Re: [PATCH] ARM: PL011: Add support for Rx DMA buffer polling

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Fri, Jan 11, 2013 at 03:13:31PM +0900, Chanho Min wrote:
> In DMA support, Received data is not triggered until the DMA buffer is filled.
> In order to actually use Rx DMA, We would like to suggest the use of the timer
> for polling DMA buffer. It makes possible character-level trigger.
> In our test, no data loss occurred at high-baudrate as compared with interrupt
> driven (We tested with 3Mbps). We know it will be because of the increased Rx
> buffer rather than DMA effect. but,It is very useful for high speed uart device.
> We changes:
> 
> - CONFIG_SERIAL_AMBA_PL011_DMAPOLL is added to select this feature.
> 
> - add timer and last_residue for polling. Every polling, timer handler checks
>   the residue in the dma buffer and transfer data to the tty. Also, last_residue
>   is updated for the next polling.
> 
> - We use consistent DMA mappings instead of streaming DMA mappings to avoid from
>   the frequent cache operation.
> 
> - pl011_dma_rx_chars is modified. the pending size is recalculated because data
>   can be taken by polling.
> 
> - add module parameter for adjusting polling interval.

Here's the question: what is the effect of having to poll every tick on
the overall system?

What I'm getting at is: what is the cost of that polling with DMA enabled
over not having DMA on the receive side at all?  What is the trade-off
vs baud rate?  Should we scale the polling interval according to baud
rate?