drm/i915/pch: Fix integer math bugs in panel fitting

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.

The problem is that you have an integer number of pixels, which means
it's usually impossible to upscale equally on both axes.  1360/768 is
1.7708, 1600/900 is 1.7777.  Since we're constrained on the one axis,
the other one wants to come out as an even number of pixels (the panel
is almost certainly even on both axes, and the x/y offsets will be
applied on both sides).  In the math above, if 'width' comes out even,
rounding down is correct; if it's odd, you'd rather round up.  So just
increment width/height in those cases.

Tested on a Lenovo T500 (Ironlake).

Signed-off-by: Adam Jackson <ajax@redhat.com>
Tested-By: Daniel Manrique <daniel.manrique@canonical.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38851
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org
Signed-off-by: Keith Packard <keithp@keithp.com>

0 files changed, 0 insertions, 0 deletions