compositor: add weston.ini option max-bpc

For working around hardware limitations as explained in the man page.
Now added for completeness' sake without knowing if anyone will ever
need this.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

compositor/main.c

@@ -2000,6 +2000,7 @@ drm_backend_output_configure(struct weston_output *output,
 	enum weston_drm_backend_output_mode mode =
 		WESTON_DRM_BACKEND_OUTPUT_PREFERRED;
 	uint32_t transform = WL_OUTPUT_TRANSFORM_NORMAL;
+	uint32_t max_bpc;
 	char *s;
 	char *modeline = NULL;
 	char *gbm_format = NULL;
@@ -2011,6 +2012,9 @@ drm_backend_output_configure(struct weston_output *output,
 		return -1;
 	}
 
+	weston_config_section_get_uint(section, "max-bpc", &max_bpc, 16);
+	api->set_max_bpc(output, max_bpc);
+
 	weston_config_section_get_string(section, "mode", &s, "preferred");
 
 	if (strcmp(s, "off") == 0) {

man/weston-drm.man

@@ -164,6 +164,15 @@ Defaults to false. Note that
 When a connector is disconnected, there is no EDID information to provide
 a list of video modes. Therefore a forced output should also have a
 detailed mode line specified.
+.TP
+\fBmax-bpc\fR=\fIN\fR
+.RB "Set \(dq" "max bpc" "\(dq KMS property to value"
+.IR N ,
+silenty clamped to the hardware driver supported range. This artificially
+limits the driver chosen link bits-per-channel which may be useful for working
+around sink hardware (e.g. monitor) limitations. The default is 16 which is
+practically unlimited. If you need to work around hardware issues, try a lower
+value like 8.
 
 .SS Section remote-output
 .TP