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

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Message-ID: <d3f47a4e-0692-4f0f-9092-cb109fc805fd@suse.de>
Date: Wed, 10 Sep 2025 14:13:49 +0200
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Nathan Chancellor <nathan@...nel.org>,
 LiangCheng Wang <zaq14760@...il.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/pixpaper: Fix return type of pixpaper_mode_valid()



Am 08.09.25 um 22:24 schrieb Nathan Chancellor:
> 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>

Reviewed-by: Thomas Zimmermann <tzimmermann@...e.de>

Thanks for the fix.

> ---
>   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>
>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)