[USB] Fixes an issue with Serial when receiving consecutive multiple 64-byte transmissions from Host #1336
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…an issue with Serial on OSX when consecutive multiple 64-byte (CDC_DATA_OUT_PACKET_SIZE) transmissions from Host occur, causing it to no longer receive any data at all if RX queue is not cleared fast enough and multiple NAKs are sent to the Host
technobly
approved these changes
May 25, 2017
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Problem
With RX queue full, the device NAKs OUT (Host->Device) transfers. With
bIntervalset to0(1 ms) this causes the device to spend too much time in USB interrupt, not giving any processing time to the application/system thread to consume RX queue. For some reason CDC driver on OSX doesn't attempt to send any data after about 300 NAKed transfers. Closing/re-opening the port doesn't help either.Solution
Set
bIntervalto a maximum value (16 ms) on OUT endpoint to give enough time for the device to consume RX queue and not send too many NAKs to Host.We should probably add some workaround as well: e.g. count the number of NAKs sent and silently drop incoming data after a certain threshold instead of NAKing it. Even though this is OSX driver's fault. 🤕
Steps to Test
The bug is easily reproducible on OSX when setting WPA Enterprise credentials over Serial. A simple copy-paste of a certificate (e.g. CA) will not complete and get stuck somewhere half-way through.
Example App
N/A
References
N/A
Completeness
Bug fix
[PR #1336]Fixes an issue with Serial when receiving consecutive multiple 64-byte transmissions from Host