linux-kernel - Re: [PATCH v7] 8250-core based serial driver for OMAP + DMA

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Date:	Fri, 15 Aug 2014 13:28:27 -0700
From:	Tony Lindgren <tony@...mide.com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	Lennart Sorensen <lsorense@...lub.uwaterloo.ca>,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	balbi@...com, Vinod Koul <vinod.koul@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v7] 8250-core based serial driver for OMAP + DMA

* Sebastian Andrzej Siewior <bigeasy@...utronix.de> [140815 12:16]:
> On 08/15/2014 08:17 PM, Lennart Sorensen wrote:
> 
> > Are you saying that with the new driver you have to respond to the RX
> > irq faster than before to avoid overflows?  It is not quite clear.
> 
> Yes. The irq fires 46 bytes giving you 16 bytes buffer before overflow
> vs 63 bytes buffer the old one had.
> 
> > I do think 40000 interrupts to handle 40000 bytes of date does seem a
> > tad inefficient, so dropping to 854 looks a lot nicer.  Was the omap
> > driver not using the fifo trigger levels at all?
> 
> It configured the trigger levels to 1 for RX and 16 for TX.

Hmm that weird RX trigger level is a workaround for lost characters.

See commit 0ba5f66836 (tty: serial: OMAP: use a 1-byte RX FIFO
threshold in PIO mode :)

There's paste test in that commit, I wonder if the 8250 drivers
can deal with it any better?

Regards,

Tony
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