Re: clipping

On Wed, 2003-01-08 at 17:54, Geert Uytterhoeven wrote:

On Tue, 7 Jan 2003, James Simmons wrote:

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BTW, do we really need clipping in fb_ops.fb_{fillrect,copyarea,imageblit}()?

Personally, I don't think we need clipping.  I tried removing it before
(circa linux-2.5.30+), but the console segfaults whenever I decrease
var->yres_virtual.  I haven't tried this again with the newest
framebuffer framework though.

Any definitive answer on this?

What happens if you resize the VT to such a large size that columns*fontwidth >
xres_virtual or rows*fontheight > yres_virtual? I guess clipping prevents a
crash there?

Correct. Actually fbcon_resize checks to make sure the user doesn't do 
something stupid like this. So we might not needed. Hm.

And what if you use fbset to reduce the resolution below what's needed to
accomodate the current console size?

Then we do need clipping :-)  I think that happened to me, I just did
not recognize it.  I accidentally lowered the window size using fbset,
and suddenly I had a console where the tail end of each row of text
wraps to the next row.  I wasn't using cfb_imageblit then so this was
done by hardware.  If I was using cfb_imageblit, this will be clipped.

So, I think we do need clipping, but instead of implementing it in
fb_{fillrect,copyarea,imageblit}, we implement it higher up, in
accel_putcs() and family.  This should also protect drivers using 
hardware-based drawing. 

So perhaps, a function something like this:

void fb_clip(struct fb_fillrect *region, struct fb_fillrect *clip);

This should not be difficult to implement, and I'll code it if everyone
agrees.

The other option (which I don't like) is just to check the passed
fb_var_screeninfo in the put_var ioctl against the current console
window size.  But this is not foolproof as we will not be sure of the
resulting window size _after_ the fb_set_var() call.

Tony




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