lavc/qsvenc: allows the SDK runtime to choose LowPower/non-LowPower modes

The SDK supports LowPower and non-LowPower modes, but some features are
available only under one of the two modes. Currently non-LowPower mode
is always chosen in FFmpeg if the mode is not set to LowPower
explicitly. User will experience some SDK errors if a LowPower related
feature is specified but the mode is not set to LowPower. With this
patch, the mode is set to unknown by default in FFmpeg, the SDK is able
to choose a workable mode for the specified features.

Reviewed-by: Soft Works <softworkz@hotmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/qsvenc.c

@@ -514,7 +514,7 @@ static int init_video_param(AVCodecContext *avctx, QSVEncContext *q)
         }
     }
 
-    if (q->low_power) {
+    if (q->low_power == 1) {
 #if QSV_HAVE_VDENC
         q->param.mfx.LowPower = MFX_CODINGOPTION_ON;
 #else
@@ -523,7 +523,9 @@ static int init_video_param(AVCodecContext *avctx, QSVEncContext *q)
         q->low_power = 0;
         q->param.mfx.LowPower = MFX_CODINGOPTION_OFF;
 #endif
-    } else
+    } else if (q->low_power == -1)
+        q->param.mfx.LowPower = MFX_CODINGOPTION_UNKNOWN;
+    else
         q->param.mfx.LowPower = MFX_CODINGOPTION_OFF;
 
     q->param.mfx.CodecProfile       = q->profile;

libavcodec/qsvenc.h

@@ -96,7 +96,7 @@
 { "adaptive_b",     "Adaptive B-frame placement",             OFFSET(qsv.adaptive_b),     AV_OPT_TYPE_INT, { .i64 = -1 }, -1,          1, VE },                         \
 { "b_strategy",     "Strategy to choose between I/P/B-frames", OFFSET(qsv.b_strategy),    AV_OPT_TYPE_INT, { .i64 = -1 }, -1,          1, VE },                         \
 { "forced_idr",     "Forcing I frames as IDR frames",         OFFSET(qsv.forced_idr),     AV_OPT_TYPE_BOOL,{ .i64 = 0  },  0,          1, VE },                         \
-{ "low_power", "enable low power mode(experimental: many limitations by mfx version, BRC modes, etc.)", OFFSET(qsv.low_power), AV_OPT_TYPE_BOOL, { .i64 = 0}, 0, 1, VE},\
+{ "low_power", "enable low power mode(experimental: many limitations by mfx version, BRC modes, etc.)", OFFSET(qsv.low_power), AV_OPT_TYPE_BOOL, { .i64 = -1}, -1, 1, VE},\
 
 extern const AVCodecHWConfigInternal *const ff_qsv_enc_hw_configs[];