[PATCH v2] drm/fourcc: introduce DRM_FOURCC_STANDALONE guard

[James Park]

On Thu, Feb 4, 2021 at 3:37 AM Emil Velikov <emil.l.velikov at gmail.com> wrote:
>
> Currently, the drm_fourcc.h header depends on drm.h for __u32 and __u64.
> At the same time drm.h pulls a lot of unneeded symbols.
>
> Add new guard DRM_FOURCC_STANDALONE, which when set will use local
> declaration of said symbols.
>
> When used on linux - we pull linux/types.h which is used either way.
> On other platforms, BSDs et al, we need a couple of typedefs.
>
> Since those can trigger a warning in some corner-cases*, add some GCC
> magic to silence them. Note that incorrect type redefinitions will still
> be flagged, and the GCC pragma is ignored by other compilers.
>
> *Corner-case:
> If one sets DRM_FOURCC_STANDALONE and compiles with C99 or earlier while
> also using -pedantic _and_ the header lives outside of the standard
> /usr/include (like BSDs normally do).
>
> v2:
>  - Add corner-case handling, based on popular demand.
>
> Cc: James Park <james.park at lagfreegames.com>
> Cc: Pekka Paalanen <pekka.paalanen at collabora.com>
> Cc: Simon Ser <contact at emersion.fr>
> Signed-off-by: Emil Velikov <emil.l.velikov at gmail.com>
> ---
>  include/uapi/drm/drm.h        | 10 ++++++++++
>  include/uapi/drm/drm_fourcc.h | 29 +++++++++++++++++++++++++++++
>  2 files changed, 39 insertions(+)
>
> diff --git a/include/uapi/drm/drm.h b/include/uapi/drm/drm.h
> index 808b48a93330..cd78950e05ce 100644
> --- a/include/uapi/drm/drm.h
> +++ b/include/uapi/drm/drm.h
> @@ -53,6 +53,15 @@ typedef unsigned int drm_handle_t;
>  #include <stdint.h>
>  #include <sys/ioccom.h>
>  #include <sys/types.h>
> +
> +/*
> + * When using C99 -pedantic the typedefs will trigger a warning.
> + * If the header is considered a system one (-isystem) those will be
> + * ignored, yet on the target platforms BSDs, et al - the headers live
> + * in a non-system location.
> + */
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wpedantic"
>  typedef int8_t   __s8;
>  typedef uint8_t  __u8;
>  typedef int16_t  __s16;
> @@ -63,6 +72,7 @@ typedef int64_t  __s64;
>  typedef uint64_t __u64;
>  typedef size_t   __kernel_size_t;
>  typedef unsigned long drm_handle_t;
> +#pragma GCC diagnostic pop
>
>  #endif
>
> diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
> index 6f0628eb13a6..84a1f96cc4ef 100644
> --- a/include/uapi/drm/drm_fourcc.h
> +++ b/include/uapi/drm/drm_fourcc.h
> @@ -24,7 +24,36 @@
>  #ifndef DRM_FOURCC_H
>  #define DRM_FOURCC_H
>
> +/*
> + * Define DRM_FOURCC_STANDALONE you're interested only FOURCC and do not want
> + * to pull drm.h into your application.
> + */
> +#ifdef DRM_FOURCC_STANDALONE
> +#if defined(__linux__)
> +
> +#include <linux/types.h>
> +
> +#else /* One of the BSDs */
> +
> +#include <stdint.h>
> +
> +/*
> + * When using C99 -pedantic the typedefs will trigger a warning.
> + * If the header is considered a system one (-isystem) those will be
> + * ignored, yet on the target platforms BSDs, et al - the headers live
> + * in a non-system location.
> + */
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wpedantic"
> +typedef uint32_t __u32;
> +typedef uint64_t __u64;
> +#pragma GCC diagnostic pop
> +
> +#endif /* __linux __ */
> +
> +#else
>  #include "drm.h"
> +#endif /* DRM_FOURCC_STANDALONE */
>
>  #if defined(__cplusplus)
>  extern "C" {
> --
> 2.30.0
>

I remember reading GCC diagnostic push/pop requires a recent enough
compiler version to be supported, which is pretty old, but I don't
know how old is old enough for Linux headers:
https://github.com/protocolbuffers/protobuf/issues/4156

Testing snippets in godbolt, I think the pragmas need to be wrapped. MSVC says:

warning C4068: unknown pragma 'GCC'

Also, Clang seems to want -Wtypedef-redefinition, not -Wpedantic. GCC
complains it doesn't know what -Wtypedef-redefinition is, so that
would also need to be wrapped.