Re: [PATCH] fbcon: Disable accelerated scrolling

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Daniel,

Replying "early" (see below), as this was applied to
drm-misc/for-linux-next.

On Sat, Oct 31, 2020 at 3:17 PM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> On Sat, Oct 31, 2020 at 11:28 AM Geert Uytterhoeven
> <geert@linux-m68k.org> wrote:
> > On Thu, 29 Oct 2020, Daniel Vetter wrote:
> > > So ever since syzbot discovered fbcon, we have solid proof that it's
> > > full of bugs. And often the solution is to just delete code and remove
> > > features, e.g.  50145474f6ef ("fbcon: remove soft scrollback code").
> > >
> > > Now the problem is that most modern-ish drivers really only treat
> > > fbcon as an dumb kernel console until userspace takes over, and Oops
> > > printer for some emergencies. Looking at drm drivers and the basic
> > > vesa/efi fbdev drivers shows that only 3 drivers support any kind of
> > > acceleration:
> > >
> > > - nouveau, seems to be enabled by default
> > > - omapdrm, when a DMM remapper exists using remapper rewriting for
> > >  y/xpanning
> > > - gma500, but that is getting deleted now for the GTT remapper trick,
> > >  and the accelerated copyarea never set the FBINFO_HWACCEL_COPYAREA
> > >  flag, so unused (and could be deleted already I think).
> > >
> > > No other driver supportes accelerated fbcon. And fbcon is the only
> > > user of this accel code (it's not exposed as uapi through ioctls),
> > > which means we could garbage collect fairly enormous amounts of code
> > > if we kill this.
> >
> > "git grep FBINFO_HWACCEL_COPYAREA" shows me there are 32 more drivers
> > using acceleration under drivers/video/fbdev/.
> >
> > > Plus because syzbot only runs on virtual hardware, and none of the
> > > drivers for that have acceleration, we'd remove a huge gap in testing.
> > > And there's no other even remotely comprehensive testing aside from
> > > syzbot.
> >
> > That sounds like a great argument to remove all hardware drivers from
> > the kernel ;-)
>
> fbdev is unmaintained, has no one volunteering to put in the work (and
> there's huge amounts of work needed), and there's no test suite. No,
> fbtest.c doesn't can't, that's not even close. We're not going to
> delete everything in the kernel, but slowly sunsetting stuff that's
> just costing and not bringing in up is a good idea.

The fbcon acceleration code is indeed not tested by fbset, and it is
purely in-kernel acceleration for the console.

> > Seriously, how hard can it be to add "software-accelerated" acceleration
> > hooks to drivers/video/fbdev/vfb.c, to enable syzbot to exercise the
> > core acceleration code paths?
>
> Just this one is 5 combinations, which means I'd need to convince
> syzbot to test 5 different machine setups.

Why 5 combinations?
Enable vfb (which can be a module) and be done with it?

> Plus we're still lacking a test suite, and judging from how much time
> it took to get something basic going for kms, that's about 2 engineer
> years of effort that no one is even close to willing to spend.

Sure, writing test suites is hard, and takes time.

> > > This patch here just disables the acceleration code by always
> > > redrawing when scrolling. The plan is that once this has been merged
> > > for well over a year in released kernels, we can start to go around
> > > and delete a lot of code.
> >
> > Have you benchmarked the performance impact on traditional fbdev
> > drivers?
>
> There's still some acceleration if you have an image blit engine for
> redrawing the screen. But the complexity is contained in the old
> drivers that no one cares about.
>
> For anything I have access to the difference is 0.

Sure, you're doing DRM drivers ;-)

> Reality is that fbdev is just there nowadays for Oops printing and
> emergency usage, and it's plenty good enough for that. If there's

That's true for systems that are supported by a DRM driver.

> anyone who cares beyond that, they're most definitely not able to put
> in time for upstream work.

There exist actual products using out-of-tree fbdev drivers that never
got the chance of being merged upstream due to the moratorium on new
fbdev drivers.

BTW, I'm trying to convert an old fbdev driver to DRM, but don't have it
working yet. I had hoped to get something working before replying to
this email, so I could provide more detailed feedback.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds