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Date:	Thu, 20 Mar 2014 23:34:11 +0000
From:	"Bollinger, Seth" <>
To:	Peter Hurley <>
CC:	"" <>,
	linux-serial <>
Subject: Re: Possible software flow problem in serial_core

>What hardware is this?

We¹re using the Freescale mxs-auart driver (i.mx28).
>A 16550 transmitter doesn't shut-off at uart->stop_tx(), so the remaining
>data in the transmitter FIFO is still transmitted.

Ok, I just reviewed the 8250 driver and you¹re correct, it appears the
transmitter is only disabled on the 16C950.  I misread before (incorrectly
saw 16C550)...

>Some hardware has (relatively) big FIFOs and allows the transmitter to be
>shutoff, but these should unconditionally re-enable the transmitter
>clearing the xmit fifo.

Do you have a driver that would be a good example of this?  From looking
at serial_core.c:uart_start(), is seems that start_tx() is only called if
the serial_core ring buffer is not empty.  If we don¹t stop the
transmitter, we slew too many characters.

>Are you seeing lost data? Maybe it's because soft flow-control is to
>slow to shutoff the sender?

No dropped bytes, but we do see a few bytes trapped in the fifo because we
received an XOFF.  When we receive the XON, start_tx() is never called
because the serial_core ring buffer is empty, and we have no more data to
transmit.  When we enable the transmitter manually, these bytes will pop

Thanks for all of your help!


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