[PATCH v2 0/4] serial: 8250_dw: Allwinner A733 UART support

[Vinicius Pedrosa <vinicius.eduardo.pedrosa@gmail.com>]

While testing the A733 UART, the console stopped taking interrupts at
the first termios change from userspace. Termios changes during console
output could also corrupt characters even when the baud rate did not
change.

Patch 1 is an independent fix for the DesignWare 8250 divisor path.
When DLF is detected, dw8250_setup_port() can replace the BUSY-safe
divisor hook with one that does not provide that protection. If the LCR
write is then dropped while the UART is busy, the retry path can try to
restore IER while DLAB is still selected, causing the value to be
written to DLH instead of IER.

Patch 2 takes Andre's suggestion to only accept a DLF readback when it
is a contiguous 2^n - 1 mask. On the A733, offset 0xc0 is the RS485
control register and reads back 0x19f; Andre observed 0x9f on the A523.

Patches 3 and 4 add the A733 compatible and select SKIP_SET_RATE for
it. Stopping the UART clock from set_termios while TX is active corrupts
output on this board. Skipping redundant reclocking in general was tried
in e5d6bd25f93d and reverted by 7dfae6cbadc1 after regressions on Bay
Trail and Cherry Trail, so the change stays specific to the A733.

Tested on a Radxa Cubie A7Z (PCB V1.11, 1 GiB) on top of tty-next
1184f87e2790e8af9be3116f608d2e86b4792d7d, using local A733 DT and
clock changes that are not part of this series.

With patch 1 alone and the generic fallback, repeated termios changes
no longer stalled the console and hardware IER remained equal to
up->ier, while the false DLF was still detected (dlf_size=9).

With the complete series and the A733 compatible, three clean runs of
both the CLOCAL and CSTOPB stress tests completed without corrupted
output. dlf_size remained 0 and offset 0xc0 remained 0 after the tests.
dlf_size and up->ier were read through a temporary debugfs test patch
which is not part of this series.

I do not have hardware with a real DLF, or Bay Trail/Cherry Trail, to
test those paths.

Changes in v2:
- rework patch 1 around a dwlib divisor helper and preserve an existing
  set_divisor hook, following Ilpo's review
- add a separate check for contiguous DLF readback, following Andre's
  suggestion
- drop the A733-specific NO_DLF quirk
- keep the generic DT fallback
- keep only SKIP_SET_RATE in the A733 platform data
- retain Conor's Acked-by on the binding
- rebase and retest on tty-next 1184f87e2790

Link to v1:
https://lore.kernel.org/all/20261005172538.398522-1-vinicius.eduardo.pedrosa@gmail.com/

Vinicius Pedrosa (4):
  serial: 8250_dw: Keep the BUSY-safe divisor hook when DLF is present
  serial: 8250_dw: Only accept a contiguous DLF readback
  dt-bindings: serial: snps-dw-apb-uart: Add Allwinner A733
  serial: 8250_dw: Add Allwinner A733 UART

 .../bindings/serial/snps-dw-apb-uart.yaml     |  1 +
 drivers/tty/serial/8250/8250_dw.c             |  3 +++
 drivers/tty/serial/8250/8250_dwlib.c          | 22 ++++++++++++++++---
 drivers/tty/serial/8250/8250_dwlib.h          |  1 +
 4 files changed, 24 insertions(+), 3 deletions(-)


base-commit: 1184f87e2790e8af9be3116f608d2e86b4792d7d