linux-kernel - Re: [PATCH v5 5/7] fbdev: Disable sysfb device registration when removing conflicting FBs

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Date:   Tue, 7 Jun 2022 17:01:00 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Javier Martinez Canillas <javierm@...hat.com>
Cc:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-fbdev@...r.kernel.org,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Helge Deller <deller@....de>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v5 5/7] fbdev: Disable sysfb device registration when
 removing conflicting FBs

On Wed, May 11, 2022 at 01:31:44PM +0200, Javier Martinez Canillas wrote:
> 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 or fbdev driver is probed and request to unregister
> the devices for drivers with conflicting framebuffes.
> 
> To prevent this, disable any future sysfb platform device registration by
> calling sysfb_disable(), if a driver requests to remove the conflicting
> framebuffers.
> 
> Suggested-by: Daniel Vetter <daniel.vetter@...ll.ch>
> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
> Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>
> ---
> 
> Changes in v5:
> - Move the sysfb_disable() call at conflicting framebuffers again to
>   avoid the need of a DRIVER_FIRMWARE capability flag.
> - Add Daniel Vetter's Reviewed-by tag again since reverted to the old
>   patch that he already reviewed in v2.
> 
> Changes in v3:
> - Call sysfb_disable() when a DRM dev and a fbdev are registered rather
>   than when conflicting framebuffers are removed (Thomas Zimmermann).
> - 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 | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 9b035ef4d552..265efa189bcc 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1789,6 +1789,17 @@ int remove_conflicting_framebuffers(struct apertures_struct *a,
>  	if (do_free)
>  		kfree(a);
>  
> +	/*
> +	 * 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.
> +	 */
> +	sysfb_disable();

So the og version had (or should have had at least) the sysfb_disable()
call before we go through the loop and try to unregister stuff. I think
this needs to be done before we call do_remove_conflicting_framebuffer()
instead. With that:

Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>


> +
>  	return 0;
>  }
>  EXPORT_SYMBOL(remove_conflicting_framebuffers);
> -- 
> 2.35.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch