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

From: Lennart Sorensen
Date: Fri Aug 15 2014 - 14:17:13 EST

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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