drm/i915: Use more optimal VTd alignment for planes

Depending on the platform and/or plane type we can get away
with a bit less alignment in the VT-d w/a. Reduce the numbers
accordingly.

Note that it's not actually clear in VLV/CHV need this w/a,
and if they do we don't actually know what kind of alignment
is sufficient. Leave the 256k alignment in place for now, but
toss in a FIXME.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250122151755.6928-3-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/display/i9xx_plane.c

@@ -785,6 +785,7 @@ unsigned int vlv_plane_min_alignment(struct intel_plane *plane,
 	if (intel_plane_can_async_flip(plane, fb->modifier))
 		return 256 * 1024;
 
+	/* FIXME undocumented so not sure what's actually needed */
 	if (intel_scanout_needs_vtd_wa(i915))
 		return 256 * 1024;
 

drivers/gpu/drm/i915/display/intel_cursor.c

@@ -375,7 +375,7 @@ static unsigned int i9xx_cursor_min_alignment(struct intel_plane *plane,
 	struct drm_i915_private *i915 = to_i915(plane->base.dev);
 
 	if (intel_scanout_needs_vtd_wa(i915))
-		return 256 * 1024;
+		return 64 * 1024;
 
 	return 4 * 1024; /* physical for i915/i945 */
 }

drivers/gpu/drm/i915/display/intel_sprite.c

@@ -983,7 +983,7 @@ static unsigned int g4x_sprite_min_alignment(struct intel_plane *plane,
 	struct drm_i915_private *i915 = to_i915(plane->base.dev);
 
 	if (intel_scanout_needs_vtd_wa(i915))
-		return 256 * 1024;
+		return 128 * 1024;
 
 	return 4 * 1024;
 }