linux-kernel - [RFC PATCH v4 08/11] fbdev: Fix race between sysfb and framebuffer devices registration

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Date:   Fri, 29 Apr 2022 10:42:50 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     dri-devel@...ts.freedesktop.org,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Alex Deucher <alexander.deucher@....com>,
        Changcheng Deng <deng.changcheng@....com.cn>,
        Daniel Vetter <daniel@...ll.ch>, Helge Deller <deller@....de>,
        Sam Ravnborg <sam@...nborg.org>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        linux-fbdev@...r.kernel.org
Subject: [RFC PATCH v4 08/11] fbdev: Fix race between sysfb and framebuffer devices registration

The platform devices registered by sysfb match with firmware-based DRM or
fbdev drivers, that are used to have early graphics using a framebuffer
provided by the system firmware.

DRM or fbdev drivers later are probed and remove all conflicting framebuffers,
leading to these platform devices for generic drivers to be unregistered.

But the current solution has a race, since the sysfb_init() function could
be called after a DRM driver is probed and requested to unregister devices
for drivers with conflicting framebuffes.

To prevent this, disable any future sysfb platform device registration by
calling sysfb_disable(), if either a framebuffer device or a DRM device is
registered. Since in that case a display will already be present.

Suggested-by: Daniel Vetter <daniel.vetter@...ll.ch>
Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
---

(no changes since v3)

Changes in v3:
- Call sysfb_disable() when a fbdev framebuffer is registered rather
  than when conflicting framebuffers are removed (Thomas Zimmermann).
- Drop Daniel Vetter's Reviewed-by tag since patch changed a lot.

Changes in v2:
- Explain in the commit message that fbmem has to unregister the device
  as fallback if a driver registered the device itself (Daniel Vetter).
- Also explain that fallback in a comment in the code (Daniel Vetter).
- Don't encode in fbmem the assumption that sysfb will always register
  platform devices (Daniel Vetter).
- Add a FIXME comment about drivers registering devices (Daniel Vetter).

 drivers/video/fbdev/core/fbmem.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index d6ae33990f40..7583296481b0 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -19,6 +19,7 @@
 #include <linux/kernel.h>
 #include <linux/major.h>
 #include <linux/slab.h>
+#include <linux/sysfb.h>
 #include <linux/mm.h>
 #include <linux/mman.h>
 #include <linux/vt.h>
@@ -1903,6 +1904,17 @@ register_framebuffer(struct fb_info *fb_info)
 	ret = do_register_framebuffer(fb_info);
 	mutex_unlock(&registration_lock);
 
+	/*
+	 * If a driver registers a framebuffer device, then it can be assumed
+	 * that a display will be present and there is no need for a generic
+	 * driver using the firmware setup system framebuffer.
+	 *
+	 * Disable sysfb and prevent registering simple framebuffer devices,
+	 * but only do it for framebuffers that are not provided by firmware.
+	 */
+	if (!(fb_info->flags & FBINFO_MISC_FIRMWARE))
+		sysfb_disable();
+
 	return ret;
 }
 EXPORT_SYMBOL(register_framebuffer);
-- 
2.35.1