Re: [RESEND RFC PATCH 4/5] fbdev: Fix some race conditions between fbmem and sysfb

On Wed, Apr 06, 2022 at 11:39:18PM +0200, Javier Martinez Canillas wrote:

The platform devices registered in sysfb match with a firmware-based fbdev or DRM driver, that are used to have early graphics using framebuffers set up by the system firmware.

Real DRM 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 two issues that this patch fixes:

1. It is a layering violation for the fbdev core to unregister a device that was registered by sysfb.

   Instead, the sysfb_try_unregister() helper function can be called for sysfb to attempt unregistering the device if is the one registered.

2. The sysfb_init() function could be called after a DRM driver is probed and requested to unregister devices for drivers with a conflicting fb.

   To prevent this, disable any future sysfb platform device registration by calling sysfb_disable(), if a driver requested to remove conflicting framebuffers with remove_conflicting_framebuffers().

Suggested-by: Daniel Vetter daniel.vetter@ffwll.ch Signed-off-by: Javier Martinez Canillas javierm@redhat.com

---

drivers/video/fbdev/core/fbmem.c | 17 ++++++++++++++++- 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c index c1bfb8df9cba..acf641b05d11 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> @@ -1588,7 +1589,10 @@ static void do_remove_conflicting_framebuffers(struct apertures_struct *a, * unregister_framebuffer() function that takes it. */ mutex_unlock(&registration_lock);

• 		platform_device_unregister(to_platform_device(device));
  

• 		if (!sysfb_try_unregister(device)) {
  

• 			/* sysfb didn't register this device, unregister it */
  

Maybe explain in the commit message that this is still needed for drivers which set up their platform_dev themselves, like vga16fb.

Also I'm not sure we want to have an assumption encoded in fbmem.c here that the sysfb device is always a platform device. I think it would be better to call sysfb_try_unregister on any device, and then fall back to the forced removal on our own if it's a platform device.

Also maybe change the comment to /* FIXME: Not all platform fb drivers use sysfb yet */

• 			platform_device_unregister(to_platform_device(device));
  

• 		}
  	mutex_lock(&registration_lock);
  	/*
  	 * Restart the removal now that the platform device
  

@@ -1781,6 +1785,17 @@ int remove_conflicting_framebuffers(struct apertures_struct *a, do_free = true; }

• /*

• * If a driver asked to unregister a platform device registered by
  

• * sysfb, then can be assumed that this is a driver for a display
  

• * that is set up by the system firmware and has a generic driver.
  

• *
  

• * Drivers for devices that don't have a generic driver will never
  

• * ask for this, so let's assume that a real driver for the display
  

• * was already probed and prevent sysfb to register devices later.
  

• */
  

Yeah it's disappointing, but no worse than the piles of hacks we have now.

With the bikesheds addressed above:

Reviewed-by: Daniel Vetter daniel.vetter@ffwll.ch

• sysfb_disable();
• mutex_lock(&registration_lock); do_remove_conflicting_framebuffers(a, name, primary); mutex_unlock(&registration_lock);

-- 2.35.1