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

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Date:	Tue, 23 Feb 2016 13:43:53 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Sekhar Nori <nsekhar@...com>,
	John Ogness <john.ogness@...utronix.de>,
	Tony Lindgren <tony@...mide.com>
Cc:	Peter Hurley <peter@...leysoftware.com>,
	gregkh@...uxfoundation.org, vinod.koul@...el.com,
	dan.j.williams@...el.com, peter.ujfalusi@...com,
	dmaengine@...r.kernel.org, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/4] serial: omap: robustify for high speed transfers

On 02/23/2016 10:59 AM, Sekhar Nori wrote:
>> With the am335x (Beaglebone Black, eDMA engine) I see 1000 DMA
>> interrupts and 1000 spurious UART interrupts. The spurious UART
>> interrupts arrive 30-50us _before_ the DMA interrupts. Always.
>>
>> If I disable UART timeout interrupts (RDI), the same test generates no
>> spurious UART interrupts. Only 1000 DMA interrupts.
> 
> To be clear, these interrupts are not caught as spurious by the
> interrupt controller (INTC). They are detected by INTC as UART
> interrupts. Just that you don't expect a timeout interrupt to happen at
> the time you see the interrupt, correct?

>From what I remember the INTC says it is UART, correct.
But UART's status register says "no interrupt" (IIR has UART_IIR_NO_INT
set). So the UART driver returns IRQ_NONE which counts as spurious.

It is just that once you disable RDI there are no more interrupts coming
during DMA transfer.

> Thanks,
> Sekhar
> 
Sebastian