[PATCH 0/2] Fix regression introduced by disabling accelerated scrolling in fbcon

[Geert Uytterhoeven]

Hi Helge,

On Wed, Jan 19, 2022 at 12:10 PM Helge Deller <deller at gmx.de> wrote:
> This series reverts two patches which disabled scrolling acceleration in
> fbcon/fbdev. Those patches introduced a regression for fbdev-supported graphic
> cards because of the performance penalty by doing screen scrolling by software
> instead of using hardware acceleration.
>
> Console scrolling acceleration was disabled by dropping code which checked at
> runtime the driver hardware possibilities for the BINFO_HWACCEL_COPYAREA or
> FBINFO_HWACCEL_FILLRECT flags and if set, it enabled scrollmode SCROLL_MOVE
> which uses hardware acceleration to move screen contents.  After dropping those
> checks scrollmode was hard-wired to SCROLL_REDRAW instead, which forces all
> graphic cards to redraw every character at the new screen position when
> scrolling.
>
> This change effectively disabled all hardware-based scrolling acceleration for
> ALL drivers, because now all kind of 2D hardware acceleration (bitblt,
> fillrect) in the drivers isn't used any longer.
>
> The original commit message mentions that only 3 DRM drivers (nouveau, omapdrm
> and gma500) used hardware acceleration in the past and thus code for checking
> and using scrolling acceleration is obsolete.
>
> This statement is NOT TRUE, because beside the DRM drivers there are around 35
> other fbdev drivers which depend on fbdev/fbcon and still provide hardware
> acceleration for fbdev/fbcon.
>
> The original commit message also states that syzbot found lots of bugs in fbcon
> and thus it's "often the solution to just delete code and remove features".
> This is true, and the bugs - which actually affected all users of fbcon,
> including DRM - were fixed, or code was dropped like e.g. the support for
> software scrollback in vgacon (commit 973c096f6a85).
>
> So to further analyze which bugs were found by syzbot, I've looked through all
> patches in drivers/video which were tagged with syzbot or syzkaller back to
> year 2005. The vast majority fixed the reported issues on a higher level, e.g.
> when screen is to be resized, or when font size is to be changed. The few ones
> which touched driver code fixed a real driver bug, e.g. by adding a check.
>
> But NONE of those patches touched code of either the SCROLL_MOVE or the
> SCROLL_REDRAW case.
>
> That means, there was no real reason why SCROLL_MOVE had to be ripped-out and
> just SCROLL_REDRAW had to be used instead. The only reason I can imagine so far
> was that SCROLL_MOVE wasn't used by DRM and as such it was assumed that it
> could go away. That argument completely missed the fact that SCROLL_MOVE is
> still heavily used by fbdev (non-DRM) drivers.
>
> Some people mention that using memcpy() instead of the hardware acceleration is
> pretty much the same speed. But that's not true, at least not for older graphic
> cards and machines where we see speed decreases by factor 10 and more and thus
> this change leads to console responsiveness way worse than before.
>
> That's why I propose to revert those patches, re-introduce hardware-based
> scrolling acceleration and fix the performance-regression for fbdev drivers.
> There isn't any impact on DRM when reverting those patches.
>
> Helge Deller (2):
>   Revert "fbdev: Garbage collect fbdev scrolling acceleration, part 1
>     (from TODO list)"
>   Revert "fbcon: Disable accelerated scrolling"

Thank you for this series, and the prior analysis!

Acked-by: Geert Uytterhoeven <geert at linux-m68k.org>

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at 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