Date:   Mon, 25 Apr 2022 11:15:51 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Javier Martinez Canillas <javierm@...hat.com>,
        linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        dri-devel@...ts.freedesktop.org,
        Alex Deucher <alexander.deucher@....com>,
        Borislav Petkov <bp@...e.de>,
        Changcheng Deng <deng.changcheng@....com.cn>,
        Daniel Vetter <daniel@...ll.ch>,
        Hans de Goede <hdegoede@...hat.com>,
        Helge Deller <deller@....de>, Johan Hovold <johan@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Miaoqian Lin <linmq006@...il.com>,
        Peter Jones <pjones@...hat.com>,
        Sam Ravnborg <sam@...nborg.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Yizhuo Zhai <yzhai003@....edu>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        linux-doc@...r.kernel.org, linux-fbdev@...r.kernel.org
Subject: Re: [PATCH v3 0/5] Fix some race conditions that exists between fbmem
 and sysfb

Hi

Am 25.04.22 um 10:54 schrieb Thomas Zimmermann:
> Hi
> 
> Am 20.04.22 um 10:52 schrieb Javier Martinez Canillas:
>> Hello,
>>
>> The patches in this series are mostly changes suggested by Daniel Vetter
>> to fix some race conditions that exists between the fbdev core (fbmem)
>> and sysfb with regard to device registration and removal.
>>
>> For example, it is currently possible for sysfb to register a platform
>> device after a real DRM driver was registered and requested to remove the
>> conflicting framebuffers.
>>
>> A symptom of this issue, was worked around with by commit fb561bf9abde
>> ("fbdev: Prevent probing generic drivers if a FB is already registered")
>> but that's really a hack and should be reverted.
> 
> As I mentioned on IRC, I think this series should be merged for the 
> reasons I give in the other comments.
> 
>>
>> This series attempt to fix it more properly and revert the mentioned 
>> hack.
>> That will also unblock a pending patch to not make the num_registered_fb
>> variable visible to drivers anymore, since that's internal to fbdev core.
> 
> Here's as far as I understand the problem:
> 
>   1) build DRM/fbdev and sysfb code into the kernel
>   2) during boot, load the DRM/fbdev modules and have them acquire I/O 
> ranges
>   3) afterwards load sysfb and have it register platform devices for the 
> generic framebuffers
>   4) these devices now conflict with the already-registered DRM/fbdev 
> devices
> 
> If that is the problem here, let's simply set a sysfb_disable flag in 
> sysfb code when the first DRM/fbdev driver first loads. With the flag 
> set, sysfb won't create any platform devices. We assume that there are 
> now DRM/fbdev drivers for the framebuffers and sysfb won't be needed.
> 
> We can set the flag internally from drm_aperture_detach_drivers() [1] 
> and do_remove_conflicting_framebuffers() [2].

And further thinking about it, it would be better to set such a flag 
after successfully registering a DRM/fbdev device.  So we know that 
there's at least one working display in the system. We don't have to 
rely on generic framebuffers after that.

Best regards
Thomas

> 
> Best regards
> Thomas
> 
> [1] 
> https://elixir.bootlin.com/linux/v5.17.4/source/drivers/gpu/drm/drm_aperture.c#L253 
> 
> [2] 
> https://elixir.bootlin.com/linux/v5.17.4/source/drivers/video/fbdev/core/fbmem.c#L1559 
> 
> 
>>
>> Patch #1 is just a trivial preparatory change.
>>
>> Patch #2 add sysfb_disable() and sysfb_try_unregister() helpers for fbmem
>> to use them.
>>
>> Patch #3 changes how is dealt with conflicting framebuffers 
>> unregistering,
>> rather than having a variable to determine if a lock should be take, it
>> just drops the lock before unregistering the platform device.
>>
>> Patch #4 fixes the mentioned race conditions and finally patch #5 is the
>> revert patch that was posted by Daniel before but he dropped from his 
>> set.
>>
>> The patches were tested on a rpi4 using different video configurations:
>> (simpledrm -> vc4 both builtin, only vc4 builtin, only simpledrm builtin
>> and simpledrm builtin with vc4 built as a module).
>>
>> Best regards,
>> Javier
>>
>> Changes in v3:
>> - Rebase on top of latest drm-misc-next branch.
>>
>> Changes in v2:
>> - Rebase on top of latest drm-misc-next and fix conflicts (Daniel 
>> Vetter).
>> - Add kernel-doc comments and include in other_interfaces.rst (Daniel 
>> Vetter).
>> - 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).
>> - Drop RFC prefix since patches were already reviewed by Daniel Vetter.
>> - Add Daniel Reviewed-by tags to the patches.
>>
>> Daniel Vetter (1):
>>    Revert "fbdev: Prevent probing generic drivers if a FB is already
>>      registered"
>>
>> Javier Martinez Canillas (4):
>>    firmware: sysfb: Make sysfb_create_simplefb() return a pdev pointer
>>    firmware: sysfb: Add helpers to unregister a pdev and disable
>>      registration
>>    fbdev: Restart conflicting fb removal loop when unregistering devices
>>    fbdev: Fix some race conditions between fbmem and sysfb
>>
>>   .../driver-api/firmware/other_interfaces.rst  |  6 ++
>>   drivers/firmware/sysfb.c                      | 77 +++++++++++++++++--
>>   drivers/firmware/sysfb_simplefb.c             | 16 ++--
>>   drivers/video/fbdev/core/fbmem.c              | 62 ++++++++++++---
>>   drivers/video/fbdev/efifb.c                   | 11 ---
>>   drivers/video/fbdev/simplefb.c                | 11 ---
>>   include/linux/fb.h                            |  1 -
>>   include/linux/sysfb.h                         | 29 +++++--
>>   8 files changed, 158 insertions(+), 55 deletions(-)
>>
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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