Re: [PATCH] drm/fb-helper: Remove drm_fb_helper_unprepare() from drm_fb_helper_fini()

Hi

Am 21.02.23 um 11:27 schrieb Javier Martinez Canillas:

Thomas Zimmermann [off-list ref] writes:

quoted

Move drm_fb_helper_unprepare() from drm_fb_helper_fini() into the
calling fbdev implementation. Avoids a possible stale mutex with
generic fbdev code.

As indicated by its name, drm_fb_helper_prepare() prepares struct
drm_fb_helper before setting up the fbdev support with a call to
drm_fb_helper_init(). In legacy fbdev emulation, this happens next
to each other. If successful, drm_fb_helper_fini() later tear down
the fbdev device and also unprepare via drm_fb_helper_unprepare().

Generic fbdev emulation prepares struct drm_fb_helper immediately
after allocating the instance. It only calls drm_fb_helper_init()
as part of processing a hotplug event. If the hotplug-handling fails,
it runs drm_fb_helper_fini(). This unprepares the fb-helper instance
and the next hotplug event runs on stale data.

Solve this by moving drm_fb_helper_unprepare() from drm_fb_helper_fini()
into the fbdev implementations. Call it right before freeing the
fb-helper instance.

Fixes: 4825797c36da ("drm/fb-helper: Introduce drm_fb_helper_unprepare()")

I think this should be Fixes: 032116bbe152 ("drm/fbdev-generic: Minimize
client unregistering") instead? Because commit 4825797c36da just added a
wrapper function for mutex_destroy(&fb_helper->lock), but it was commit
032116bbe152 that made drm_fbdev_cleanup() to call that helper function.

Good point. After looking through the recent fbdev commits, I'll use 
commit 643231b28380 ("drm/fbdev-generic: Minimize hotplug error 
handling") for the tag. This is the one that added the call to 
drm_fb_helper_fini() to the client's hotplug handler. And _fini() 
currently does the _unprepare(), when it shouldn't.

quoted

Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <redacted>
Cc: dri-devel@lists.freedesktop.org

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
---

The change itself looks good to me.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>

Thanks a lot.

Best regards
Thomas


-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
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