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From:   Daniel Vetter <daniel@ffwll.ch>
Date:   Thu, 20 Jan 2022 13:33:13 +0100
Message-ID: <CAKMK7uEwDdaR7kDVi9Oah0w8qFEoAywp_wj2eH7DtttVD2L00A@mail.gmail.com>
Subject: Re: [PATCH] MAINTAINERS: Add Helge as fbdev maintainer
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Gerd Hoffmann <kraxel@redhat.com>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
        Helge Deller <deller@gmx.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Sven Schnelle <svens@stackframe.org>
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On Thu, Jan 20, 2022 at 1:13 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> Hi Daniel,
>
> On Thu, Jan 20, 2022 at 12:33 PM Daniel Vetter <daniel@ffwll.ch> wrote:
> > On Thu, Jan 20, 2022 at 10:06 AM Geert Uytterhoeven
> > <geert@linux-m68k.org> wrote:
> > > On Thu, Jan 20, 2022 at 4:29 AM Gerd Hoffmann <kraxel@redhat.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@stackframe.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.

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

> > Like do people actually know what drm can and cannot do, or would that
> > take out all the fun?
>
> ;-)
>
> 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


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch