Hi, I have written recently a patch that solve the FIQ issue with RT Preempt on the 4-core Raspberry Pi board. Please see this article for full description on the solution. https://www.osadl.org/Single-View.111+M5c03315dc57.0.html Regards, Oussama On Thu, Sep 22, 2016 at 3:10 PM, Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> wrote: > 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 -- 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
