What to do with rtems_cache_disable_data()?

[Sebastian Huber]

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.

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