linux-kernel - Re: omap uart + dma issues (Re: [PATCH] tty: serial: 8250

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Message-ID: <570EEEF0.2070102@hurleysoftware.com>
Date:	Wed, 13 Apr 2016 18:14:24 -0700
From:	Peter Hurley <peter@...leysoftware.com>
To:	Sekhar Nori <nsekhar@...com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	John Ogness <john.ogness@...utronix.de>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:	Tony Lindgren <tony@...mide.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
	linux-omap@...r.kernel.org
Subject: Re: omap uart + dma issues (Re: [PATCH] tty: serial: 8250_omap: do
 not defer termios changes)

On 04/13/2016 04:11 AM, Sekhar Nori wrote:
> On Wednesday 13 April 2016 05:30 AM, Peter Hurley wrote:
> 
>>>> - generates spurious uart interrupt for every rx dma transaction
>>>>   (ie., necessitates acking every UART interrupt, even UART_IIR_NO_INT)
>>>>   _Even with this workaround_, it still generates spurious interrupt warning
>>>>   which shuts off interrupts for several ms while logging the error
>>>>   message to the console, virtually guaranteeing lost data.
>>>
>>> as I wrote in my other email I think RDI should be disabled with DMA
>>
>>
>> I'll test to see if disabling RDI eliminates the UART_IIR_NO_INT spurious
>> interrupts.

Ok; disabling UART_IER_RDI eliminates the UART_IIR_NO_INT spurious
interrupts.

However, disabling RDI disables RX timeout as well, so data just sits in
the RX fifo with no way to get it out. AFAICT that's a showstopper.


>>> according the Intel manual and I *think* someone here reported that
>>> they see the same problem.
>>
>> Let's confirm with the Intel folks that this is true, which would argue
>> for using the omap-style rx dma flow.
> 
> Andy Shevchenko pointed this out here: https://lkml.org/lkml/2016/2/23/588

which Andy noted as well:

On 02/23/2016 08:56 AM, Andy Shevchenko wrote:
> The problem is that we have no separate bit to control timeout
> interrupts from UART.