Re: RT_PREEMPT on Raspberry PI 3

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2016-09-18 12:03:42 [+0200], Giuliano Colla wrote:
> I have managed to achieve both good performance and good stability, but I
> have been obliged to overcome some difficulties which IMHO could have been
> avoided if the Raspberry platform peculiarities had been given more
> attention. Being a very popular platform, particularly suited also for
> real-time applications, I believe it deserves it.

who should give Raspberry more attention?

> The main items are:
> 
> 1) The RT patches are not in sync with mainline Raspbian kernel versions. At
> the time of writing, the stock 4.x kernel supported is 4.4.21, while the
> last available patch is 4.4.19-rt27. Even going backwards it's impossible to
> find a perfect match: 4.4.14 against 4.4.12-rt20 or 4.4.15-rt23, and so on.

we do have v4.4.21-rt30 as of the time of writing. I hope that you do
not ask to align the -RT development & stable updates on the Raspberry
tree.

> I'm currently using a 4.4.13 kernel with a 4.4.12-rt19 patch, which was the
> best match I could find, and which required some minor manual adjustments.
> 
> Is there a way to provide a better sync?

You can always take the latest -RT patch from the v4.4 branch and
manually apply it on top of your BSP. And then fixup what broke.

> 2) The RT_PREEMPT patch is not compatible with FIQ (or maybe FIQ is not
> compatible with RT_PREEMPT). Leaving FIQ enabled generates intolerably
> frequent freezes. It appears to work properly only on the basic Raspberry pi
> (single core) but not on the multicore Pi2 and Pi3. Currently the only way
> to make a real-time kernel run on Raspberry PI 2 or 3 is to disable FIQ on
> the command line.

I wouldn't say that this is PREEMPT-RT's fault. FIQ is a non-maskable
interrupt which we do have on x86 and we call it NMI. As long as you
don't do anything illegal like taking locks which may lead to a dead
lock then no harm will happen to you. Therefore I assume that PREEMPT-RT
triggers the bugs that are already there more reliably.

> If the RT_PREEMPT patch could somehow support this incompatibility (either
> by fully disabling FIQ or fixing the code) this would be a great help, thus
> avoiding populating forums with "my raspberry pi2/3 freezes with rt patch"

We could disable FIQ on rpi3 if PREEMPT_RT is enabled but this would
require that the rpi3 is supported upstream. There is no way I suck the
rpi3 bsp into the RT queue. 

> Can something be done to solve those issues?

You could talk to the rpi3 people to put a link next to their bsp which
describes the common pitfalls with -RT.

> Thanks,
> 
> Giuliano

Sebastian