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 -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
