config/output: Add support for 6-bit render fmt

GUD devices uses RGB565 by default for performance reasons. Allow
specifying render_bit_depth 6 to pick this format. The definition works
out if you consider the maximum number of bits per channel instead of
the average.

include/sway/config.h

@@ -262,6 +262,7 @@ enum scale_filter_mode {
 
 enum render_bit_depth {
 	RENDER_BIT_DEPTH_DEFAULT, // the default is currently 8
+	RENDER_BIT_DEPTH_6,
 	RENDER_BIT_DEPTH_8,
 	RENDER_BIT_DEPTH_10,
 };

sway/commands/output/render_bit_depth.c

@@ -11,7 +11,10 @@ struct cmd_results *output_cmd_render_bit_depth(int argc, char **argv) {
 		return cmd_results_new(CMD_INVALID, "Missing bit depth argument.");
 	}
 
-	if (strcmp(*argv, "8") == 0) {
+	if (strcmp(*argv, "6") == 0) {
+		config->handler_context.output_config->render_bit_depth =
+			RENDER_BIT_DEPTH_6;
+	} else if (strcmp(*argv, "8") == 0) {
 		config->handler_context.output_config->render_bit_depth =
 			RENDER_BIT_DEPTH_8;
 	} else if (strcmp(*argv, "10") == 0) {
@@ -19,7 +22,7 @@ struct cmd_results *output_cmd_render_bit_depth(int argc, char **argv) {
 			RENDER_BIT_DEPTH_10;
 	} else {
 		return cmd_results_new(CMD_INVALID,
-			"Invalid bit depth. Must be a value in (8|10).");
+			"Invalid bit depth. Must be a value in (6|8|10).");
 	}
 
 	config->handler_context.leftovers.argc = argc - 1;

sway/config/output.c

@@ -398,9 +398,15 @@ static int compute_default_scale(struct wlr_output *output,
 	return 2;
 }
 
-static bool render_format_is_10bit(uint32_t render_format) {
-	return render_format == DRM_FORMAT_XRGB2101010 ||
-		render_format == DRM_FORMAT_XBGR2101010;
+static enum render_bit_depth bit_depth_from_format(uint32_t render_format) {
+	if (render_format == DRM_FORMAT_XRGB2101010 || render_format == DRM_FORMAT_XBGR2101010) {
+		return RENDER_BIT_DEPTH_10;
+	} else if (render_format == DRM_FORMAT_XRGB8888 || render_format == DRM_FORMAT_ARGB8888) {
+		return RENDER_BIT_DEPTH_8;
+	} else if (render_format == DRM_FORMAT_RGB565) {
+		return RENDER_BIT_DEPTH_6;
+	}
+	return RENDER_BIT_DEPTH_DEFAULT;
 }
 
 static bool render_format_is_bgr(uint32_t fmt) {
@@ -496,11 +502,13 @@ static void queue_output_config(struct output_config *oc,
 
 	if (oc && oc->render_bit_depth != RENDER_BIT_DEPTH_DEFAULT) {
 		if (oc->render_bit_depth == RENDER_BIT_DEPTH_10 &&
-			render_format_is_10bit(output->wlr_output->render_format)) {
+			bit_depth_from_format(output->wlr_output->render_format) == oc->render_bit_depth) {
 			// 10-bit was set successfully before, try to save some tests by reusing the format
 			wlr_output_state_set_render_format(pending, output->wlr_output->render_format);
 		} else if (oc->render_bit_depth == RENDER_BIT_DEPTH_10) {
 			wlr_output_state_set_render_format(pending, DRM_FORMAT_XRGB2101010);
+		} else if (oc->render_bit_depth == RENDER_BIT_DEPTH_6){
+			wlr_output_state_set_render_format(pending, DRM_FORMAT_RGB565);
 		} else {
 			wlr_output_state_set_render_format(pending, DRM_FORMAT_XRGB8888);
 		}
@@ -785,6 +793,7 @@ static bool search_render_format(struct search_context *ctx, size_t output_idx)
 		DRM_FORMAT_XBGR2101010,
 		DRM_FORMAT_XRGB8888,
 		DRM_FORMAT_ARGB8888,
+		DRM_FORMAT_RGB565,
 		DRM_FORMAT_INVALID,
 	};
 	if (render_format_is_bgr(wlr_output->render_format)) {
@@ -795,9 +804,13 @@ static bool search_render_format(struct search_context *ctx, size_t output_idx)
 
 	const struct wlr_drm_format_set *primary_formats =
 		wlr_output_get_primary_formats(wlr_output, WLR_BUFFER_CAP_DMABUF);
-	bool need_10bit = cfg->config && cfg->config->render_bit_depth == RENDER_BIT_DEPTH_10;
+	enum render_bit_depth needed_bits = RENDER_BIT_DEPTH_8;
+	if (cfg->config && cfg->config->render_bit_depth != RENDER_BIT_DEPTH_DEFAULT) {
+		needed_bits = cfg->config->render_bit_depth;
+	}
 	for (size_t idx = 0; fmts[idx] != DRM_FORMAT_INVALID; idx++) {
-		if (!need_10bit && render_format_is_10bit(fmts[idx])) {
+		enum render_bit_depth format_bits = bit_depth_from_format(fmts[idx]);
+		if (needed_bits < format_bits) {
 			continue;
 		}
 		if (!wlr_drm_format_set_get(primary_formats, fmts[idx])) {

sway/sway-output.5.scd

@@ -163,9 +163,9 @@ must be separated by one space. For example:
 	adaptive sync can improve latency, but can cause flickering on some
 	hardware.
 
-*output* <name> render_bit_depth 8|10
-	Controls the color channel bit depth at which frames are rendered; the
-	default is currently 8 bits per channel.
+*output* <name> render_bit_depth 6|8|10
+	Controls the maximum color channel bit depth at which frames are
+	rendered; the default is currently 8 bits per channel.
 
 	Setting higher values will not have an effect if hardware and software lack
 	support for such bit depths. Successfully increasing the render bit depth