SPI: performance regression when using the common message queuing infrastructure

[Grygorii Strashko]

On 07/06/2016 12:50 PM, Cyrille Pitchen wrote:
> Hi Mark,
>
> recently Heiko reported to us a performance regression with Atmel SPI
> controllers. He noticed the issue on a sam9g15ek board and I was also able to
> reproduce it on a sama5d36ek board.
>
> We found out that the performance regression was introduced in 3.14 by commit:
> 8090d6d1a415d3ae1a7208995decfab8f60f4f36
> spi: atmel: Refactor spi-atmel to use SPI framework queue
>
> For the test, I connected a Spansion S25FL512 memory on the SPI1 controller of
> a sama5d36ek board. Then with an oscilloscope I monitored the chip-select, clock
> and MOSI signals on the SPI bus.
>
>
> 1 - Reading 512 bytes from the memory
>
> # dd if=/dev/mtd6 bs=512 count=1 of=/dev/null
>
> With the oscilloscope, I measured the time between the chip-select fell before
> the Read Status command (05h) and the chip-select rose after all data had been
> read by the 4-byte address Fast Read 1-1-1 command (13h).
>
> 3.14 vanilla                      : 305 µs
> 3.14 commit 8090d6d1a415 reverted : 242 µs   -21%
>
> 2 - Reading 1000 x 1024 bytes from the memory
>
> # dd if=/dev/mtd6 bs=1024 count=1000 of=/dev/null
>
> Still with the scope, I measured the time to read all data.
>
> 3.14 vanilla                      : 435 ms
> 3.14 commit 8090d6d1a415 reverted : 361 ms   -17%
>
>
> Indeed the oscilloscope shows that more time is spent between messages and
> transfers.
>
> commit 8090d6d1a415 replaced the tasklet used to manage a SPI message/transfer
> queue by a workqueue provided by the SPI framework.
>
> The support of this (optional) workqueue was introduced by commit:
> ffbbdd21329f3e15eeca6df2d4bc11c04d9d91c0
> spi: create a message queuing infrastructure
>
> Though the commit message claims that is common infrastructure is optional,
> the patch also claims the .transfer() hook is deprecated, suggesting drivers
> should implement the new .transfer_one_message() hook instead.
>
> This is the reason why commit 8090d6d1a415 was submitted. However we lost
> quite amount of performances moving from our tasklet to the generic workqueue.
>
> So do you recommend us to keep our current generic implementation relying on
> the SPI framework workqueue or to go back to a custom implementation using
> tasklet?
> If we keep the current implementation, is there a way to improve the
> performances so we go back to something close to what he had before?
>
> We saw in commit ffbbdd21329f that we can change the workqueue thread
> scheduling policy to SCHED_FIFO by setting master->rt.
>

master->rt is not a good choice as i know and
you may find thread [1] useful for you.

[1] http://www.spinics.net/lists/linux-rt-users/msg14347.html

-- 
regards,
-grygorii