Re: fbcon: remove soft scrollback code (missing Doc. patch)

[Daniel Vetter]

On Fri, Jan 22, 2021 at 01:55:04PM -0500, Phillip Susi wrote:
> 
> Geert Uytterhoeven writes:
> 
> Judging from some of the comments in the code, it looks like you were
> one of the original authors of fbcon?  I haven't been able to find any
> of these sczbot crash reports, and am not sure how fuzzing syscalls
> would really affect this code ( it's not really handling a buch of
> ioctls or otherwise taking arguments from user space ) , but I am a bit
> confused as to why the softback was implemented the way that it was.
> 
> vgacon simply copies the main buffer to vram in ->set_origin() and then
> changes the pointers to operate out of the much larger vram while that
> virtual terminal is active.  If I understand it correctly, it looks like
> fbcon instead opts to operate out of the main buffer but rescue lines as
> they are scrolled off and relocate them to the softback buffer.  This
> seems to be rather more convoluted.
> 
> I'm thinking of re-implementing scrollback more like the way vgacon does
> it: allocate a big "vram" buffer and operate out of that.  Obviously
> ->scroll() and ->scrolldelta() have to actually repaint the screen rather
> than simply change the pointer register, but that should be about the
> only difference.
> 
> I have also noticed that there was some code to use hardware panning of
> the video buffer rather than having to do a block bitblt to scroll the
> contents of the screen, but that it was disabled because virtually no
> video drivers actually implemented it?  That seems like a shame, but if
> it is so, then there's no sense carrying the dead code so I think I'll
> clean that up now.
> 
> Now that I look at it again, everything is simply always redrawn now
> instead of even doing a simple bitblt.  Daniel, you mentioned that
> almost nobody supports hardware acceleration, but even without any
> specific hardware support, surely even if bitblt() is implemented just
> as a memcpy(), it has to be faster than redrawing all of the characters
> doesn't it?  Getting rid of the panning if it isn't generally supported
> I can see, but I don't understand killing bitblt even if most devices
> don't accelerate it.

Just a quick comment on this: Since most framebuffers are write-combining,
and reads from that tend to be ~3 orders of magnitude slower than writes
(at least on the pile of machines I looked at here, there's big
differences, and some special streaming cpu instructions to make the
reading side not so slow).

So scrolling by copying tends to be significantly slower than just
redrawing everything.

And once you're at that point it's really hard to write a 2d acceleration
which is consistently faster than just cpu rendering.

If you're interested in why 2d acceleration is rather hard as a general
problem, not just specific to fbcon, I wrote a blog on that a while ago:

https://blog.ffwll.ch/2018/08/no-2d-in-drm.html

Cheers, Daniel

> In addition, I noticed that ->screen_pos() was changed to just return
> vc_origin+offset.  fbcon is the only console driver to implement
> ->screenpos() and if not implemented, vt defaults to using
> vc_visible_origin+offset, so it looks like this function isn't needed at
> all anymore and ->screen_pos() can be removed from struct consw.
> 
> Does this make sense or am I talking crazy?

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