usb: gadget: u_audio: set volume value to uevent

Different Windwos version has different volume gain
range [0], set UAC_VOLUME_MAX/MIN to maximum according
UAC 1.0 spec. And Windows volume slider position /
Loundness is not linear [1], it's better to set
c/p_volume to uevent than percentage.

[0]: https://docs.microsoft.com/en-us/windows-hardware/drivers/audio/default-audio-volume-settings
[1]: https://docs.microsoft.com/en-us/windows/win32/coreaudio/audio-tapered-volume-controls

Signed-off-by: Zain Wang <wzz@rock-chips.com>
Change-Id: I41d03cfe1d8d348720c89083707070c68a158bcb

Author: zainwangrockchips

drivers/usb/gadget/function/u_audio.c

@@ -687,7 +687,6 @@ static void g_audio_work(struct work_struct *data)
 	struct device *dev = &gadget->dev;
 	char *uac_event[4]  = { NULL, NULL, NULL, NULL };
 	char str[19];
-	signed short volume;
 	int i;
 
 	for (i = 0; i < SET_USB_STATE_MAX; i++) {
@@ -736,17 +735,13 @@ static void g_audio_work(struct work_struct *data)
 		case SET_VOLUME_OUT:
 			uac_event[0] = "USB_STATE=SET_VOLUME";
 			uac_event[1] = "STREAM_DIRECTION=OUT";
-			volume = (signed short)params->c_volume;
-			volume /= UAC_VOLUME_RES;
-			snprintf(str, sizeof(str), "VOLUME=%d%%", volume + 50);
+			snprintf(str, sizeof(str), "VOLUME=%6d", (int16_t)params->c_volume);
 			uac_event[2] = str;
 			break;
 		case SET_VOLUME_IN:
 			uac_event[0] = "USB_STATE=SET_VOLUME";
 			uac_event[1] = "STREAM_DIRECTION=IN";
-			volume = (signed short)params->p_volume;
-			volume /= UAC_VOLUME_RES;
-			snprintf(str, sizeof(str), "VOLUME=%d%%", volume + 50);
+			snprintf(str, sizeof(str), "VOLUME=%6d", (int16_t)params->p_volume);
 			uac_event[2] = str;
 			break;
 		default:

drivers/usb/gadget/function/u_audio.h

@@ -22,10 +22,9 @@
 #include <linux/usb/composite.h>
 
 #define UAC_VOLUME_CUR			0x0000
-#define UAC_VOLUME_RES			0x0080 /* 0.5 dB */
-#define UAC_VOLUME_MAX			0x1900 /* 25 dB */
-#define UAC_VOLUME_MIN			0xE700 /* -25 dB */
-#define UAC_VOLUME_NEGATIVE_INFINITY	0x8000
+#define UAC_VOLUME_RES			0x0001 /* 1/256 dB */
+#define UAC_VOLUME_MAX			0x7FFF /* +127.9961 dB */
+#define UAC_VOLUME_MIN			0x8001 /* -127.9961 dB */
 #define UAC_MAX_RATES 10
 struct uac_params {
 	/* playback */