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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