drm/ingenic: ipu: Search for scaling coefs up to 102% of the screen

Increase the scaled image's theorical width/height until we find a
configuration that has valid scaling coefficients, up to 102% of the
screen's resolution. This makes sure that we can scale from almost
every resolution possible at the cost of a very small distorsion.
The CRTC_W / CRTC_H are not modified.

This algorithm was already in place but would not try to go above the
screen's resolution, and as a result would only work if the CRTC_W /
CRTC_H were smaller than the screen resolution. It will now try until it
reaches 102% of the screen's resolution.

Note that this algorithm exists mostly as a band-aid for a missing
functionality: it is not possible for userspace to request the closest
mode that would encapsulate the provided one, because the GEM buffer is
created beforehand. If there was a way to let the kernel tweak the mode,
I could write a better algorithm that would result in a better looking
picture.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201105083905.8780-1-paul@crapouillou.net

drivers/gpu/drm/ingenic/ingenic-ipu.c

@@ -516,7 +516,7 @@ static void ingenic_ipu_plane_atomic_update(struct drm_plane *plane,
 static int ingenic_ipu_plane_atomic_check(struct drm_plane *plane,
 					  struct drm_plane_state *state)
 {
-	unsigned int num_w, denom_w, num_h, denom_h, xres, yres;
+	unsigned int num_w, denom_w, num_h, denom_h, xres, yres, max_w, max_h;
 	struct ingenic_ipu *ipu = plane_to_ingenic_ipu(plane);
 	struct drm_crtc *crtc = state->crtc ?: plane->state->crtc;
 	struct drm_crtc_state *crtc_state;
@@ -558,19 +558,26 @@ static int ingenic_ipu_plane_atomic_check(struct drm_plane *plane,
 	xres = state->src_w >> 16;
 	yres = state->src_h >> 16;
 
-	/* Adjust the coefficients until we find a valid configuration */
-	for (denom_w = xres, num_w = state->crtc_w;
-	     num_w <= crtc_state->mode.hdisplay; num_w++)
+	/*
+	 * Increase the scaled image's theorical width/height until we find a
+	 * configuration that has valid scaling coefficients, up to 102% of the
+	 * screen's resolution. This makes sure that we can scale from almost
+	 * every resolution possible at the cost of a very small distorsion.
+	 * The CRTC_W / CRTC_H are not modified.
+	 */
+	max_w = crtc_state->mode.hdisplay * 102 / 100;
+	max_h = crtc_state->mode.vdisplay * 102 / 100;
+
+	for (denom_w = xres, num_w = state->crtc_w; num_w <= max_w; num_w++)
 		if (!reduce_fraction(&num_w, &denom_w))
 			break;
-	if (num_w > crtc_state->mode.hdisplay)
+	if (num_w > max_w)
 		return -EINVAL;
 
-	for (denom_h = yres, num_h = state->crtc_h;
-	     num_h <= crtc_state->mode.vdisplay; num_h++)
+	for (denom_h = yres, num_h = state->crtc_h; num_h <= max_h; num_h++)
 		if (!reduce_fraction(&num_h, &denom_h))
 			break;
-	if (num_h > crtc_state->mode.vdisplay)
+	if (num_h > max_h)
 		return -EINVAL;
 
 	ipu->num_w = num_w;