[PATCH v3] spi: bitbang: only toggle bitchanges

[Marc Kleine-Budde]

On April 18, 2015 2:09:26 PM GMT+02:00, Mark Brown <broonie at kernel.org> wrote:
>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.

IIRC we had a reduction of about 30% when programming an FPGA via SPI. But Michael can give you the exact numbers. 

Marc