GP-2221 Prevent receive errors on wwan0 when MTU is less than 1500

Changing the MTU for wwan affects the "rx_urb_size" value. When
"rx_urb_size" (effectively an MRU) is too low, the host rejects
packets from the radio, causing a packet loss, connection issues,
increasing interface error counters, and (potentially) high load on
the host CPU to the point when the host effectively stops
responding to user input (reproduced on MTCDT-L4E1 with Atmel).

Decouple MTU and "MRU" by setting "rx_usb_size" to a relatively
high value that is different from MTU.

NOTE: The target kernel in 6.3.x is linux-at91_5.4.199 as of the
time of writing.

Assumptions:

- Cellular networks never send packets bigger than 1500 bytes;
- the radio may send a packet bigger than the MTU size
  reported in wds-get-current-settings;
- mLinux and mPower do not use packet aggreggation (like qmap),
  hence no need in MTU/MRU sizes bigger than 1500.

0 files changed, 0 insertions, 0 deletions