On 18/6/2024 12:02 am, Sebastian Huber wrote: > On 17.06.24 03:35, Chris Johns wrote: >> On 14/6/2024 10:42 pm, Peter Dufault wrote: >>> >>> >>>> On Jun 14, 2024, at 5:47 AM, Sebastian Huber >>>> <sebastian.hu...@embedded-brains.de> wrote: >>>> >>>> Hello, >>>> >>>> an user noticed that for example on the Xilinx Zynq 7000 BSP, the >>>> rtems_cache_disable_data() doesn't work. >>>> >>>> I had a look at this and managed to disable the L1 and L2 caches, however, >>>> afterwards I got not that far. On the Cortex-A cores it seems at least the >>>> L1 data cache is required to provide support for atomic operations: >>>> >>>> https://stackoverflow.com/questions/76207164/disabled-dcache-will-prevent-atomic-flag-from-being-set >>>> >>>> I guess we have this situation on most modern chips with caches since the >>>> reservation granule is usually a cache line. How do we want to deal with >>>> rtems_cache_disable_data() in this case? From my point of view, this >>>> function as no real use case and adding it in the first place was a >>>> mistake. >>>> >>>> We have a couple of options: >>>> >>>> * Make the rtems_cache_disable_data() directive a no-operation. Afterwards >>>> the cache is still enabled, and an user may get confused. >>>> >>>> * Issue a fatal error if someone calls rtems_cache_disable_data(). >>>> >>>> * Really disable the cache and let the user figure out that the atomic >>>> operations no longer work. Disabling the data cache can be a quite complex >>>> thing to do. >>>> >>>> * Add a return status code to rtems_cache_disable_data() and let it return >>>> RTEMS_UNSATISFIED for example. >>> >>> Assuming "this function has no real use case and adding it in the first >>> place >>> was a mistake" how about: >>> >>> - In the active release continue to disable the data cache but add a warning >>> attribute 'warning ("Disabling data cache breaks atomic functionality")'; >>> - In the next release change it to an error attribute and change the >>> function >>> behavior to do nothing except return RTEMS_UNSATISFIED (in case someone >>> somehow still calls it), or better change it to call an RTEMS fatal >>> function. >>> >> >> I am not so sure it can be easily simplified. I know of a Zynq or ZynqMP >> application in a critical system (bare metal, no OS) with the caches disabled >> for a specific reason. I cannot go into or remember the specific detail. > > Even in such a scenario you would probably not enable the data cache and then > disable it with a function call? You would simply configure the system to not > enable the cache at all.
Yes this would be correct, it held off. >> We cannot silently disable functionality. A call must do what it says or >> return >> or raise an error. If disabling the data cache is not implement there should >> be >> an error. The error would draw the user to the documentation. It is not yet >> clear we can disable the code in that function. >> >> If our kernel code uses atomics then the data cache cannot be disabled. >> However >> we should look into the use of atomics and if it is SMP specific and then >> handle >> the specific cases. > > The atomic operations are defined by the C/C++ standard. This is not an > SMP-specific issue. It is also not about the usage of the cache in general. It > is about the rtems_cache_disable_data() directive. My comments about not having silent things happens is common to all things we do. We need an error if what we have is not workable on a platform. >> A user can use atomics even if the kernel does not but leaving that to the >> user >> to figure out and deal is at best unhelpful and as an OS provider we should >> help >> users in these complex corner cases. At a minimum our documentation should >> cover >> this and we could update rtems-exeinfo to report a potential issue. > > Which of the above four options would be your favourite? Would there be > another > option to deal with rtems_cache_disable_data()? > > Currently, calling this function crashes the system on a Zynq 7000. The return code but is it realistic to make such a change for such a long standing interface? I suppose it is then the fatal error. It is a major thing to do disabling the cache. Out of interest why do we have the interface? What is the use case? Chris _______________________________________________ devel mailing list devel@rtems.org http://lists.rtems.org/mailman/listinfo/devel
