linux-kernel - Re: [PATCH 05/15] tty: serial: Add 8250-core based omap driver

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Date:	Fri, 15 Aug 2014 22:20:53 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Lennart Sorensen <lsorense@...lub.uwaterloo.ca>
CC:	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	tony@...mide.com, balbi@...com, Vinod Koul <vinod.koul@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 05/15] tty: serial: Add 8250-core based omap driver

On 08/15/2014 09:33 PM, Lennart Sorensen wrote:
>> On the other hand if you use DMA then it can handle transfers > 64bytes
>> in one go and you can start transfers while the FIFO is not completely
>> empty.
> 
> You can dma more than the fifo size?

Yes. The UART asserts the DMA line as long as there is room for
$TRESHOLD number of bytes. So we never overflow the FIFO. That is why we
can't take any DMA channel but only *the* one.

>> There is patch in Greg's tty tree already where you are able to
>> configure the RX trigger level. We could wire this up once we agree
>> which levels we want support. The OMAP supports all levels from 1…63.
> 
> All? or just every 4 (that's what I just read in the DRA7xx docs).

All. Take a look at the RX_TRIGGER constant while comparing source vs
manual :)

Sebastian
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