Re: [PATCH 0/4] serial: omap: robustify for high speed transfers

[Sekhar Nori]

On Wednesday 24 February 2016 08:50 AM, Peter Hurley wrote:
> Just to follow-up on what Sebastian wrote.
> 
> As he pointed out, these spurious interrupts are not timeout interrupts.
> Since IIR_UART[0] == 1, no uart interrupt is pending.
> 
> As he wrote, these count as spurious interrupts and trigger
> interrupt shutdown at 100000 (unless acked as uart interrupts).

Okay, by adding a printk to where the check for UART_IIR_NO_INT is in
omap_8250_dma_handle_irq(), I do see that UART irq handler is called
when there is apparently no interrupt.

I don't see the error interrupt count in /proc/interrupts go up although
the code is returning IRQ_NONE when this happens. I initially thought
that must be because of the interrupt being IRQF_SHARED. But getting rid
of IRQF_SHARED still does not lead to error count going up. I need to
spend some more time to see what is going on.

> These spurious interrupts very nearly correspond 1:1 (but not quite)
> with each dma submission. So, for example, one test run had:
> 
>     @3Mbaud line rate
>     195826 submits
>     195823 completions
> 
>     195704 spurious interrupts (ie., interrupts with IIR_UART[0] == 1)
>          0 RLSI interrupts (no line errors) (IIR_UART == 0xc6)
>          2 RX timeout interrupts (IIR_UART == 0xcc),
>            one during i/o test and one at the end of i/o test
>          6 RDI interrupts (IIR_UART == 0xc4)
> 
> The spurious interrupts occur with nearly 1:1 correspondence at _all_
> line rates.
> 
> Presumbably, the 6 RDI interrupts are from too-slow submission of
> the next DMA and the uart rx fifo has reached rx trigger level already.
> [NOTE: we should at least be using ping-pong dma buffers for rx so that
> there is always a next DMA buffer when the current buffer is completed].
> 
> There is no documentation in any of the OMAP TRMs regarding RDI
> interrupts while in DMA mode. Some guidance from TI would be appreciated.

UART interrupts triggering while UART_IIR_NO_INT is set is weird enough.
I will check around internally with hardware folks here. Getting an
answer might take time. But this is easily reproducible so I am
optimistic we will get an answer soon.

Regards,
Sekhar