drm/radeon/kms: force legacy pll algo for RV515 LVDS

There has been periodic evidence that LVDS, on at least some
panels, prefers the dividers selected by the legacy pll algo.
This patch forces the use of the legacy pll algo on RV515
LVDS panels.  The old behavior (new pll algo) can be selected
by setting the new_pll module parameter to 1.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/atombios_crtc.c

@@ -534,6 +534,20 @@ static u32 atombios_adjust_pll(struct drm_crtc *crtc,
 					pll->algo = PLL_ALGO_LEGACY;
 					pll->flags |= RADEON_PLL_PREFER_CLOSEST_LOWER;
 				}
+				/* There is some evidence (often anecdotal) that RV515 LVDS
+				 * (on some boards at least) prefers the legacy algo.  I'm not
+				 * sure whether this should handled generically or on a
+				 * case-by-case quirk basis.  Both algos should work fine in the
+				 * majority of cases.
+				 */
+				if ((radeon_encoder->active_device & (ATOM_DEVICE_LCD_SUPPORT)) &&
+				    (rdev->family == CHIP_RV515)) {
+					/* allow the user to overrride just in case */
+					if (radeon_new_pll == 1)
+						pll->algo = PLL_ALGO_NEW;
+					else
+						pll->algo = PLL_ALGO_LEGACY;
+				}
 			} else {
 				if (encoder->encoder_type != DRM_MODE_ENCODER_DAC)
 					pll->flags |= RADEON_PLL_NO_ODD_POST_DIV;