drm/format-helper: Move drm_fb_build_fourcc_list() to sysfb helpers

Only sysfb drivers use drm_fb_build_fourcc_list(). Move the function
to sysfb helpers and rename it accordingly. Update drivers and tests.

v3:
- update naming in tests
v2:
- select DRM_SYSFB_HELPER (kernel test robot)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250616083846.221396-4-tzimmermann@suse.de

drivers/gpu/drm/sysfb/drm_sysfb_modeset.c

@@ -47,6 +47,144 @@ EXPORT_SYMBOL(drm_sysfb_mode);
  * Plane
  */
 
+static u32 to_nonalpha_fourcc(u32 fourcc)
+{
+	/* only handle formats with depth != 0 and alpha channel */
+	switch (fourcc) {
+	case DRM_FORMAT_ARGB1555:
+		return DRM_FORMAT_XRGB1555;
+	case DRM_FORMAT_ABGR1555:
+		return DRM_FORMAT_XBGR1555;
+	case DRM_FORMAT_RGBA5551:
+		return DRM_FORMAT_RGBX5551;
+	case DRM_FORMAT_BGRA5551:
+		return DRM_FORMAT_BGRX5551;
+	case DRM_FORMAT_ARGB8888:
+		return DRM_FORMAT_XRGB8888;
+	case DRM_FORMAT_ABGR8888:
+		return DRM_FORMAT_XBGR8888;
+	case DRM_FORMAT_RGBA8888:
+		return DRM_FORMAT_RGBX8888;
+	case DRM_FORMAT_BGRA8888:
+		return DRM_FORMAT_BGRX8888;
+	case DRM_FORMAT_ARGB2101010:
+		return DRM_FORMAT_XRGB2101010;
+	case DRM_FORMAT_ABGR2101010:
+		return DRM_FORMAT_XBGR2101010;
+	case DRM_FORMAT_RGBA1010102:
+		return DRM_FORMAT_RGBX1010102;
+	case DRM_FORMAT_BGRA1010102:
+		return DRM_FORMAT_BGRX1010102;
+	}
+
+	return fourcc;
+}
+
+static bool is_listed_fourcc(const u32 *fourccs, size_t nfourccs, u32 fourcc)
+{
+	const u32 *fourccs_end = fourccs + nfourccs;
+
+	while (fourccs < fourccs_end) {
+		if (*fourccs == fourcc)
+			return true;
+		++fourccs;
+	}
+	return false;
+}
+
+/**
+ * drm_sysfb_build_fourcc_list - Filters a list of supported color formats against
+ *                               the device's native formats
+ * @dev: DRM device
+ * @native_fourccs: 4CC codes of natively supported color formats
+ * @native_nfourccs: The number of entries in @native_fourccs
+ * @fourccs_out: Returns 4CC codes of supported color formats
+ * @nfourccs_out: The number of available entries in @fourccs_out
+ *
+ * This function create a list of supported color format from natively
+ * supported formats and additional emulated formats.
+ * At a minimum, most userspace programs expect at least support for
+ * XRGB8888 on the primary plane. Sysfb devices that have to emulate
+ * the format should use drm_sysfb_build_fourcc_list() to create a list
+ * of supported color formats. The returned list can be handed over to
+ * drm_universal_plane_init() et al. Native formats will go before
+ * emulated formats. Native formats with alpha channel will be replaced
+ * by equal formats without alpha channel, as primary planes usually
+ * don't support alpha. Other heuristics might be applied to optimize
+ * the sorting order. Formats near the beginning of the list are usually
+ * preferred over formats near the end of the list.
+ *
+ * Returns:
+ * The number of color-formats 4CC codes returned in @fourccs_out.
+ */
+size_t drm_sysfb_build_fourcc_list(struct drm_device *dev,
+				   const u32 *native_fourccs, size_t native_nfourccs,
+				   u32 *fourccs_out, size_t nfourccs_out)
+{
+	/*
+	 * XRGB8888 is the default fallback format for most of userspace
+	 * and it's currently the only format that should be emulated for
+	 * the primary plane. Only if there's ever another default fallback,
+	 * it should be added here.
+	 */
+	static const u32 extra_fourccs[] = {
+		DRM_FORMAT_XRGB8888,
+	};
+	static const size_t extra_nfourccs = ARRAY_SIZE(extra_fourccs);
+
+	u32 *fourccs = fourccs_out;
+	const u32 *fourccs_end = fourccs_out + nfourccs_out;
+	size_t i;
+
+	/*
+	 * The device's native formats go first.
+	 */
+
+	for (i = 0; i < native_nfourccs; ++i) {
+		/*
+		 * Several DTs, boot loaders and firmware report native
+		 * alpha formats that are non-alpha formats instead. So
+		 * replace alpha formats by non-alpha formats.
+		 */
+		u32 fourcc = to_nonalpha_fourcc(native_fourccs[i]);
+
+		if (is_listed_fourcc(fourccs_out, fourccs - fourccs_out, fourcc)) {
+			continue; /* skip duplicate entries */
+		} else if (fourccs == fourccs_end) {
+			drm_warn(dev, "Ignoring native format %p4cc\n", &fourcc);
+			continue; /* end of available output buffer */
+		}
+
+		drm_dbg_kms(dev, "adding native format %p4cc\n", &fourcc);
+
+		*fourccs = fourcc;
+		++fourccs;
+	}
+
+	/*
+	 * The extra formats, emulated by the driver, go second.
+	 */
+
+	for (i = 0; (i < extra_nfourccs) && (fourccs < fourccs_end); ++i) {
+		u32 fourcc = extra_fourccs[i];
+
+		if (is_listed_fourcc(fourccs_out, fourccs - fourccs_out, fourcc)) {
+			continue; /* skip duplicate and native entries */
+		} else if (fourccs == fourccs_end) {
+			drm_warn(dev, "Ignoring emulated format %p4cc\n", &fourcc);
+			continue; /* end of available output buffer */
+		}
+
+		drm_dbg_kms(dev, "adding emulated format %p4cc\n", &fourcc);
+
+		*fourccs = fourcc;
+		++fourccs;
+	}
+
+	return fourccs - fourccs_out;
+}
+EXPORT_SYMBOL(drm_sysfb_build_fourcc_list);
+
 int drm_sysfb_plane_helper_atomic_check(struct drm_plane *plane,
 					struct drm_atomic_state *new_state)
 {