Add Option "Backlight" to override the probed backlight control interface

The automatic selection may not correspond with the correct backlight
(such as in a multi-gpu, multi-panel device) or the user may simply
prefer another control interface. This allows them to override the
chosen interface using

  Option "Backlight" "my-backlight"

to specify '/sys/class/backlight/my-backlight' as the interface to use
instead.

Suggested-by: Alon Levy <alevy@redhat.com>
References: https://bugs.freedesktop.org/show_bug.cgi?id=29273
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

man/intel.man

@@ -132,6 +132,15 @@ have options for selecting adaptors.
 .IP
 Default: Textured video adaptor is preferred.
 .TP
+.BI "Option \*Backlight\*q \*q" string \*q
+Override the probed backlight control interface. Sometimes the automatically
+selected backlight interface may not correspond to the correct, or simply
+most useful, interface available on the system. This allows you to override
+by specifying the entry under /sys/class/backlight to use.
+server log.
+.IP
+Default: Automatic selection.
+.TP
 .BI "Option \*qFallbackDebug\*q \*q" boolean \*q
 Enable printing of debugging information on acceleration fallbacks to the
 server log.

src/intel_display.c

@@ -41,6 +41,7 @@
 
 #include "intel.h"
 #include "intel_bufmgr.h"
+#include "intel_options.h"
 #include "xf86drm.h"
 #include "xf86drmMode.h"
 #include "X11/Xatom.h"
@@ -252,19 +253,36 @@ static void
 intel_output_backlight_init(xf86OutputPtr output)
 {
 	struct intel_output *intel_output = output->driver_private;
+	intel_screen_private *intel = intel_get_screen_private(output->scrn);
+	char path[BACKLIGHT_PATH_LEN];
+	struct stat buf;
+	char *str;
 	int i;
 
-	for (i = 0; backlight_interfaces[i] != NULL; i++) {
-		char path[BACKLIGHT_PATH_LEN];
-		struct stat buf;
+	str = xf86GetOptValString(intel->Options, OPTION_BACKLIGHT);
+	if (str == NULL) {
+		sprintf(path, "%s/%s", BACKLIGHT_CLASS, str);
+		if (!stat(path, &buf)) {
+			intel_output->backlight_iface = backlight_interfaces[i];
+			intel_output->backlight_max = intel_output_backlight_get_max(output);
+			if (intel_output->backlight_max > 0) {
+				xf86DrvMsg(output->scrn->scrnIndex, X_CONFIG,
+					   "found backlight control interface %s\n", path);
+				return;
+			}
+		}
+		xf86DrvMsg(output->scrn->scrnIndex, X_ERROR,
+			   "unrecognised backlight control interface %s\n", str);
+	}
 
+	for (i = 0; backlight_interfaces[i] != NULL; i++) {
 		sprintf(path, "%s/%s", BACKLIGHT_CLASS, backlight_interfaces[i]);
 		if (!stat(path, &buf)) {
 			intel_output->backlight_iface = backlight_interfaces[i];
 			intel_output->backlight_max = intel_output_backlight_get_max(output);
 			if (intel_output->backlight_max > 0) {
 				intel_output->backlight_active_level = intel_output_backlight_get(output);
-				xf86DrvMsg(output->scrn->scrnIndex, X_INFO,
+				xf86DrvMsg(output->scrn->scrnIndex, X_PROBED,
 					   "found backlight control interface %s\n", path);
 				return;
 			}

src/intel_options.c

@@ -7,6 +7,7 @@
 const OptionInfoRec intel_options[] = {
 	{OPTION_ACCEL_DISABLE,	"NoAccel",	OPTV_BOOLEAN,	{0},	0},
 	{OPTION_ACCEL_METHOD,	"AccelMethod",	OPTV_STRING,	{0},	0},
+	{OPTION_BACKLIGHT,	"Backlight",	OPTV_STRING,	{0},	0},
 	{OPTION_DRI,		"DRI",		OPTV_BOOLEAN,	{0},	1},
 	{OPTION_COLOR_KEY,	"ColorKey",	OPTV_INTEGER,	{0},	0},
 	{OPTION_VIDEO_KEY,	"VideoKey",	OPTV_INTEGER,	{0},	0},

