From: Thomas Zimmermann <tzimmermann@suse.de>
To: Geert Uytterhoeven <geert@linux-m68k.org>,
Daniel Vetter <daniel@ffwll.ch>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Phillip Susi <phill@thesusis.net>, Pavel Machek <pavel@ucw.cz>,
Randy Dunlap <rdunlap@infradead.org>,
LKML <linux-kernel@vger.kernel.org>,
"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
dri-devel <dri-devel@lists.freedesktop.org>,
Linux Fbdev development list <linux-fbdev@vger.kernel.org>
Subject: Re: fbcon: remove soft scrollback code (missing Doc. patch)
Date: Wed, 3 Feb 2021 09:03:47 +0100 [thread overview]
Message-ID: <7d666c65-80ea-905e-b50f-76af5a6dceb0@suse.de> (raw)
In-Reply-To: <CAMuHMdUf3eeK=Fr_pasUdXkk_zuicFBqNY+jSnH6EgUgmG+3hA@mail.gmail.com>
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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@ffwll.ch> wrote:
>> On Thu, Jan 14, 2021 at 4:56 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>>> On Tue, Jan 12, 2021 at 5:00 PM Daniel Vetter <daniel@ffwll.ch> wrote:
>>>> On Sat, Jan 9, 2021 at 12:11 AM Linus Torvalds
>>>> <torvalds@linux-foundation.org> wrote:
>>>>> On Fri, Jan 8, 2021 at 11:13 AM Phillip Susi <phill@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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next prev parent reply other threads:[~2021-02-03 8:04 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20200916205434.GA10389@duo.ucw.cz>
[not found] ` <87czyf5jjp.fsf@vps.thesusis.net>
[not found] ` <CAHk-=wjsjC1h7fskwYaaRLykN1ms6ZtxGvucQgmL-zZTfxPdBA@mail.gmail.com>
[not found] ` <CAKMK7uEGXOC_ci=Drm=Hz+xPGdcoxv8YZ-gcOckoPmu2XijiSA@mail.gmail.com>
2021-01-14 15:56 ` fbcon: remove soft scrollback code (missing Doc. patch) Geert Uytterhoeven
2021-01-14 16:11 ` Daniel Vetter
2021-01-15 8:06 ` Geert Uytterhoeven
2021-02-03 8:03 ` Thomas Zimmermann [this message]
2021-01-22 18:55 ` Phillip Susi
2021-01-25 15:39 ` Geert Uytterhoeven
2021-02-02 14:18 ` Daniel Vetter
2021-02-02 15:13 ` Phillip Susi
2021-02-02 15:23 ` Daniel Vetter
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