linux-kernel - Re: [PATCH] MAINTAINERS: Add Helge as fbdev maintainer

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Date:   Thu, 20 Jan 2022 13:46:13 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     Gerd Hoffmann <kraxel@...hat.com>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Helge Deller <deller@....de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Sven Schnelle <svens@...ckframe.org>
Subject: Re: [PATCH] MAINTAINERS: Add Helge as fbdev maintainer

Hi Daniel,

On Thu, Jan 20, 2022 at 1:33 PM Daniel Vetter <daniel@...ll.ch> wrote:
> On Thu, Jan 20, 2022 at 1:13 PM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > On Thu, Jan 20, 2022 at 12:33 PM Daniel Vetter <daniel@...ll.ch> wrote:
> > > On Thu, Jan 20, 2022 at 10:06 AM Geert Uytterhoeven
> > > <geert@...ux-m68k.org> wrote:
> > > > On Thu, Jan 20, 2022 at 4:29 AM Gerd Hoffmann <kraxel@...hat.com> wrote:
> > > > > On Tue, Jan 18, 2022 at 10:33:23AM +0200, Pekka Paalanen wrote:
> > > > > > On Mon, 17 Jan 2022 19:47:39 +0100
> > > > > > Sven Schnelle <svens@...ckframe.org> wrote:
> > > > > > > I also tested the speed on my Thinkpad X1 with Intel graphics, and there
> > > > > > > a dmesg with 919 lines one the text console took about 2s to display. In
> > > > > > > x11, i measure 22ms. This might be unfair because encoding might be
> > > > > > > different, but i cannot confirm the 'memcpy' is faster than hardware
> > > > > > > blitting' point. I think if that would be the case, no-one would care
> > > > > > > about 2D acceleration.
> > > > > >
> > > > > > I think that is an extremely unfair comparison, because a graphical
> > > > > > terminal app is not going to render every line of text streamed to it.
> > > > > > It probably renders only the final view alone if you simply run
> > > > > > 'dmesg', skipping the first 800-900 lines completely.
> > > > >
> > > > > Probably more like "render on every vblank", but yes, unlike fbcon it
> > > > > surely wouldn't render every single character sent to the terminal.
> > > > >
> > > > > Also acceleration on modern hardware is more like "compose window
> > > > > content using the 3d engine" than "use 2d blitter to scroll the window".
> > > > >
> > > > > > Maybe fbcon should do the same when presented with a flood of text,
> > > > > > but I don't know how or why it works like it works.
> > > > >
> > > > > fbcon could do the same, i.e. render to fbdev in a 60Hz timer instead of
> > > > > doing it synchronously.
> > > >
> > > > Hopefully only the parts of the screen which need a redraw?
> > > >
> > > > Not all displays can be updated that fast. For a "modern" example, see
> > > > https://patchwork.freedesktop.org/series/93070/.
> > >
> > > drm does damage tracking throughout the stack, e.g.
> > >
> > > https://dri.freedesktop.org/docs/drm/gpu/drm-kms.html#damage-tracking-properties
> > >
> > > And unlike fbdev, it's explicit (so less overhead since userspace
> > > generally knows what it's drawn) and doesn't rely on page fault
> > > intercepting and fun stuff like that.
> >
> > My reply was to a paragraph about rendering text by fbcon, not about
> > userspace rendering graphics.
>
> Yeah, and ofc when I say "throughout the stack" this also includes the
> fbdev emulation, including the mmap intercepting with fbdev_defio and
> all that. They all get remapped to that damage tracking property,
> which drivers can then inspect using a bunch of helpers.

And I really meant the text rendering part, not the copy of the shadow
buffer after the rendering.

> But reading code&docs is too hard I guess, safer to assume it's just
> broken and not supported.

Don't worry, I'm actually writing a larger rebuttal _and_ code...

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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