compositor-drm: rewrite crtc picking for clone mode

To support shared-CRTC clone mode, the chosen CRTC needs to support
driving all the attached connectors. Replace the old algorithm with a
new one that takes into account all associated connectors.

Ideally it should use possible_clones mask to check which encoders (and
therefore connectors) actually can be in a cloned set. However, the DRM
documentation says about possible_clones and possible_crtcs masks both:
"In reality almost every driver gets this wrong."
- https://01.org/linuxgraphics/gfx-docs/drm/gpu/drm-kms.html#c.drm_encoder

Looking at a target device and its kernel where clone mode is desired,
possible_clones is indeed self-conflicting and would not allow cloning
at all. Therefore the implemented algorithm replaces the checking of
possible_clones with luck. It even goes out of its way to find any CRTC
for a configuration, even if not advertised by the kernel as not
supported.

Libweston would need infrastructure to allow trial-and-error CRTC
allocation: rather than picking one CRTC in advance and do or die, it
should try all available CRTCs one by one. Unfortunately that is not yet
possible, so this patch implements what it can. It is also the DRM
upstream opinion that trial-and-error with ATOMIC_TEST would be the way
to go.

Unlike the old algorithm, the new algorithm prefers routings that were
in place when Weston started instead of when enabling an output. When
you never temporarily disable an output, this makes no difference.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Acked-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>

libweston/compositor-drm.c

@@ -417,6 +417,7 @@ struct drm_head {
 	struct backlight *backlight;
 
 	drmModeModeInfo inherited_mode;	/**< Original mode on the connector */
+	uint32_t inherited_crtc_id;	/**< Original CRTC assignment */
 };
 
 struct drm_output {
@@ -3975,51 +3976,6 @@ make_connector_name(const drmModeConnector *con)
 	return name;
 }
 
-static int
-find_crtc_for_connector(struct drm_backend *b,
-			drmModeRes *resources, drmModeConnector *connector)
-{
-	drmModeEncoder *encoder;
-	int i, j;
-	int ret = -1;
-
-	for (j = 0; j < connector->count_encoders; j++) {
-		uint32_t possible_crtcs, encoder_id, crtc_id;
-
-		encoder = drmModeGetEncoder(b->drm.fd, connector->encoders[j]);
-		if (encoder == NULL) {
-			weston_log("Failed to get encoder.\n");
-			continue;
-		}
-		encoder_id = encoder->encoder_id;
-		possible_crtcs = encoder->possible_crtcs;
-		crtc_id = encoder->crtc_id;
-		drmModeFreeEncoder(encoder);
-
-		for (i = 0; i < resources->count_crtcs; i++) {
-			if (!(possible_crtcs & (1 << i)))
-				continue;
-
-			if (drm_output_find_by_crtc(b, resources->crtcs[i]))
-				continue;
-
-			/* Try to preserve the existing
-			 * CRTC -> encoder -> connector routing; it makes
-			 * initialisation faster, and also since we have a
-			 * very dumb picking algorithm, may preserve a better
-			 * choice. */
-			if (!connector->encoder_id ||
-			    (encoder_id == connector->encoder_id &&
-			     crtc_id == resources->crtcs[i]))
-				return i;
-
-			ret = i;
-		}
-	}
-
-	return ret;
-}
-
 static void drm_output_fini_cursor_egl(struct drm_output *output)
 {
 	unsigned int i;
@@ -4690,8 +4646,11 @@ drm_head_read_current_setup(struct drm_head *head, struct drm_backend *backend)
 	 * this connector. */
 	encoder = drmModeGetEncoder(drm_fd, head->connector->encoder_id);
 	if (encoder != NULL) {
+		head->inherited_crtc_id = encoder->crtc_id;
+
 		crtc = drmModeGetCrtc(drm_fd, encoder->crtc_id);
 		drmModeFreeEncoder(encoder);
+
 		if (crtc == NULL)
 			return -1;
 		if (crtc->mode_valid)
@@ -4778,14 +4737,134 @@ drm_output_init_gamma_size(struct drm_output *output)
 	return 0;
 }
 
