[PATCH v1 3/4] libcamera: controls: Arrays of arrays are not supported

[Barnabás Pőcze]

Hi


2025. 04. 01. 23:45 keltezéssel, Laurent Pinchart írta:
> On Tue, Apr 01, 2025 at 03:19:38PM +0200, Barnabás Pőcze wrote:
>> Arrays of arrays, even arrays of strings, are not supported by
>> the current `ControlValue` mechanism, so disable them to trigger
>> comptime errors if attempts are made to use them.
> 
> What currently happens if we try to use them ?

There are two options I can see:

  * runtime assertion failure in `set()` because `elementSize == ControlValueSize[type]`
    is not true, e.g. `Span<const std::string>`
  * stores spans inside `ControlValue` (with arbitrarily dangling pointers),
    e.g. `Span<Span<Rectangle>>` (since `sizeof(Span<Rectangle>) == sizeof(Rectangle)`
    on x86-64)

So they don't really work in any reasonable capacity,
this is why I want to disable them for now.


Regards,
Barnabás Pőcze

> 
>> Signed-off-by: Barnabás Pőcze <barnabas.pocze at ideasonboard.com>
>> ---
>>   include/libcamera/controls.h    | 2 +-
>>   src/apps/cam/capture_script.cpp | 4 ----
>>   2 files changed, 1 insertion(+), 5 deletions(-)
>>
>> diff --git a/include/libcamera/controls.h b/include/libcamera/controls.h
>> index d35347f4c..85c724ec1 100644
>> --- a/include/libcamera/controls.h
>> +++ b/include/libcamera/controls.h
>> @@ -120,7 +120,7 @@ struct control_type<Point> {
>>   };
>>   
>>   template<typename T, std::size_t N>
>> -struct control_type<Span<T, N>> : public control_type<std::remove_cv_t<T>> {
>> +struct control_type<Span<T, N>, std::enable_if_t<control_type<std::remove_cv_t<T>>::size == 0>> : public control_type<std::remove_cv_t<T>> {
>>   	static constexpr std::size_t size = N;
>>   };
>>   
>> diff --git a/src/apps/cam/capture_script.cpp b/src/apps/cam/capture_script.cpp
>> index fc1dfa75f..f3262af98 100644
>> --- a/src/apps/cam/capture_script.cpp
>> +++ b/src/apps/cam/capture_script.cpp
>> @@ -600,10 +600,6 @@ ControlValue CaptureScript::parseArrayControl(const ControlId *id,
>>   		value = Span<const float>(values.data(), values.size());
>>   		break;
>>   	}
>> -	case ControlTypeString: {
>> -		value = Span<const std::string>(repr.data(), repr.size());
>> -		break;
>> -	}
>>   	default:
>>   		std::cerr << "Unsupported control type" << std::endl;
>>   		break;
>