fbcon: remove soft scrollback code (missing Doc. patch)

[Thomas Zimmermann]

Hi

Am 15.01.21 um 09:06 schrieb Geert Uytterhoeven:
> Hi Daniel,
> 
> On Thu, Jan 14, 2021 at 5:11 PM Daniel Vetter <daniel at ffwll.ch> wrote:
>> On Thu, Jan 14, 2021 at 4:56 PM Geert Uytterhoeven <geert at linux-m68k.org> wrote:
>>> On Tue, Jan 12, 2021 at 5:00 PM Daniel Vetter <daniel at ffwll.ch> wrote:
>>>> On Sat, Jan 9, 2021 at 12:11 AM Linus Torvalds
>>>> <torvalds at linux-foundation.org> wrote:
>>>>> On Fri, Jan 8, 2021 at 11:13 AM Phillip Susi <phill at thesusis.net> wrote:
>>>>>>> Could we pause this madness? Scrollback is still useful. I needed it
>>>>>>> today... it was too small, so command results I was looking for
>>>>>>> already scrolled away, but... life will be really painful with 0
>>>>>>> scrollback.
>>>>>>
>>>>>>> You'll need it, too... as soon as you get oops and will want to see
>>>>>>> errors just prior to that oops.
>>>>>>
>>>>>>> If it means I get to maintain it... I'm not happy about it but that's
>>>>>>> better than no scrollback.
>>>>>>
>>>>>> Amen!  What self respecting admin installs a gui on servers?  What do we
>>>>>> have to do to get this back in?  What was so buggy with this code that
>>>>>> it needed to be removed?  Why was it such a burden to just leave it be?
>>>>>
>>>>> It really was buggy, with security implications. And we have no maintainers.
>>>>>
>>>>> So the scroll-back code can't come back until we have a maintainer and
>>>>> a cleaner and simpler implementation.
>>>>>
>>>>> And no, maintaining it really doesn't mean "just get it back to the
>>>>> old broken state".
>>>>>
>>>>> So far I haven't actually seen any patches, which means that it's not
>>>>> coming back.
>>>>>
>>>>> The good news? If you have an actual text VGA console, that should
>>>>> still work just fine.
>>>
>>> IIRC, all of this was written for systems lacking VGA text consoles
>>> in the first place...
>>>
>>>> Also on anything that is remotely modern (i.e. runs a drm kernel
>>>> modesetting driver undearneath the fbdev/fbcon stack) there's a pile
>>>> more issues on top of just the scrollback/fbcon code being a mess.
>>>
>>> Would it help to remove DRM_FBDEV_EMULATION (instead)?

Of the fbdev code, DRM's fbdev emulation is the cleanest. We now even 
have test cases for the userspace I/O.

>>
>> It's a problem with the hardware. "Write some registers and done"
>> isn't how display blocks work nowadays. So your proposal amounts to
>> "no fbdev/fbcon for anything modern-ish".
> 
> With "modern-ish" actually meaning: "desktop/gaming/mobile-style
> 3D-accelerated wide-color display hardware".  There's plenty of display
> hardware that doesn't fall into that class, and is served by fbdev (also
> out-of-tree due to the moratorium) because of that.

Userspace has been moving away from fbdev. Writing an fbdev driver locks 
you into a legacy userspace. I also found that DRM drivers are smaller, 
because of all the DRM helper libraries. Using DRM + fbdev emulation is 
a win in almost any case. We did have some complaints about performance 
of the emulation. So that might be worth looking into.

Best regards
Thomas

> 
>> Also I said "a pile more", most of the issues in fbcon/fbdev code
>> apply for all drivers.
>>
>>>> Specifically the locking is somewhere between yolo and outright
>>>> deadlocks. This holds even more so if the use case here is "I want
>>>> scrollback for an oops". There's rough sketches for how it could be
>>>> solved, but it's all very tricky work.
>>>
>>> When an oops happens, all bets are off.  At that point, all information
>>> you can extract from the system is valuable, and additional locking
>>> issues are moot.
>>
>> Except the first oops then scrolls aways because it's getting buried
>> under further fail. Your locking needs to be minimally good enough to
>> not make the situation worse.
> 
> When an oops happens, all bets are off...
> 
> Gr{oetje,eeting}s,
> 
>                          Geert
> 

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

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