[Xenomai] Only 2 serial ports at a time with xeno_16550A driver

C Smith csmithquestions at gmail.com
Wed May 23 07:25:27 CEST 2018

I'm not familiar with the chaining of shared interrupts in the RTDM
architecture. I think the EOI should somehow be done in the very last
invocation of rt_16550_interrupt() by the last peripheral needing service.

I read this from an old interrupt handler:
(assuming IBM PC (Intel) architecture)
An interrupt handler issues an EOI by writing the value 0x20 to the PIC at
address 0x20. If the interrupt was number 8-15, do the same thing again,
write 0x20 to address 0x20.

So perhaps something like this at the end of rt_16550_interrupt() ?

if (ctx->irq_handle.irq <= 7)        /* non-specific EOI to 8259 */
  rt_16550_reg_out(mode, 0, 0x20, 0x20);
if (ctx->irq_handle.irq <= 15)          /* extra EOI for upper ISA IRQs */
  rt_16550_reg_out(mode, 0, 0x20, 0x20);

But again I don't know if that will work with shared interrupts. Surely
RTDM implements some "master" handler which is already doing an EOI?
Remember lower interrupts like IRQ 5 work fine with the xeno_16550A driver
unless another port is sharing the same IRQ.

-C Smith

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