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

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Message-ID: <20140815181704.GH17769@csclub.uwaterloo.ca>
Date:	Fri, 15 Aug 2014 14:17:04 -0400
From:	"Lennart Sorensen" <lsorense@...lub.uwaterloo.ca>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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 v7] 8250-core based serial driver for OMAP + DMA

On Fri, Aug 15, 2014 at 07:42:28PM +0200, Sebastian Andrzej Siewior wrote:
> This is my complete queue fo the omap serial driver based on the 8250 core
> code. I played with it on beagle bone, am335x-evm and dra7xx including DMA.
> The uncertain remain the runtime-pm pieces.
> I hacked a small serial testing application which sent 10x 4KiB of data in
> raw mode. The number of interrupts in comparison:
> 
>     serial-omap | 8250 omap | 8250 omap + dma |
>    --------------------------------------------
> TX |       2558 |       641 |         0 +  30 |
> RX |      40960 |       854 |         1 + 853 |
> 
> So the 8250 version uses less interrupts for the same amount of data.
> The consequence is that in TX mode there should be "short" periods where
> no data is sent (before the CPU gets to re-fill the FIFO). On RX we have
> a smaller time frame where we have to start to purge the FIFO before it
> overflows.

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.

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?

-- 
Len Sorensen
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