[PATCH weston v6 04/12] compositor: Version the backend configuration structures

[Pekka Paalanen]

On Fri, 15 Apr 2016 20:28:29 -0700
Bryce Harrington <bryce at osg.samsung.com> wrote:

> With this struct versioning, it is possible to add new options without
> breaking the ABI, as long as all additions are made to the end of a
> struct and nothing existing is modified or removed.  When things are
> added, the structure's size will increase, and we'll use this size as
> our minor version number.  If existing things need to be changed, then
> the major version, struct_version, is incremented to indicate the ABI
> break.
> 
> From our call sites in main these major and minor version will be
> recorded as struct_version and struct_size.  Each backend will then
> verify these against its own assumptions.  So long as the backend's
> struct is equal or larger than what was passed in and the major versions
> are equal, we're good; but if it is larger, then this is a fatal error.
> 
> Signed-off-by: Bryce Harrington <bryce at osg.samsung.com>
> Reviewed-by: Pekka Paalanen <pekka.paalanen at collabora.co.uk>
> 
> v6:
>  - Document refs for alternatives/assumptions for backend configs
> v5:
>  - Move the header changes to a pre-requisite patch from the drm backend
> 
> Signed-off-by: Bryce Harrington <bryce at osg.samsung.com>
> ---
>  src/compositor.h | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/src/compositor.h b/src/compositor.h
> index 5ca497c..a329dbe 100644
> --- a/src/compositor.h
> +++ b/src/compositor.h
> @@ -684,8 +684,30 @@ struct weston_backend_output_config {
>   * passed to the backend's init entry point. The backend will
>   * likely want to subclass this in order to handle backend specific
>   * data.
> + *
> + * NOTE: Alternate designs were proposed (Feb 2016) for using opaque
> + * structures and for section+key/value getter/setters.  The rationale
> + * for selecting the transparent structure design is based on several
> + * assumptions which may require re-evaluating the design choice if they
> + * fail to hold.
>   */
>  struct weston_backend_config {
> +   /** Major version for the backend-specific config struct
> +    *
> +    * This version must match exactly what the backend expects, otherwise
> +    * the struct is incompatible.
> +    */
> +   uint32_t struct_version;
> +
> +   /** Minor version of the backend-specific config struct
> +    *
> +    * This must be set to sizeof(struct backend-specific config).
> +    * If the value here is smaller than what the backend expects, the
> +    * extra config members will assume their default values.
> +    *
> +    * A value greater than what the backend expects is incompatible.
> +    */
> +   size_t struct_size;
>  };
>  
>  struct weston_backend {

Hi Bryce,

I brought back the links to the discussions you dug up earlier, and
pushed with that change:
   e9b8a2b..20b66c3  master -> master


Thanks,
pq
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