src/intel_options.h

@@ -13,6 +13,7 @@
 enum intel_options {
 	OPTION_ACCEL_DISABLE,
 	OPTION_ACCEL_METHOD,
+	OPTION_BACKLIGHT,
 	OPTION_DRI,
 	OPTION_VIDEO_KEY,
 	OPTION_COLOR_KEY,

src/sna/sna_display.c

@@ -290,6 +290,31 @@ enum {
 	NAMED,
 };
 
+static char *
+has_user_backlight_override(xf86OutputPtr output)
+{
+	struct sna_output *sna_output = output->driver_private;
+	struct sna *sna = to_sna(output->scrn);
+	char *str;
+	int max;
+
+	str = xf86GetOptValString(sna->Options, OPTION_BACKLIGHT);
+	if (str == NULL)
+		return NULL;
+
+	sna_output->backlight_iface = str;
+	max = sna_output_backlight_get_max(output);
+	sna_output->backlight_iface = NULL;
+	if (max <= 0) {
+		xf86DrvMsg(output->scrn->scrnIndex, X_ERROR,
+			   "unrecognised backlight control interface '%s'\n",
+			   str);
+		return NULL;
+	}
+
+	return str;
+}
+
 static void
 sna_output_backlight_init(xf86OutputPtr output)
 {
@@ -307,14 +332,17 @@ sna_output_backlight_init(xf86OutputPtr output)
 		"acpi_video0",
 		"intel_backlight",
 	};
+	MessageType from = X_PROBED;
 	struct sna_output *sna_output = output->driver_private;
 	char *best_iface;
 	int best_type;
 	DIR *dir;
 	struct dirent *de;
 
-	best_iface = NULL;
 	best_type = INT_MAX;
+	best_iface = has_user_backlight_override(output);
+	if (best_iface)
+		goto skip;
 
 	dir = opendir(BACKLIGHT_CLASS);
 	if (dir == NULL)
@@ -371,6 +399,7 @@ sna_output_backlight_init(xf86OutputPtr output)
 
 			sna_output->backlight_iface = de->d_name;
 			max = sna_output_backlight_get_max(output);
+			sna_output->backlight_iface = NULL;
 			if (max <= 0)
 				continue;
 
@@ -384,24 +413,23 @@ sna_output_backlight_init(xf86OutputPtr output)
 	}
 	closedir(dir);
 
-	sna_output->backlight_iface = NULL;
+	if (!best_iface)
+		return;
 
-	if (best_iface) {
-		const char *str;
-
-		sna_output->backlight_iface = best_iface;
-		sna_output->backlight_max = sna_output_backlight_get_max(output);
-		sna_output->backlight_active_level = sna_output_backlight_get(output);
-		switch (best_type) {
-		case FIRMWARE: str = "firmware"; break;
-		case PLATFORM: str = "platform"; break;
-		case RAW: str = "raw"; break;
-		default: str = "unknown"; break;
-		}
-		xf86DrvMsg(output->scrn->scrnIndex, X_INFO,
-			   "found backlight control interface %s (type '%s')\n",
-			   best_iface, str);
+skip:
+	sna_output->backlight_iface = best_iface;
+	sna_output->backlight_max = sna_output_backlight_get_max(output);
+	sna_output->backlight_active_level = sna_output_backlight_get(output);
+	switch (best_type) {
+	case INT_MAX: best_iface = "user"; from = X_CONFIG; break;
+	case FIRMWARE: best_iface = "firmware"; break;
+	case PLATFORM: best_iface = "platform"; break;
+	case RAW: best_iface = "raw"; break;
+	default: best_iface = "unknown"; break;
 	}
+	xf86DrvMsg(output->scrn->scrnIndex, from,
+		   "found backlight control interface %s (type '%s')\n",
+		   sna_output->backlight_iface, best_iface);
 }