serial: 8250: Toggle IER bits on only after irq has been set up

Invoking TIOCVHANGUP on 8250_mid port on Ice Lake-D and then reopening
the port triggers these faults during serial8250_do_startup():

  DMAR: DRHD: handling fault status reg 3
  DMAR: [DMA Write NO_PASID] Request device [00:1a.0] fault addr 0x0 [fault reason 0x05] PTE Write access is not set

If the IRQ hasn't been set up yet, the UART will have zeroes in its MSI
address/data registers. Disabling the IRQ at the interrupt controller
won't stop the UART from performing a DMA write to the address programmed
in its MSI address register (zero) when it wants to signal an interrupt.

The UARTs (in Ice Lake-D) implement PCI 2.1 style MSI without masking
capability, so there is no way to mask the interrupt at the source PCI
function level, except disabling the MSI capability entirely, but that
would cause it to fall back to INTx# assertion, and the PCI specification
prohibits disabling the MSI capability as a way to mask a function's
interrupt service request.

The MSI address register is zeroed by the hangup as the irq is freed.
The interrupt is signalled during serial8250_do_startup() performing a
THRE test that temporarily toggles THRI in IER. The THRE test currently
occurs before UART's irq (and MSI address) is properly set up.

Refactor serial8250_do_startup() such that irq is set up before the
THRE test. The current irq setup code is intermixed with the timer
setup code. As THRE test must be performed prior to the timer setup,
extract it into own function and call it only after the THRE test.

The ->setup_timer() needs to be part of the struct uart_8250_ops in
order to not create circular dependency between 8250 and 8250_base
modules.

Fixes: 40b36daad0ac ("[PATCH] 8250 UART backup timer")
Reported-by: Lennert Buytenhek <buytenh@arista.com>
Tested-by: Lennert Buytenhek <buytenh@arista.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220922070005.2965-1-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/serial_8250.h b/include/linux/serial_8250.h
index 43edd10e8c96..19376bee9667 100644
--- a/include/linux/serial_8250.h
+++ b/include/linux/serial_8250.h
@@ -74,6 +74,7 @@ struct uart_8250_port;
 struct uart_8250_ops {
 	int		(*setup_irq)(struct uart_8250_port *);
 	void		(*release_irq)(struct uart_8250_port *);
+	void		(*setup_timer)(struct uart_8250_port *);
 };
 
 struct uart_8250_em485 {