OpenCores Ethernet MAC has a relatively simple interface, and is
already supported in QEMU. This makes it a good candidate for enabling
network support when running IDF apps in QEMU, compared to the
relatively more complex task of writing a QEMU model of ESP32 EMAC.
This driver is written with QEMU in mind: it does not implement or
handle things that aren't implemented or handled in the QEMU model:
error flags, error interrupts. The transmit part of the driver also
assumes that the TX operation is done immediately when the TX
descriptor is written (which is the case with QEMU), hence waiting for
the TX operation to complete is not necessary.
For simplicity, the driver assumes that the peripheral register
occupy the same memory range as the ESP32 EMAC registers, and the
same interrupt source number is used.
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