[Intel-gfx] [PATCH] drm/i915/pch: Fix integer math bugs in panel fitting

[Chris Wilson]

On Wed, 13 Jul 2011 16:32:32 -0400, Adam Jackson <ajax at redhat.com> wrote:
> Consider a 1600x900 panel, upscaling a 1360x768 mode, full-aspect.  The
> old math would give you:
> 
>     scaled_width  = 1600 * 768;         /* 1228800 */
>     scaled_height = 1360 * 900;         /* 1224000 */
>     if (scaled_width > scaled_height) { /* pillarbox, and true */
>         width  = 1224000 / 768;         /* int(1593.75) = 1593 */
>         x      = (1600 - 1593 + 1) / 2; /* 4 */
>         y      = 0;
>         height = 768;
>     } /* ... */
> 
> This is broken.  The total width of scanout would then be 1593 + 4 + 4,
> or 1601, which is wider than the panel itself.  The hardware very
> dutifully implements this, and you end up with a black 45° diagonal from
> the top-left corner to the bottom edge of the screen.  It's a cool
> effect and all, but not what you wanted.  Similar things happen for the
> letterbox case.

I'm going to show my ignorance but why did the hw apply the offset to both
sides of the output and generate a new mode line? That's probably inherent
in the nature of the hblanks, I guess? The entirety of the description I
have is that the position is simply the origin of the top-left pixel of the
fitted window.

So a comment here is necessary, but otherwise:

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38851
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>

And a big thank you.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre