[PATCH v3 0/4] serial: 8250_dw: Fix ref clock usage

[Serge Semin <Sergey.Semin@baikalelectronics.ru>]

It might be dangerous if an UART port reference clock rate is suddenly
changed. In particular the 8250 port drivers (and AFAICS most of the tty
drivers using common clock framework clocks) rely either on the
exclusive reference clock utilization or on the ref clock rate being
always constant. Needless to say that it turns out not true and if some
other service suddenly changes the clock rate behind an UART port driver
back it's no good. So the port might not only end up with an invalid
uartclk value saved, but may also experience a distorted output/input
data since such action will effectively update the programmed baud-clock.
We discovered such problem on Baikal-T1 SoC where two DW 8250 ports have
got a shared reference clock. Allwinner SoC is equipped with an UART,
which clock is derived from the CPU PLL clock source, so the CPU frequency
change might be propagated down up to the serial port reference clock.
This patchset provides a way to fix the problem to the 8250 serial port
controllers and mostly fixes it for the DW 8250-compatible UART. I say
mostly because due to not having a facility to pause/stop and resume/
restart on-going transfers we implemented the UART clock rate update
procedure executed post factum of the actual reference clock rate change.

In addition the patchset includes a few fixes we discovered when were
working the issue. First one concerns the maximum baud rate setting used
to determine a serial port baud based on the current UART port clock rate.
Another one simplifies the ref clock rate setting procedure a bit.

This patchset is rebased and tested on the mainline Linux kernel 5.7-rc4:
0e698dfa2822 ("Linux 5.7-rc4")
tag: v5.7-rc4

Changelog v3:
- Refactor the original patch to adjust the UART port divisor instead of
  requesting an exclusive ref clock utilization.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Maxim Kaurkin <Maxim.Kaurkin@baikalelectronics.ru>
Cc: Pavel Parkhomenko <Pavel.Parkhomenko@baikalelectronics.ru>
Cc: Alexey Kolotnikov <Alexey.Kolotnikov@baikalelectronics.ru>
Cc: Ramil Zaripov <Ramil.Zaripov@baikalelectronics.ru>
Cc: Ekaterina Skachko <Ekaterina.Skachko@baikalelectronics.ru>
Cc: Vadim Vlasov <V.Vlasov@baikalelectronics.ru>
Cc: Alexey Kolotnikov <Alexey.Kolotnikov@baikalelectronics.ru>
Cc: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Long Cheng <long.cheng@mediatek.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: linux-mips@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
Cc: linux-serial@vger.kernel.org
Cc: linux-kernel@vger.kernel.org

Serge Semin (4):
  serial: 8250: Fix max baud limit in generic 8250 port
  serial: 8250: Add 8250 port clock update method
  serial: 8250_dw: Simplify the ref clock rate setting procedure
  serial: 8250_dw: Fix common clocks usage race condition

 drivers/tty/serial/8250/8250_dw.c   | 125 +++++++++++++++++++++++++---
 drivers/tty/serial/8250/8250_port.c |  42 +++++++++-
 include/linux/serial_8250.h         |   2 +
 3 files changed, 156 insertions(+), 13 deletions(-)

-- 
2.25.1