serial: msm: Use single character mode

serial: msm: Use single character mode

The UART DM controller supports different channel data packing modes,
either the 4-character packing mode (where 32-bit are read/written at once)
or the single-character mode (where only a single character is read/written
at a time). The 4-character mode can be more efficient, but the
single-character mode is much easier to implement.

At the moment, serial_msm uses the 4-character mode. Since the
dm_serial_ops operate on one character at the time, the code goes through
quite some hoops in order to break this down to single characters. This
code is prone to race conditions (e.g. priv->chars_cnt is read from the
registers, then a command is issued, what if another char came in
inbetween?). It also seems to cause another subtle issue with autoboot:

Unlike the previous autoboot failures that happened when UART was
disconnected, this problem occurs when UART is connected and open in a
terminal: For EFI boot, the console size is queried in efi_console.c
query_console_serial() by sending an ANSI escape code via UART. For some
reason, with the current driver we get yet another 0x00 byte (UART break
event?) when reading the reply from serial input. Because of that, reading
the console size fails in efi_console.c, the actual reply remains in the
UART buffer, and later the boot flow aborts because it detects input after
printing a prompt.

Rather than trying to fix the issue in the current complicated approach,
switch the driver to use the single-character mode. This is simple and
straightforward to implement without race conditions:

 - We write one character at a time to UARTDM_TF, as long as the TX FIFO
   has space available (TX_READY). To flush the console before starting
   Linux, we wait for TX_EMPTY.

 - We read one character at a time from UARTDM_RF and strip off the
   additional error information (assuming there is something in the
   RX FIFO, as indicated by RX_READY).

In this mode, querying the serial console size works and autoboot is no
longer interrupted. The overall code is also much shorter.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Stephan Gerhold <stephan.gerhold@linaro.org>
Tested-by: Alexey Minnekhanov <alexeymin@minlexx.ru>
Acked-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250908-db410c-autoboot-fixes-v2-6-316ed98e0143@linaro.org
Signed-off-by: Casey Connolly <kcxt@postmarketos.org>