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

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Date:	Mon, 8 Sep 2014 17:15:01 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Frans Klaver <frans.klaver@...ns.com>
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

* Frans Klaver | 2014-09-08 16:46:18 [+0200]:

>- ncurses based applications (vi, less) don't play nice for me on the
>  console with this series. less doesn't show me anything. vi doesn't
>  return to console properly.

Can you give a test case 

>- 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?

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);

>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?

>Thanks,
>Frans

Sebastian
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