What to do with rtems_cache_disable_data()?

[Gedare Bloom]

On Mon, Jun 17, 2024 at 11:20 PM Chris Johns <chrisj at rtems.org> wrote:
>
> 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.huber at 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.
>
Given the age of the interface, I would be fine with making it a fatal
error if called with SMP enabled. Currently it gets called during some
bsp restart procedures. most likely, invalidating the cache would be
effective in those cases.

I would be in favor of deprecating the interface now, and make it
obsolete in the future.

> Out of interest why do we have the interface? What is the use case?
>
Joel may know.

> Chris
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