n_tty: Process echoes in blocks
Byte-by-byte echo output is painfully slow, requiring a lock/unlock cycle for every input byte. Instead, perform the echo output in blocks of 256 characters, and at least once per flip buffer receive. Enough space is reserved in the echo buffer to guarantee a full block can be saved without overrunning the echo output. Overrun is prevented by discarding the oldest echoes until enough space exists in the echo buffer to receive at least a full block of new echoes. Signed-off-by: NPeter Hurley <peter@hurleysoftware.com> Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
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