[PATCH] fbuffer: improve toggle cursor performance

[Greg Kurz]

On Wed, 27 May 2015 10:41:06 +0530
Nikunj A Dadhania <nikunj at linux.vnet.ibm.com> wrote:

> Greg Kurz <gkurz at linux.vnet.ibm.com> writes:
> 
> > 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.
> >
> > 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 * +
> 
> Why did you drop "char-width screen-depth * -" in the new code? This is
> not me mentioned in the description.
> 

This is because the current inner loop increments the address. When the loop
ends, we're pointing at the next char, that is char-width * screen-depth bytes
too far.

In the new code, the address is duped on the stack before calling hv-logical-memop,
so we don't need to fix it when proceeding to next line.

In my first attempt, I forgot to drop the subtraction and got an interesting
visual result :)

> Regards
> Nikunj

Cheers.

--
Greg