What to do with rtems_cache_disable_data()?

[Sebastian Huber]

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.

> 
> 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.

> 
> 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.

-- 
embedded brains GmbH & Co. KG
Herr Sebastian HUBER
Dornierstr. 4
82178 Puchheim
Germany
email: sebastian.huber at embedded-brains.de
phone: +49-89-18 94 741 - 16
fax:   +49-89-18 94 741 - 08

Registergericht: Amtsgericht München
Registernummer: HRB 157899
Vertretungsberechtigte Geschäftsführer: Peter Rasmussen, Thomas Dörfler
Unsere Datenschutzerklärung finden Sie hier:
https://embedded-brains.de/datenschutzerklaerung/