usb-device-cdc: Fix lost data in read() path if short reads happened. #886
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If the CDC receive buffer was full and some code read less than 64 bytes (wMaxTransferSize), the CDC code would submit an OUT transfer with N<64 bytes length to fill the buffer back up. However if the host had more than N bytes to send then it would still send the full 64 bytes (correctly) in the transfer. The remaining (64-N) bytes would be lost. Adds the restriction that CDCInterface rxbuf has to be at least 64 bytes. Fixes issue micropython#885. This work was funded through GitHub Sponsors. Signed-off-by: Angus Gratton <[email protected]>
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If the CDC receive buffer was full and some code read less than 64 bytes (wMaxTransferSize), the CDC code would submit an OUT transfer with N<64 bytes length to fill the buffer back up.
However if the host had more than N bytes to send then it would still send the full 64 bytes (correctly) in the transfer. The remaining (64-N) bytes would be lost.
Fix is to not send a new transfer until 64 bytes is free. Adds the restriction that CDCInterface rxbuf has to be at least 64 bytes.
Closes #885.
This work was funded through GitHub Sponsors.