linux-kernel - Re: [PATCH v8 00/18] 8250-core based serial driver for OMAP + DMA

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Date:	Mon, 8 Sep 2014 20:33:53 +0200
From:	Frans Klaver <frans.klaver@...ns.com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
CC:	<linux-serial@...r.kernel.org>, <tony@...mide.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<linux-kernel@...r.kernel.org>, <balbi@...com>,
	<linux-omap@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v8 00/18] 8250-core based serial driver for OMAP + DMA

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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