+static uint32_t
+drm_head_get_possible_crtcs_mask(struct drm_head *head)
+{
+	uint32_t possible_crtcs = 0;
+	drmModeEncoder *encoder;
+	int i;
+
+	for (i = 0; i < head->connector->count_encoders; i++) {
+		encoder = drmModeGetEncoder(head->backend->drm.fd,
+					    head->connector->encoders[i]);
+		if (!encoder)
+			continue;
+
+		possible_crtcs |= encoder->possible_crtcs;
+		drmModeFreeEncoder(encoder);
+	}
+
+	return possible_crtcs;
+}
+
+static int
+drm_crtc_get_index(drmModeRes *resources, uint32_t crtc_id)
+{
+	int i;
+
+	for (i = 0; i < resources->count_crtcs; i++) {
+		if (resources->crtcs[i] == crtc_id)
+			return i;
+	}
+
+	assert(0 && "unknown crtc id");
+	return -1;
+}
+
+/** Pick a CRTC that might be able to drive all attached connectors
+ *
+ * @param output The output whose attached heads to include.
+ * @param resources The DRM KMS resources.
+ * @return CRTC index, or -1 on failure or not found.
+ */
+static int
+drm_output_pick_crtc(struct drm_output *output, drmModeRes *resources)
+{
+	struct drm_backend *backend;
+	struct weston_head *base;
+	struct drm_head *head;
+	uint32_t possible_crtcs = 0xffffffff;
+	int existing_crtc[32];
+	unsigned j, n = 0;
+	uint32_t crtc_id;
+	int best_crtc_index = -1;
+	int i;
+
+	backend = to_drm_backend(output->base.compositor);
+
+	/* This algorithm ignores drmModeEncoder::possible_clones restriction,
+	 * because it is more often set wrong than not in the kernel. */
+
+	/* Accumulate a mask of possible crtcs and find existing routings. */
+	wl_list_for_each(base, &output->base.head_list, output_link) {
+		head = to_drm_head(base);
+
+		possible_crtcs &= drm_head_get_possible_crtcs_mask(head);
+
+		crtc_id = head->inherited_crtc_id;
+		if (crtc_id > 0 && n < ARRAY_LENGTH(existing_crtc))
+			existing_crtc[n++] = drm_crtc_get_index(resources,
+								crtc_id);
+	}
+
+	/* Find a crtc that could drive each connector individually at least,
+	 * and prefer existing routings. */
+	for (i = 0; i < resources->count_crtcs; i++) {
+		crtc_id = resources->crtcs[i];
+
+		/* Could the crtc not drive each connector? */
+		if (!(possible_crtcs & (1 << i)))
+			continue;
+
+		/* Is the crtc already in use? */
+		if (drm_output_find_by_crtc(backend, crtc_id))
+			continue;
+
+		/* Try to preserve the existing CRTC -> connector routing;
+		 * it makes initialisation faster, and also since we have a
+		 * very dumb picking algorithm, may preserve a better
+		 * choice. */
+		for (j = 0; j < n; j++) {
+			if (existing_crtc[j] == i)
+				return i;
+		}
+
+		best_crtc_index = i;
+	}
+
+	if (best_crtc_index != -1)
+		return best_crtc_index;
+
+	/* Likely possible_crtcs was empty due to asking for clones,
+	 * but since the DRM documentation says the kernel lies, let's
+	 * pick one crtc anyway. Trial and error is the only way to
+	 * be sure if something doesn't work. */
+
+	/* First pick any existing assignment. */
+	for (j = 0; j < n; j++) {
+		crtc_id = resources->crtcs[existing_crtc[j]];
+		if (!drm_output_find_by_crtc(backend, crtc_id))
+			return existing_crtc[j];
+	}
+
+	/* Otherwise pick any available crtc. */
+	for (i = 0; i < resources->count_crtcs; i++) {
+		crtc_id = resources->crtcs[i];
+
+		if (!drm_output_find_by_crtc(backend, crtc_id))
+			return i;
+	}
+
+	return -1;
+}
+
 /** Allocate a CRTC for the output
  *
  * @param output The output with no allocated CRTC.
  * @param resources DRM KMS resources.
- * @param connector The DRM KMS connector data.
  * @return 0 on success, -1 on failure.
  *
- * Finds a free CRTC that can drive the given connector, reserves the CRTC
+ * Finds a free CRTC that might drive the attached connectors, reserves the CRTC
  * for the output, and loads the CRTC properties.
  *
  * Populates the cursor and scanout planes.
@@ -4793,8 +4872,7 @@ drm_output_init_gamma_size(struct drm_output *output)
  * On failure, the output remains without a CRTC.
  */
 static int
-drm_output_init_crtc(struct drm_output *output,
-		     drmModeRes *resources, drmModeConnector *connector)
+drm_output_init_crtc(struct drm_output *output, drmModeRes *resources)
 {
 	struct drm_backend *b = to_drm_backend(output->base.compositor);
 	drmModeObjectPropertiesPtr props;
@@ -4802,9 +4880,10 @@ drm_output_init_crtc(struct drm_output *output,
 
 	assert(output->crtc_id == 0);
 
-	i = find_crtc_for_connector(b, resources, connector);
+	i = drm_output_pick_crtc(output, resources);
 	if (i < 0) {
-		weston_log("No usable crtc/encoder pair for connector.\n");
+		weston_log("Output '%s': No available CRTCs.\n",
+			   output->base.name);
 		return -1;
 	}
 
@@ -4917,7 +4996,6 @@ drm_output_enable(struct weston_output *base)
 {
 	struct drm_output *output = to_drm_output(base);
 	struct drm_backend *b = to_drm_backend(base->compositor);
-	struct drm_head *head = to_drm_head(weston_output_get_first_head(base));
 	drmModeRes *resources;
 	int ret;
 
@@ -4926,7 +5004,7 @@ drm_output_enable(struct weston_output *base)
 		weston_log("drmModeGetResources failed\n");
 		return -1;
 	}
-	ret = drm_output_init_crtc(output, resources, head->connector);
+	ret = drm_output_init_crtc(output, resources);
 	drmModeFreeResources(resources);
 	if (ret < 0)
 		return -1;