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

[Mark Brown]

On Thu, Mar 23, 2017 at 01:02:57PM +0100, Lukas Wunner wrote:
> On Thu, Mar 23, 2017 at 11:07:18AM +0000, Mark Brown wrote:

> > This is going to depend on the software running on the board, I'd expect
> > it to be something configured at runtime rather than fixed in DT.

> On the Revolution Pi, the SPI master is hardwired to two KSZ8851 Ethernet
> controllers which are connected to various fieldbus gateways in a
> plug-and-play fashion.  Low latencies are needed to achieve a decent
> cycle rate when reading/writing data to the fieldbus devices.

> Hence for this use case, the RT priority is mandated by the hardware
> platform, which is why I put it in the devicetree.  It is needed
> regardless of which software is running.

Caring about having low latency access to fieldbus devices is a property
of the application, and achieving that via the use of RT priority for
the SPI pump thread is a property of current Linux implementations.
Neither of these things is a description of the hardware, the SPI pump
thread bit is already changing (for a very large proportion of use cases
we only use the thread to shut down devices once they become idle these
days).

> We could accommodate to setting the RT priority on the command line but
> we'd like to avoid always having to set it at runtime.  That is already
> possible by invoking chrt(1) from user space after the SPI master driver
> has loaded and wouldn't require a kernel patch.

Right, there's already mechanisms to do this at runtime.

> Also, if you question setting the RT priority in the devicetree, why
> was that functionality allowed for pl022 in the first place?

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.
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