linux-kernel - [PATCH v2 1/6] drm/plane: Make format_mod

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Date:   Wed, 22 Dec 2021 10:05:47 +0100
From:   José Expósito <jose.exposito89@...il.com>
To:     contact@...rsion.fr
Cc:     dmitry.baryshkov@...aro.org, maarten.lankhorst@...ux.intel.com,
        mripard@...nel.org, tzimmermann@...e.de, airlied@...ux.ie,
        daniel@...ll.ch, jani.nikula@...ux.intel.com,
        joonas.lahtinen@...ux.intel.com, rodrigo.vivi@...el.com,
        marex@...x.de, stefan@...er.ch, shawnguo@...nel.org,
        s.hauer@...gutronix.de, kernel@...gutronix.de, festevam@...il.com,
        linux-imx@....com, yannick.fertre@...s.st.com,
        philippe.cornu@...s.st.com, benjamin.gaignard@...aro.org,
        mcoquelin.stm32@...il.com, alexandre.torgue@...s.st.com,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-stm32@...md-mailman.stormreply.com,
        José Expósito <jose.exposito89@...il.com>
Subject: [PATCH v2 1/6] drm/plane: Make format_mod_supported truly optional

The documentation for "drm_plane_funcs.format_mod_supported" reads:

  This *optional* hook is used for the DRM to determine if the given
  format/modifier combination is valid for the plane. This allows the
  DRM to generate the correct format bitmask (which formats apply to
  which modifier), and to validate modifiers at atomic_check time.

  *If not present*, then any modifier in the plane's modifier
  list is allowed with any of the plane's formats.

However, where the function is not present, an invalid IN_FORMATS blob
property with modifiers but no formats is exposed to user-space.

This breaks the latest Weston [1]. For testing purposes, I extracted the
affected code to a standalone program [2].

Make "create_in_format_blob" behave as documented.

[1] https://gitlab.freedesktop.org/wayland/weston/-/blob/9.0/libweston/backend-drm/kms.c#L431
[2] https://github.com/JoseExposito/drm-sandbox/blob/main/in_formats.c

Signed-off-by: José Expósito <jose.exposito89@...il.com>
---
 drivers/gpu/drm/drm_plane.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
index 82afb854141b..c1186b7215ee 100644
--- a/drivers/gpu/drm/drm_plane.c
+++ b/drivers/gpu/drm/drm_plane.c
@@ -202,17 +202,13 @@ static int create_in_format_blob(struct drm_device *dev, struct drm_plane *plane
 
 	memcpy(formats_ptr(blob_data), plane->format_types, formats_size);
 
-	/* If we can't determine support, just bail */
-	if (!plane->funcs->format_mod_supported)
-		goto done;
-
 	mod = modifiers_ptr(blob_data);
 	for (i = 0; i < plane->modifier_count; i++) {
 		for (j = 0; j < plane->format_count; j++) {
-			if (plane->funcs->format_mod_supported(plane,
+			if (!plane->funcs->format_mod_supported ||
+			    plane->funcs->format_mod_supported(plane,
 							       plane->format_types[j],
 							       plane->modifiers[i])) {
-
 				mod->formats |= 1ULL << j;
 			}
 		}
-- 
2.25.1