V4L/DVB (13331): mxl5005s: provide ability to override QAM gain for HVR-1600

The HVR-1600 doesn't use the standard mechanism for computing the gain when
in QAM mode, instead always forcing it to be 0x02.  Provide the ability to
override the algorithm on a per-board basis.

This change results in an improvement of 0.4-0.6 dB for QAM tuning.

This work was sponsored by ONELAN Limited.

Cc: Steven Toth <stoth@kernellabs.com>
Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/common/tuners/mxl5005s.c

@@ -2789,7 +2789,10 @@ static u16 MXL_TuneRF(struct dvb_frontend *fe, u32 RF_Freq)
 
 	/* add for 2.6.5 Special setting for QAM */
 	if (state->Mod_Type == MXL_QAM) {
-		if (state->RF_IN < 680000000)
+		if (state->config->qam_gain != 0)
+			status += MXL_ControlWrite(fe, RFSYN_CHP_GAIN,
+						   state->config->qam_gain);
+		else if (state->RF_IN < 680000000)
 			status += MXL_ControlWrite(fe, RFSYN_CHP_GAIN, 3);
 		else
 			status += MXL_ControlWrite(fe, RFSYN_CHP_GAIN, 2);

drivers/media/common/tuners/mxl5005s.h

@@ -108,6 +108,10 @@ struct mxl5005s_config {
 #define MXL_LOW_IF  1
 	u8 if_mode;
 
+	/* Some boards need to override the built-in logic for determining
+	   the gain when in QAM mode (the HVR-1600 is one such case) */
+	u8 qam_gain;
+
 	/* Stuff I don't know what to do with */
 	u8 AgcMasterByte;
 };

drivers/media/video/cx18/cx18-dvb.c

@@ -61,6 +61,7 @@ static struct mxl5005s_config hauppauge_hvr1600_tuner = {
 	.top		 = MXL5005S_TOP_25P2,
 	.mod_mode        = MXL_DIGITAL_MODE,
 	.if_mode         = MXL_ZERO_IF,
+	.qam_gain        = 0x02,
 	.AgcMasterByte   = 0x00,
 };