Subject: Re: [PATCH 2/2] fbdev: Remove hot-unplug workaround for framebuffers without device

From: Javier Martinez Canillas <[email-masked]>
Date: Tue, 19 Apr 2022 09:22:24 +0200

Hello Thomas,

On 4/13/22 20:09, Thomas Zimmermann wrote:

[snip]

>>>> index bc6ed750e915..bdd00d381bbc 100644
>>>> --- a/drivers/video/fbdev/core/fbmem.c
>>>> +++ b/drivers/video/fbdev/core/fbmem.c
>>>> @@ -1579,14 +1579,7 @@ static void do_remove_conflicting_framebuffers(struct apertures_struct *a,
>>>>   			 * If it's not a platform device, at least print a warning. A
>>>>   			 * fix would add code to remove the device from the system.
>>>>   			 */
>>>> -			if (!device) {
>>>> -				/* TODO: Represent each OF framebuffer as its own
>>>> -				 * device in the device hierarchy. For now, offb
>>>> -				 * doesn't have such a device, so unregister the
>>>> -				 * framebuffer as before without warning.
>>>> -				 */
>>>> -				do_unregister_framebuffer(registered_fb[i]);
>>>
>>> Maybe we could still keep this for a couple of releases but with a big
>>> warning that's not supported in case there are out-of-tree drivers out
>>> there that still do this ?
>>>
>>> Or at least a warning if the do_unregister_framebuffer() call is removed.
>>
>> Yeah dying while holding console_lock isn't fun, and not having a WARN_ON
>> + bail-out code pretty much forces bug reporters to do a bisect here to
>> give us something more than "machine dies at boot with no messages".
>>
>> I'd just outright keep the WARN_ON here for 1-2 years even to really make
>> sure we got all the bug reports, since often these older machines only
>> update onto LTS releases.
> 
> If that's what the consent is, I'll go with it.
> 
> I'm just not sure if we talk about the same problem. offb didn't have a 
> platform device, so we recently added this workaround with 'if 
> (!device)'.  All the other fbdev drivers have a platform device; and 
> anything else that could fail is out-of-tree. We don't really care about 
> those AFAIK.
>

Yes, agreed on the offb change but I'm not really sure if we don't care
about out-of-tree modules. I mean, you are right in theory but I still
feel that we are changing a core behavior without giving people time to
sort out if needed.

Since before commit 27599aacbaef ("fbdev: Hot-unplug firmware fb devices
on forced removal") registered FBs didn't need to have a device, but now
that will lead to a NULL pointer dereference in dev_is_platform(device).

And that change only landed in v5.18-rc1, so it is fairly recent.

I know that we follow https://www.kernel.org/doc/Documentation/process/stable-api-nonsense.rst
but still my opinion is that having a warning for a couple of releases
if registered_fb[i]->device is NULL, instead of just crashing would be
a better way to handle this.
 
> With offb converted, we could practically remove all of the checks here 
> and call platform_device_unregister() unconditionally.
>

Yes for mainline, but as mentioned I thought mostly about out-of-tree. If
folks agree that we shouldn't care about these, I'm Ok with that as well.

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat