[libcamera-devel] [PATCH v4] ipa: core.mojom: Rework core file documentation

[Kieran Bingham]

Hi Jacopo,

On 27/07/2021 08:50, Jacopo Mondi wrote:
> The comment block at the beginning of the core.mojom file is meant to
> provide an overview of how to use libcamera defined types in the definition
> of mojom interfaces.
> 
> As the IPA/IPC interface definition mechanism evolved, the documentation
> has not been updated accordingly.
> 
> Update the file comments to match the most recent IPA/IPC
> interface definition and generation mechanism.
> 
> Signed-off-by: Jacopo Mondi <jacopo at jmondi.org>
> ---
> v3->v4:
> - Apply wording suggestions from Paul
> 
> v2-v3:
> - Address Paul's comment:
>   - s/[Mojo core|build system]/code generator/
>   - Swap points in skipSerdes description to make clear the flag instruct
>     the code generator not to generate a deserializer
>   - Clarify that nested types can *solely* be used as map/array members
> 
> v1->v2:
> - Address Paul's comment and clarify points that were not clear to me when I
>   wrote v1 :)
>   - (de)serializers implementations go in ipa_data_serializer.cpp
>   - types used as array/map members do not require a mojom definition
>   - remove duplications
>   - s/the library/libcamera
> - While at it, make all statements start with a capital letter and end without a
>   full-stop.
> ---
> ---

Lots of extra --- to watch out for when applying there ;-)

Sounds good.

Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>

>  include/libcamera/ipa/core.mojom | 68 +++++++++++++++++++-------------
>  1 file changed, 41 insertions(+), 27 deletions(-)
> 
> diff --git a/include/libcamera/ipa/core.mojom b/include/libcamera/ipa/core.mojom
> index b32f30939454..29ba35482b39 100644
> --- a/include/libcamera/ipa/core.mojom
> +++ b/include/libcamera/ipa/core.mojom
> @@ -15,40 +15,54 @@ module libcamera;
>   *
>   * Attributes:
>   * - skipHeader - structs only, and only in core.mojom
> - *   - designate that this struct shall not have a C++ header definition
> - *     generated
> + *   - Do not generate a C++ definition for the structure
> + *   - Any type used in a mojom interface definition must have a corresponding
> + *     definition in a mojom file for the code generator to accept it, except
> + *     for types solely used as map/array members for which a definition is not
> + *     required
> + *   - This attribute allows defining a symbol for the code generator that
> + *     corresponds to a libcamera type without duplicating its definition in the
> + *     generated C++ headers
>   * - skipSerdes - structs only, and only in core.mojom
> - *   - designate that this struct shall not have a (de)serializer generated
> - *   - all fields need a (de)serializer to be defined, either hand-written
> - *     in ipa_data_serializer.h
> + *   - All types need a (de)serializer to be defined in order to be transported
> + *     over IPC. The (de)serializer can be:
> + *     - Manually implemented as a template specialization in
> + *       ipa_data_serializer.cpp in the libcamera sources
> + *     - Generated at build time for types defined in a mojom file
> + *   - This attribute instructs the build system that a (de)serializer is
> + *     available for the type and there's no need to generate one
>   * - hasFd - struct fields or empty structs only
> - *   - designate that this field or empty struct contains a FileDescriptor
> + *   - Designate that this field or empty struct contains a FileDescriptor
>   *
>   * Rules:
> - * - Any struct that is used in a struct definition in mojom must also be
> - *   defined in mojom
> - *   - If the struct has both a definition in a C++ header and a (de)serializer
> - *     in ipa_data_serializer.h, then the struct shall be declared as empty,
> - *     with both the [skipHeader] and [skipSerdes] attributes
> - *   - If the struct only has a definition in a C++ header, but no
> - *     (de)serializer, then the struct definition should have the [skipHeader]
> - *     attribute
> - * - Nested structures (e.g. FrameBuffer::Plane) cannot be defined in mojom.
> - *   - Avoid them, by defining them in a header in C++ and a (de)serializer in
> - *     ipa_data_serializer.h
> - * - If a struct is in an array/map inside a struct, then the struct that is
> - *   the member of the array/map does not need a mojom definition if it is
> - *   defined in a C++ header.
> - *   - This can be used to embed nested structures. The C++ double colon is
> - *     replaced with a dot (e.g. FrameBuffer::Plane -> FrameBuffer.Plane)
> - *   - The struct must still be defined in a header in C++ and a (de)serializer
> - *     implemented in ipa_data_serializer.h, as it cannot be defined in mojom
> + * - If the type is defined in a libcamera C++ header *and* a (de)serializer is
> + *   available then the type shall be declared as empty with both attributes
> + *   associated and specified as: [skipHeader, skipSerdes]
> + *   - Example: [skipHeader, skipSerdes] ControlList {};
> + * - If the type is defined in libcamera but no (de)serializer is available
> + *   then the type definition in the core.mojom file should have the
> + *   [skipHeader] attribute only
> + *   - A (de)serializer will be generated for the type
> + * - If a type definition has [skipHeader], then the header where the type is
> + *   defined must be included in ipa_interface.h
> + * - Types that are solely used as array/map members do not require a mojom
> + *   definition if one exists in libcamera
> + * - Nested types (e.g. FrameBuffer::Plane) cannot be defined in mojom
> + *   - If used in mojom, the nested type shall be defined in a C++ header
> + *     and a (de)serializer shall be provided
> + *   - Nested types can only be used as array/map members
> + *   - When using the type, the C++ namespace separator :: is replaced with a
> + *     dot
> + *   - In example, to use the FrameBuffer::Plane type in mojom:
> + *     - Provide a definition of the FrameBuffer::Plane type in a C++ header
> + *     - Include the header in ipa_interface.h
> + *     - Provide a (de)serializer implementation ipa_data_serializer.cpp
> + *     - In mojom, reference the type as FrameBuffer.Plane and only as map/array
> + *       member
>   * - [skipHeader] and [skipSerdes] only work here in core.mojom.
> - * - If a struct definition has [skipHeader], then the header where the
> - *   struct is defined must be #included in ipa_interface.h
>   * - If a field in a struct has a FileDescriptor, but is not explicitly
>   *   defined so in mojom, then the field must be marked with the [hasFd]
> - *   attribute.
> + *   attribute
>   *
>   * \todo Generate documentation from Doxygen comments in .mojom files
>   * \todo Figure out how to keep the skipHeader structs in sync with their
> --
> 2.32.0
>