serial: tegra: don't clear the Tx FIFO on an Rx-only reset

tegra_uart_fifo_reset() applies the Tegra30 workaround for
"cannot clear the Tx FIFO while FIFO mode is enabled"
unconditionally: it leaves FIFO mode, writes the requested FCR
clear bits, and re-enters FIFO mode. Leaving FIFO mode empties
both FIFOs, so an Rx-only reset discards queued Tx data as well.

The break handler in tegra_uart_decode_rx_error() calls it with
UART_FCR_CLEAR_RCVR only. On a half-duplex RS485 board whose Rx
line is pulled low while the transceiver drives the bus, every
transmission raises a spurious break, and all but the first
character of the frame is lost.

Only take the FIFO-mode path when the caller actually asked for
CLEAR_XMIT. Likewise only wait for TEMT in that case: with the Tx
FIFO deliberately left intact, that loop would otherwise spin for
a full frame time in hard IRQ context with the port lock held.

Tested on a Colibri T30 (Tegra30) with Rx DMA:

 - Break during transmission: the reset still fires from
   tegra_uart_decode_rx_error(), and the complete frame now
   reaches the peer. Before this change only the character in
   the shift register was sent.
 - Internal loopback with a generated break: the Rx FIFO is
   correctly cleared by the plain FCR write, and subsequent
   receive works with no frame, parity or overrun errors.

Fixes: e9ea096dd225 ("serial: tegra: add serial driver")
Cc: stable@kernel.org
Signed-off-by: Simon Gassner <simon.gassner@noxsystems.com>
Link: https://patch.msgid.link/20261001060539.32659-1-simon.gassner@noxsystems.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

0 files changed, 0 insertions, 0 deletions