[REQUEST] Backport 8250_dw BUSY deassert series to 6.12.y stable

["Ionut Nechita (Wind River)" <ionut.nechita@windriver.com>]

From: Ionut Nechita <ionut.nechita@windriver.com>

Hi Ilpo, Greg,

We're running kernel 6.12 (LTS) on production systems with DesignWare
8250 UARTs and are hitting the BUSY assertion issue that your recent
series addresses in 6.18:

  Ilpo Järvinen (7):
    serial: 8250: Protect LCR write in shutdown
    serial: 8250_dw: Avoid unnecessary LCR writes
    serial: 8250: Add serial8250_handle_irq_locked()
    serial: 8250_dw: Rework dw8250_handle_irq() locking and IIR handling
    serial: 8250_dw: Rework IIR_NO_INT handling to stop interrupt storm
    serial: 8250: Add late synchronize_irq() to shutdown to handle DW UART BUSY
    serial: 8250_dw: Ensure BUSY is deasserted

Patch 7/7 has Cc: stable, but it depends on patches 1-6 for the new
infrastructure (serial8250_handle_irq_locked(), the reworked locking in
dw8250_handle_irq(), etc.).

Could the full series be nominated for 6.12.y stable backport?  Or if
that's too invasive for stable, could you advise on the minimal subset
that would allow patch 7 to apply cleanly on 6.12?

We've attempted a standalone backport of patch 7 (squashing the
necessary helpers from 8250_port.c), but the IRQ handler refactoring
in patches 3-5 changes the locking model significantly, and we'd
rather carry the maintainer-blessed version than risk introducing
subtle races.

Our environment:
  - Kernel: 6.12.57-rt/6.12.87-rt (PREEMPT_RT)
  - Hardware: Intel platforms with DW APB UART (snps,dw-apb-uart)
  - Symptom: LCR writes silently ignored under Rx load, causing
    baud rate / framing mismatches after set_termios

We are available to test any backport candidates on our hardware.

Thanks,
Ionut
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