linux.git/drivers/tty, branch master

vt: save/restore unicode screen buffer for alternate screen

2026-03-13T08:15:58+00:00

The alternate screen support added by commit 23743ba64709 ("vt: add
support for smput/rmput escape codes") only saves and restores the
regular screen buffer (vc_origin), but completely ignores the corresponding
unicode screen buffer (vc_uni_lines) creating a messed-up display.

Add vc_saved_uni_lines to save the unicode screen buffer when entering
the alternate screen, and restore it when leaving. Also ensure proper
cleanup in reset_terminal() and vc_deallocate().

Fixes: 23743ba64709 ("vt: add support for smput/rmput escape codes")
Cc: stable 
Signed-off-by: Nicolas Pitre 
Link: https://patch.msgid.link/5o2p6qp3-91pq-0p17-or02-1oors4417ns7@onlyvoer.pbz
Signed-off-by: Greg Kroah-Hartman

serial: 8250_dw: Ensure BUSY is deasserted

2026-03-12T14:34:29+00:00

DW UART cannot write to LCR, DLL, and DLH while BUSY is asserted.
Existance of BUSY depends on uart_16550_compatible, if UART HW is
configured with it those registers can always be written.

There currently is dw8250_force_idle() which attempts to achieve
non-BUSY state by disabling FIFO, however, the solution is unreliable
when Rx keeps getting more and more characters.

Create a sequence of operations that ensures UART cannot keep BUSY
asserted indefinitely. The new sequence relies on enabling loopback mode
temporarily to prevent incoming Rx characters keeping UART BUSY.

Ensure no Tx in ongoing while the UART is switches into the loopback
mode (requires exporting serial8250_fifo_wait_for_lsr_thre() and adding
DMA Tx pause/resume functions).

According to tests performed by Adriana Nicolae ,
simply disabling FIFO or clearing FIFOs only once does not always
ensure BUSY is deasserted but up to two tries may be needed. This could
be related to ongoing Rx of a character (a guess, not known for sure).
Therefore, retry FIFO clearing a few times (retry limit 4 is arbitrary
number but using, e.g., p->fifosize seems overly large). Tests
performed by others did not exhibit similar challenge but it does not
seem harmful to leave the FIFO clearing loop in place for all DW UARTs
with BUSY functionality.

Use the new dw8250_idle_enter/exit() to do divisor writes and LCR
writes. In case of plain LCR writes, opportunistically try to update
LCR first and only invoke dw8250_idle_enter() if the write did not
succeed (it has been observed that in practice most LCR writes do
succeed without complications).

This issue was first reported by qianfan Zhao who put lots of debugging
effort into understanding the solution space.

Fixes: c49436b657d0 ("serial: 8250_dw: Improve unwritable LCR workaround")
Fixes: 7d4008ebb1c9 ("tty: add a DesignWare 8250 driver")
Cc: stable 
Reported-by: qianfan Zhao 
Link: https://lore.kernel.org/linux-serial/289bb78a-7509-1c5c-2923-a04ed3b6487d@163.com/
Reported-by: Adriana Nicolae 
Link: https://lore.kernel.org/linux-serial/20250819182322.3451959-1-adriana@arista.com/
Reported-by: Bandal, Shankar 
Tested-by: Bandal, Shankar 
Tested-by: Murthy, Shanth 
Reviewed-by: Andy Shevchenko 
Signed-off-by: Ilpo Järvinen 
Link: https://patch.msgid.link/20260203171049.4353-8-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman

serial: 8250: Add late synchronize_irq() to shutdown to handle DW UART BUSY

2026-03-12T14:34:25+00:00

When DW UART is !uart_16550_compatible, it can indicate BUSY at any
point (when under constant Rx pressure) unless a complex sequence of
steps is performed. Any LCR write can run a foul with the condition
that prevents writing LCR while the UART is BUSY, which triggers
BUSY_DETECT interrupt that seems unmaskable using IER bits.

Normal flow is that dw8250_handle_irq() handles BUSY_DETECT condition
by reading USR register. This BUSY feature, however, breaks the
assumptions made in serial8250_do_shutdown(), which runs
synchronize_irq() after clearing IER and assumes no interrupts can
occur after that point but then proceeds to update LCR, which on DW
UART can trigger an interrupt.

