Subject: FAILED: patch "[PATCH] drm/fb-helper: defer setup when fbdev_probe() fails, not just" failed to apply to 5.15-stable tree
To: ngocthang2710.1999@gmail.com,tzimmermann@suse.de
Cc: <stable@vger.kernel.org>
From: <gregkh@linuxfoundation.org>
Date: Mon, 12 Oct 2026 17:29:46 +0200
Message-ID: <2026101246-reason-finished-ac79@gregkh>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=ANSI_X3.4-1968
Content-Transfer-Encoding: 8bit


The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 755c94d178b7e972bb766199d34e4ba4f63a89eb
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026101246-reason-finished-ac79@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:


thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 755c94d178b7e972bb766199d34e4ba4f63a89eb Mon Sep 17 00:00:00 2001
From: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>
Date: Sun, 27 Sep 2026 14:54:34 +0700
Subject: [PATCH] drm/fb-helper: defer setup when fbdev_probe() fails, not just
 on -EAGAIN

__drm_fb_helper_initial_config_and_unlock() calls
drm_client_modeset_probe() first, which already fills in each
mode_set->mode/num_connectors for every connected output, and only
afterwards calls drm_fb_helper_single_fb_probe() to create
fb_helper->fb and wire it into those mode_sets via
drm_setup_crtcs_fb().

If drm_fb_helper_single_fb_probe() fails with anything other than
-EAGAIN (e.g. dev->driver->fbdev_probe() itself fails), the function
bails out before drm_setup_crtcs_fb() runs and before setting
fb_helper->deferred_setup. The client is left with mode_sets that
have a mode and connectors but mode_set->fb == NULL, and nothing
marks this fb_helper as needing a retry.

__drm_fb_helper_restore_fbdev_mode_unlocked() only skips committing
when fb_helper->deferred_setup is set, so a later .restore() call
(drm_lastclose() on process exit, in this report) commits the stale
mode_set through drm_client_modeset_commit() ->
drm_client_modeset_commit_atomic() -> __drm_atomic_helper_set_config(),
which hits WARN_ON(!set->fb).

Mark fb_helper->deferred_setup on any drm_fb_helper_single_fb_probe()
failure, not just -EAGAIN, so restore keeps skipping the commit until
a later hotplug event successfully re-probes and re-populates
mode_set->fb. The real error code is still returned to the caller
unchanged.

Reported-by: syzbot+ca23c8570669ead78867@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=ca23c8570669ead78867
Fixes: ca91a2758fce ("drm/fb-helper: Support deferred setup")
Cc: stable@vger.kernel.org
Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20260927075434.72658-1-ngocthang2710.1999@gmail.com
Link: https://patch.msgid.link/20260917154743.1621-1-shechenglong@xfusion.com

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index d4664ed468b2..0817b639b3fa 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -1724,10 +1724,10 @@ __drm_fb_helper_initial_config_and_unlock(struct drm_fb_helper *fb_helper)
 
 	ret = drm_fb_helper_single_fb_probe(fb_helper);
 	if (ret < 0) {
-		if (ret == -EAGAIN) {
-			fb_helper->deferred_setup = true;
+		/* modesets are probed but fb_helper->fb isn't; defer restore too */
+		fb_helper->deferred_setup = true;
+		if (ret == -EAGAIN)
 			ret = 0;
-		}
 		mutex_unlock(&fb_helper->lock);
 
 		goto err_drm_fb_helper_release_info;