[PATCH] fbuffer: improve toggle cursor performance

[Greg Kurz]

On Wed, 27 May 2015 07:59:34 +0200
Thomas Huth <thuth at redhat.com> wrote:

> On Wed, 27 May 2015 02:11:13 +0200
> Greg Kurz <gkurz at linux.vnet.ibm.com> wrote:
> 
> > SLOF currently calls hv-logical-load and hv-logical-store for every pixel
> > when enabling or disabling the cursor. This is suboptimal when writing one
> > char at a time to the console since terminal-write always toggles the cursor.
> > And this is precisely what grub is doing when the user wants to edit a menu
> > entry... the result is an incredibly slow and barely usable interface.
> > 
> > The inner loop in fb8-toggle-cursor handles a contiguous region: it can be
> > converted to hv-logical-memop. The result is 32 times less hcalls per char
> > and a serious improvement in grub usability.
> 
> Good idea for an optimization!
> 

Heh no big deal... the hardest part was to find that the LOAD/STORE avalanche
was coming from these rb@ and rb! words. SLOF is still a mysterious beast to
me :)

> > Signed-off-by: Greg Kurz <gkurz at linux.vnet.ibm.com>
> > ---
> >  slof/fs/fbuffer.fs |    4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/slof/fs/fbuffer.fs b/slof/fs/fbuffer.fs
> > index 756f05a..46b59bf 100644
> > --- a/slof/fs/fbuffer.fs
> > +++ b/slof/fs/fbuffer.fs
> > @@ -99,8 +99,8 @@ CREATE bitmap-buffer 400 4 * allot
> >  : fb8-toggle-cursor ( -- )
> >  	line# fb8-line2addr column# fb8-columns2bytes +
> >  	char-height 0 ?DO
> > -		char-width screen-depth * 0 ?DO dup dup rb@ -1 xor swap rb! 1+ LOOP
> > -		screen-width screen-depth * + char-width screen-depth * -
> > +		dup dup 0 char-width screen-depth * 1 hv-logical-memop drop
> > +		screen-width screen-depth * +
> >  	LOOP drop
> >  ;
> 
> If you use hv-logical-memop in this file here, you definitely break
> board-js2x, since this is bare metal and hv-logical-memop is not
> defined there.
> 

Of course, this is common code... I'll remember for next time. :)

> I think you should either move the new function to board-qemu and handle
> it there like it is done for hcall-invert-screen already, or we could
> think of introducing a helper function that is defined by each board
> which does the xor operation on a memory region (that way we could
> maybe also unify hcall-invert-screen and fb8-invert-screen again).
> 

I guess the first proposal is the obvious fix. From there, we can
work out a patchset for the second proposal.

>  Thomas
> 

Cheers.

--
Greg