If serial8250_do_shutdown() releases the interrupt handler before the
handler has run and processed the BUSY_DETECT condition by read the USR
register, the IRQ is not deasserted resulting in interrupt storm that
triggers "irq x: nobody cared" warning leading to disabling the IRQ.

Add late synchronize_irq() into serial8250_do_shutdown() to ensure
BUSY_DETECT from DW UART is handled before port's interrupt handler is
released. Alternative would be to add DW UART specific shutdown
function but it would mostly duplicate the generic code and the extra
synchronize_irq() seems pretty harmless in serial8250_do_shutdown().

Fixes: 7d4008ebb1c9 ("tty: add a DesignWare 8250 driver")
Cc: stable 
Reported-by: Bandal, Shankar 
Tested-by: Bandal, Shankar 
Tested-by: Murthy, Shanth 
Reviewed-by: Andy Shevchenko 
Signed-off-by: Ilpo Järvinen 
Link: https://patch.msgid.link/20260203171049.4353-7-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman

serial: 8250_dw: Rework IIR_NO_INT handling to stop interrupt storm

2026-03-12T14:34:22+00:00

INTC10EE UART can end up into an interrupt storm where it reports
IIR_NO_INT (0x1). If the storm happens during active UART operation, it
is promptly stopped by IIR value change due to Rx or Tx events.
However, when there is no activity, either due to idle serial line or
due to specific circumstances such as during shutdown that writes
IER=0, there is nothing to stop the storm.

During shutdown the storm is particularly problematic because
serial8250_do_shutdown() calls synchronize_irq() that will hang in
waiting for the storm to finish which never happens.

This problem can also result in triggering a warning:

  irq 45: nobody cared (try booting with the "irqpoll" option)
  [...snip...]
  handlers:
    serial8250_interrupt
  Disabling IRQ #45

Normal means to reset interrupt status by reading LSR, MSR, USR, or RX
register do not result in the UART deasserting the IRQ.

Add a quirk to INTC10EE UARTs to enable Tx interrupts if UART's Tx is
currently empty and inactive. Rework IIR_NO_INT to keep track of the
number of consecutive IIR_NO_INT, and on fourth one perform the quirk.
Enabling Tx interrupts should change IIR value from IIR_NO_INT to
IIR_THRI which has been observed to stop the storm.

Fixes: e92fad024929 ("serial: 8250_dw: Add ACPI ID for Granite Rapids-D UART")
Cc: stable 
Reported-by: Bandal, Shankar 
Tested-by: Bandal, Shankar 
Tested-by: Murthy, Shanth 
Reviewed-by: Andy Shevchenko 
Signed-off-by: Ilpo Järvinen 
Link: https://patch.msgid.link/20260203171049.4353-6-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman

serial: 8250_dw: Rework dw8250_handle_irq() locking and IIR handling

2026-03-12T14:34:18+00:00

dw8250_handle_irq() takes port's lock multiple times with no good
reason to release it in between and calls serial8250_handle_irq()
that also takes port's lock.

Take port's lock only once in dw8250_handle_irq() and use
serial8250_handle_irq_locked() to avoid releasing port's lock in
between.

As IIR_NO_INT check in serial8250_handle_irq() was outside of port's
lock, it has to be done already in dw8250_handle_irq().

DW UART can, in addition to IIR_NO_INT, report BUSY_DETECT (0x7) which
collided with the IIR_NO_INT (0x1) check in serial8250_handle_irq()
(because & is used instead of ==) meaning that no other work is done by
serial8250_handle_irq() during an BUSY_DETECT interrupt.

This allows reorganizing code in dw8250_handle_irq() to do both
IIR_NO_INT and BUSY_DETECT handling right at the start simplifying
the logic.

