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

[Jacopo Mondi]

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 comment to match the most recent of th IPA/IPC
interface definition and generation mechanism.

Signed-off-by: Jacopo Mondi <jacopo at jmondi.org>
---
 include/libcamera/ipa/core.mojom | 60 +++++++++++++++++++-------------
 1 file changed, 35 insertions(+), 25 deletions(-)

diff --git a/include/libcamera/ipa/core.mojom b/include/libcamera/ipa/core.mojom
index b32f30939454..d2017369b597 100644
--- a/include/libcamera/ipa/core.mojom
+++ b/include/libcamera/ipa/core.mojom
@@ -15,37 +15,47 @@ 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 mojo file for the Mojo core to accept it
+ *   - this attribute allows to define a symbol for the Mojo core that
+ *     corresponds to a library-defined 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
+ *   - do not generate a (de)serializer for the structure
+ *   - all types need a (de)serializer to be defined in order to be transported
+ *     over the IPA protocol. The (de)serializer can be:
+ *     - manually implemented in the core library as in example the
+ *       ControlSerializer class that handles libcamera control-related types
+ *     - provided as a template specialization as done in ipa_data_serializer.h
+ *       for POD types and C++ containers
+ *     - generated at build time for types defined in a mojo interfaces
+ *       definition
+ *   - 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
  *
  * 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 library C++ header and a (de)serializer is
+ *   available, either manually written in the library or in
+ *   ipa_data_serializer.h, then the struct shall be declared as empty,
+ *   with both the [skipHeader] and [skipSerdes] attributes associated
+ * - If the type is defined in the library but no (de)serializer is available
+ *   then the type definition in the core.mojom file should have the
+ *   [skipHeader] attribute only
+ * - If a type definition has [skipHeader], then the header where the type is
+ *   defined must be included in ipa_interface.h
+ * - Nested types (e.g. FrameBuffer::Plane) cannot be directly defined in mojom
+ *   - Avoid them, by defining the nested type in a C++ header and provide a
+ *     (de)serializer
+ *   - The C++ namespace separator :: is replaced with a dot
+ *   - In example, to refer to FrameBuffer::Plane provide a definition of the
+ *     Plane type in a C++ header to be included, provide a deserializer and
+ *     reference it as FrameBuffer.Plane
+ * - Types that are contained in an array/map do not require a mojom definition
+ *   if one exists in the library.
  * - [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.
-- 
2.32.0