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serial: imx: implement the flush_buffer hook
The current driver does not implement the flush_buffer hook for
uart_ops. When we enable the DMA for the driver, and test it with Bluetooth,
we may meet the following bug for TX:
[1] User application may call the flush operation at any time.
The uart_flush_buffer() calls the uart_circ_clear() to set
the xmit->head and xmit->tail with 0.
[2] The TX DMA callback can be called at any time too.
The dma_tx_call() will update the xmit->tail.
If [2] occurs just after the [1], we will get the wrong xmit->tail.
This patch implements the flush_buffer hook to fix this issue.
Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>