[PATCH v3] spi: bitbang: only toggle bitchanges

[Mark Brown]

On Mon, Apr 13, 2015 at 12:59:30PM +0200, Michael Grzeschik wrote:
> The current implementation of bitbang_txrx_be_cpha0 and
> bitbang_txrx_be_cpha1 always call setmosi. That runs into several
> unnecessary calls into the gpiolib when the level of the GPIO actually
> has not to be changed.
> 
> This patch changes the routines to remember the last GPIO level
> and only calls setmosi if an change has to be made. This
> way it improves the transfer throughput.

Do you have any benchmarking or anything showing that this offers a
practical benefit?  Generally a bitbanging system would be using memory
mapped GPIOs for the bus so we're talking about replacing a memory
access with some shifts and logic operations here and while I can
believe that something that runs in cache is going to be an overall win
over going out to a device it's not quite so clear cut as it could be.
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