[PATCH 2/2] spi: bcm2835: Allow platform to set realtime priority

Mark Brown broonie at kernel.org
Thu Mar 23 09:22:11 PDT 2017

On Thu, Mar 23, 2017 at 03:08:01PM +0100, kernel at martin.sperl.org wrote:

> * we use an “inline” spi-pump if there are no messages in the message
>   queue - the spi-message-pump-thread is not used in those cases.
>   So those spi messages are NOT handled with the proposed RT priority,
>   as they run within the context of the “caller” - which may even be
>   the priority of userspace if spidev is used.

Yeah, this might actually make things worse if the RT thread is used
since unless there's multiple contending users or async users the pump
is only responsible for idling the controller when it goes idle so
making it RT can result in more idle/resume cycles (though they are
fairly cheap usually) and context thrashing doing that.

> > This was being done via platform data not via device tree, unlike
> > platform data device tree should provide a long term stable OS neutral
> > ABI.

> Well - if it is really a necessity and as the mantra for DT is:
> "it should describe the HW only", why not use a module parameter
> instead or use chrt() from userspace?

Yes, those are fine.
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