platform/DRMFormat: Don't require a FormatInfo for each format. (#3463

)

We're now using `DRMFormat` in a situation (linux-dmabuf) where:
* We don't actually need to care about many the properties of the
format, and
* We're processing formats submitted by cilents.

This means that the existing “throw an exception if we haven't got a
`FormatInfo`” approach works badly; we will disconnect clients with a
Wayland internal error if they submit buffers in a format we haven't
explicitly listed, even though it *would* work if we just treated the
format as an opaque identifier.

So, don't do that anymore. Instead, store the fourcc code directly in
`DRMFormat`, only look up the `FormatInfo` (now `DRMFormat::Info`) when
code asks for it.

Additionally, we log the first time code asks for the `DRMFormat::Info`
for a format we haven't got info for; this way we can fill out the
needed formats on an as-noticed basis.

Fixes: #3462

debian/control

@@ -77,7 +77,7 @@ Description: Display server for Ubuntu - server library
  .
  Contains the shared library needed by server applications for Mir.
 
-Package: libmirplatform28
+Package: libmirplatform29
 Section: libs
 Architecture: linux-any
 Multi-Arch: same
@@ -140,7 +140,7 @@ Section: libdevel
 Architecture: linux-any
 Multi-Arch: same
 Pre-Depends: ${misc:Pre-Depends}
-Depends: libmirplatform28 (= ${binary:Version}),
+Depends: libmirplatform29 (= ${binary:Version}),
          libmircommon-dev (= ${binary:Version}),
          libboost-program-options-dev,
          ${misc:Depends},

debian/libmirplatform29.install

@@ -0,0 +1 @@
+usr/lib/*/libmirplatform.so.29

include/platform/mir/graphics/drm_formats.h

@@ -27,34 +27,47 @@ namespace mir::graphics
 class DRMFormat
 {
 public:
-    struct RGBComponentInfo
+    struct FormatInfo;
+    class Info
     {
-        uint32_t red_bits;
-        uint32_t green_bits;
-        uint32_t blue_bits;
-        std::optional<uint32_t> alpha_bits;
-    };
+    public:
+        struct RGBComponentInfo
+        {
+            uint32_t red_bits;
+            uint32_t green_bits;
+            uint32_t blue_bits;
+            std::optional<uint32_t> alpha_bits;
+        };
 
-    // This could be constexpr, at the cost of moving a bunch of implementation into the header
-    explicit DRMFormat(uint32_t fourcc_format);
+        auto opaque_equivalent() const -> std::optional<DRMFormat>;
+        auto alpha_equivalent() const -> std::optional<DRMFormat>;
 
-    auto name() const -> char const*;
+        bool has_alpha() const;
 
-    auto opaque_equivalent() const -> std::optional<DRMFormat> const;
-    auto alpha_equivalent() const -> std::optional<DRMFormat> const;
+        auto components() const -> std::optional<RGBComponentInfo> const&;
+    private:
+        friend class DRMFormat;
+        Info(FormatInfo const* info);
 
-    bool has_alpha() const;
+        FormatInfo const* info;
+    };
 
-    auto components() const -> std::optional<RGBComponentInfo> const&;
+    constexpr explicit DRMFormat(uint32_t fourcc_format)
+        : fourcc{fourcc_format}
+    {
+    }
+
+    auto name() const -> char const*;
+
+    auto info() const -> std::optional<Info const>;
 
     operator uint32_t() const;
 
     auto as_mir_format() const -> std::optional<MirPixelFormat>;
     static auto from_mir_format(MirPixelFormat format) -> DRMFormat;
 
-    struct FormatInfo;
 private:
-    FormatInfo const* info;
+    uint32_t fourcc;
 };
 
 auto drm_modifier_to_string(uint64_t modifier) -> std::string;

src/CMakeLists.txt

@@ -1,5 +1,5 @@
 # We need MIRPLATFORM_ABI in both libmirplatform and the platform implementations.
-set(MIRPLATFORM_ABI 28)
+set(MIRPLATFORM_ABI 29)
 
 set(MIRAL_VERSION_MAJOR 5)
 set(MIRAL_VERSION_MINOR 1)