before the doorbell signals
the consumer is done with the ring.
- release fence before the TSD write (TX doorbell) so the
device sees the buffer writes before claiming ownership of
the descriptor.
The 8139 is a legacy 100 Mb/s chip with a 64 KB ring rather
than a 16-entry descriptor table, but the same hand-off
semantics apply: ownership bit is cleared on read, set on
write, and the device accesses buffer memory after the doorbell.
The fences are the same compiler-level ordering the Linux
8139too driver places around its MMIO writes; the 8139 chip
itself does not need anything more.
Red Bear OS
c71e518391