[teensy4] Why are the same USB endpoints used for both RX & TX

[forderud]

The teensy4 usb_desc.h contain a mix of configurations where the same USB endpoint number is used for both transmit & receive for a given interface, and configurations where different endpoint numbers are used. The former puzzles me, since I though USB data endpoints were supposed to be unidirectional.

Some examples of same endpoint numbers:

• USB_DUAL_SERIAL defines both CDC_RX_ENDPOINT and CDC_TX_ENDPOINT to 3.
• USB_DUAL_SERIAL defines both CDC2_RX_ENDPOINT and CDC2_TX_ENDPOINT to 5.
• USB_KEYBOARDONLY defines both SEREMU_TX_ENDPOINT and SEREMU_RX_ENDPOINT to 2.
• USB_MIDI defines both SEREMU_TX_ENDPOINT and SEREMU_RX_ENDPOINT to 2.
• USB_MIDI defines both MIDI_TX_ENDPOINT and MIDI_RX_ENDPOINT to 3.
• USB_FLIGHTSIM defines both FLIGHTSIM_TX_ENDPOINT and FLIGHTSIM_RX_ENDPOINT to 3.

It seems endpoint number reuse is enabled by a bEndpointAddress =| 0x80 in the USB configuration descriptor, which increases the actual endpoint number by 128.

Some examples of different endpoint numbers:

• USB_SERIAL defines CDC_RX_ENDPOINT to 3 and CDC_TX_ENDPOINT to 4.
• USB_RAWHID defines RAWHID_TX_ENDPOINT to 3 and RAWHID_RX_ENDPOINT to 4.
• USB_EVERYTHING contains a commented-out MTP_TX_ENDPOINT define to 11 and MTP_RX_ENDPOINT define to 3.

Could it make sense to either always use the same endpoint number for TX & RX, or always use different numbers?

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