linux-kernel - [PATCH] drm/pixpaper: Fix return type of pixpaper_mode

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Message-Id: <20250908-drm-pixpaper-fix-mode_valid-return-type-v1-1-705ceaf03757@kernel.org>
Date: Mon, 08 Sep 2025 13:24:59 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: LiangCheng Wang <zaq14760@...il.com>, 
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
 Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH] drm/pixpaper: Fix return type of pixpaper_mode_valid()

When building with -Wincompatible-function-pointer-types-strict, a
warning designed to catch kernel control flow integrity (kCFI) issues at
build time, there is an instance in the new tiny DRM pixpaper driver:

  drivers/gpu/drm/tiny/pixpaper.c:982:16: error: incompatible function pointer types initializing 'enum drm_mode_status (*)(struct drm_crtc *, const struct drm_display_mode *)' with an expression of type 'int (struct drm_crtc *, const struct drm_display_mode *)' [-Werror,-Wincompatible-function-pointer-types-strict]
    982 |         .mode_valid = pixpaper_mode_valid,
        |                       ^~~~~~~~~~~~~~~~~~~

While 'int' and 'enum drm_mode_status' are ABI compatible, hence no
regular warning from -Wincompatible-function-pointer-types, the mismatch
will trigger a kCFI violation when pixpaper_mode_valid() is called
indirectly.

Update the return type of pixpaper_mode_valid() to be
'enum drm_mode_status' to clear up the warning and kCFI violation.

Fixes: c9e70639f591 ("drm: tiny: Add support for Mayqueen Pixpaper e-ink panel")
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---
 drivers/gpu/drm/tiny/pixpaper.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/tiny/pixpaper.c b/drivers/gpu/drm/tiny/pixpaper.c
index b1379cb5f030..32598fb2fee7 100644
--- a/drivers/gpu/drm/tiny/pixpaper.c
+++ b/drivers/gpu/drm/tiny/pixpaper.c
@@ -968,8 +968,8 @@ static const struct drm_crtc_funcs pixpaper_crtc_funcs = {
 	.atomic_destroy_state = drm_atomic_helper_crtc_destroy_state,
 };
 
-static int pixpaper_mode_valid(struct drm_crtc *crtc,
-			       const struct drm_display_mode *mode)
+static enum drm_mode_status
+pixpaper_mode_valid(struct drm_crtc *crtc, const struct drm_display_mode *mode)
 {
 	if (mode->hdisplay == PIXPAPER_WIDTH &&
 	    mode->vdisplay == PIXPAPER_HEIGHT) {

---
base-commit: 490b30fbaca2abbd6afa8bdc7e2df329b5d82412
change-id: 20250908-drm-pixpaper-fix-mode_valid-return-type-4228e531193b

Best regards,
--  
Nathan Chancellor <nathan@...nel.org>