Tested-by: Bandal, Shankar 
Tested-by: Murthy, Shanth 
Cc: stable 
Reviewed-by: Andy Shevchenko 
Signed-off-by: Ilpo Järvinen 
Link: https://patch.msgid.link/20260203171049.4353-5-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman

serial: 8250: Add serial8250_handle_irq_locked()

2026-03-12T14:34:15+00:00

8250_port exports serial8250_handle_irq() to HW specific 8250 drivers.
It takes port's lock within but a HW specific 8250 driver may want to
take port's lock itself, do something, and then call the generic
handler in 8250_port but to do that, the caller has to release port's
lock for no good reason.

Introduce serial8250_handle_irq_locked() which a HW specific driver can
call while already holding port's lock.

As this is new export, put it straight into a namespace (where all 8250
exports should eventually be moved).

Tested-by: Bandal, Shankar 
Tested-by: Murthy, Shanth 
Cc: stable 
Reviewed-by: Andy Shevchenko 
Signed-off-by: Ilpo Järvinen 
Link: https://patch.msgid.link/20260203171049.4353-4-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman

serial: 8250_dw: Avoid unnecessary LCR writes

2026-03-12T14:34:12+00:00

When DW UART is configured with BUSY flag, LCR writes may not always
succeed which can make any LCR write complex and very expensive.
Performing write directly can trigger IRQ and the driver has to perform
complex and distruptive sequence while retrying the write.

Therefore, it's better to avoid doing LCR write that would not change
the value of the LCR register. Add LCR write avoidance code into the
8250_dw driver's .serial_out() functions.

Reported-by: Bandal, Shankar 
Tested-by: Bandal, Shankar 
Tested-by: Murthy, Shanth 
Cc: stable 
Reviewed-by: Andy Shevchenko 
Signed-off-by: Ilpo Järvinen 
Link: https://patch.msgid.link/20260203171049.4353-3-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman

serial: 8250: Protect LCR write in shutdown

2026-03-12T14:34:07+00:00

The 8250_dw driver needs to potentially perform very complex operations
during LCR writes because its BUSY handling prevents updates to LCR
while UART is BUSY (which is not fully under our control without those
complex operations). Thus, LCR writes should occur under port's lock.

Move LCR write under port's lock in serial8250_do_shutdown(). Also
split the LCR RMW so that the logic is on a separate line for clarity.

Reported-by: Bandal, Shankar 
Tested-by: Bandal, Shankar 
Tested-by: Murthy, Shanth 
Cc: stable 
Reviewed-by: Andy Shevchenko 
Signed-off-by: Ilpo Järvinen 
Link: https://patch.msgid.link/20260203171049.4353-2-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman

serial: 8250_pci: add support for the AX99100

2026-03-12T14:31:51+00:00

This is found in popular brands such as StarTech.com or Delock, and has
been a source of frustration to quite a few people, if I can trust
Amazon comments complaining about Linux support via the official
out-of-the-tree driver.

Signed-off-by: Martin Roukala (né Peres) 
Cc: stable 
Link: https://patch.msgid.link/20260309-8250_pci_ax99100-v1-1-3328bdfd8e94@mupuf.org
Signed-off-by: Greg Kroah-Hartman

serial: core: fix infinite loop in handle_tx() for PORT_UNKNOWN

2026-03-12T14:31:41+00:00

uart_write_room() and uart_write() behave inconsistently when
xmit_buf is NULL (which happens for PORT_UNKNOWN ports that were
never properly initialized):

- uart_write_room() returns kfifo_avail() which can be > 0
- uart_write() checks xmit_buf and returns 0 if NULL

This inconsistency causes an infinite loop in drivers that rely on
tty_write_room() to determine if they can write:

  while (tty_write_room(tty) > 0) {
      written = tty->ops->write(...);
      // written is always 0, loop never exits
  }

For example, caif_serial's handle_tx() enters an infinite loop when
used with PORT_UNKNOWN serial ports, causing system hangs.

Fix by making uart_write_room() also check xmit_buf and return 0 if
it's NULL, consistent with uart_write().

Reproducer: https://gist.github.com/mrpre/d9a694cc0e19828ee3bc3b37983fde13

Signed-off-by: Jiayuan Chen 
Cc: stable 
Link: https://patch.msgid.link/20260204074327.226165-1-jiayuan.chen@linux.dev
Signed-off-by: Greg Kroah-Hartman