mxs-auart gives data from previous run after close and reopen

[Peter Hurley]

On 03/14/2014 04:11 PM, Stanislav Meduna wrote:
> Hi,
>
> following scenario:
>
> - a Freescale i.MX28 machine
> - RS232 AUART looped back Rx - Tx or two different ports cross-connected
> - a test program sending data in one thread and receiving in the other:
>    thread A periodically sends "Quick brown fox jumps over the lazy dog",
>    thread B receives
> - stop the program using ctrl-C
> - restart
>
> => sometimes the receiving thread gets "dogQuick", receiving
>     characters from the previous transmission
>
> The extra characters definitely come from the receiver - the transmission
> is OK. This was verified by a scope.
>
> My theory is that this behaviour is caused by mxs_auart_shutdown
> function gating and mxs_auart_startup reenabling the clock instead
> of doing a soft reset. If the clock is gated while the AUART already
> has something in the FIFO, but did not generate the interrupt yet,
> the internal state machine is frozen in this state. As soon as it
> is reenabled, the characters are delivered to a new user.
>
> I am using a 3.4.77 kernel, but the relevant code looks the same in
> the recent kernels (when not using DMA). I also backported patches
> waiting for the transmission FIFO to clear at mxs_auart_shutdown.
>
> I did not find any possibility to fully clear the receiver - even
> if one disables the AUART and reads everything from the FIFO,
> the character currently in transmit might be still somewhere (the
> reference manual states that disabling is effective after the
> current character is received). Which might be 1 ms at 9600 and one
> has no clue whether it is the case.
>
> Also note that the function mxs_auart_reset is in fact not doing
> a reset - it just makes sure that the AUART comes out of one after
> the initialization. Maybe a full reset at startup is the solution...
>
> Pleas Cc: me when answering.

Does disabling the fifo on shutdown clear the fifo?