On Mon, Sep 08, 2014 at 05:15:01PM +0200, Sebastian Andrzej Siewior wrote:
> * Frans Klaver | 2014-09-08 16:46:18 [+0200]:
>
> >- I seem seem to get stuck in a "serial8250: too much work for irq%d"
> > loop somewhat reliably. We have a rather demanding application with
> > typically somewhere between 600 and 1000 byte packets being sent at
> > 240Hz (roughly somewhere between 1.5 and 2 Mb/s). We run at baudrate
> > 3500k. I get into this "too much work" thing already when running at
> > 300 bytes per packet.
>
> Do you get this message also at lower baud rates, say 115200?
I don't get this message at lower data rates. Haven't tested lower baud
rates yet.
> What I am trying to understand is why you are spinning in the handler.
> _With_ DMA you should hardly get into the serial handler under normal
> conditions. Running at 3.5MB/sec should give one byte every 2.8us and
> 48 Bytes every ~137us. This looks like plenty of time to get out of
> the handler. My *guess* is that serial8250_handle_irq() has IIR often
> set to timeout and you end up fetching byte after byte.
>
> This patch should protocol when and why you got into the handler.
>
> diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
> index 7111b22de000..59852069e4a0 100644
> --- a/drivers/tty/serial/8250/8250_core.c
> +++ b/drivers/tty/serial/8250/8250_core.c
> @@ -1583,6 +1583,7 @@ int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
> status = serial_port_in(port, UART_LSR);
>
> DEBUG_INTR("status = %x...", status);
> + trace_printk("l%d IIR %x LSR %x\n", port->line, iir, status);
>
> if (status & (UART_LSR_DR | UART_LSR_BI)) {
> if (up->dma)
> @@ -1707,6 +1708,7 @@ static irqreturn_t serial8250_interrupt(int irq, void *dev_id)
>
> spin_unlock(&i->lock);
>
> + trace_printk("%d e\n", irq);
> DEBUG_INTR("end.\n");
>
> return IRQ_RETVAL(handled);
>
Thanks. I'll give it a spin on Wednesday.
> >I hope this is of some use to you. I'll do more testing later.
>
> Which SoC do you use and do you have DMA enabled?
am335x, DMA is enabled, unless I need to do something extra in the
device tree. We depend on am335x.dtsi, so I would think that would be
automatic if CONFIG_SERIAL_8250_DMA=y.
Thanks,
Frans
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