Better gain support for RTLSDR

[ratay]

Is your feature request related to a problem?

I'm still not satisfied with gain settings on RTLSDR device
so I made little research of it
after looking sources of rtlsdr.dll I found:

• there are 6 basic types of RTLSDR (E4000,FC0012,FC0013,FC2580,R820T,R828D)
• some of them allows random value of gain, some of them only predefined
• for example my tuner (fc0012) allows only several predefined values:

int fc0012_set_gain(void *dev, int gain)
{
	int ret;
	uint8_t tmp = 0;

	ret = fc0012_readreg(dev, 0x13, &tmp);
	tmp &= 0xe0;

	switch (gain) {
	case -99:		/* -9.9 dB */
		tmp |= 0x02;
		break;
	case -40:		/* -4 dB */
		break;
	case 71:
		tmp |= 0x08;	/* 7.1 dB */
		break;
	case 179:
		tmp |= 0x17;	/* 17.9 dB */
		break;
	case 192:
	default:
		tmp |= 0x10;	/* 19.2 dB */
		break;
	}

	ret = fc0012_writereg(dev, 0x13, tmp);
	return ret;
} 

• as you can see if another value is used driver choose value 19.2dB

• similar situation in driver E4000 - only error appears when non listed value is used:

  static const int32_t lnagain[] = {
	-50,	0,
	-25,	1,
	0,	4,
	25,	5,
	50,	6,
	75,	7,
	100,	8,
	125,	9,
	150,	10,
	175,	11,
	200,	12,
	250,	13,
	300,	14,
};

int e4k_set_lna_gain(struct e4k_state *e4k, int32_t gain)
{
	uint32_t i;
	for(i = 0; i < ARRAY_SIZE(lnagain)/2; ++i) {
		if(lnagain[i*2] == gain) {
			e4k_reg_set_mask(e4k, E4K_REG_GAIN1, 0xf, lnagain[i*2+1]);
			return gain;
		}
	}
	return -EINVAL;
}

Describe the solution you'd like

so for correct operation rtlsdr_get_tuner_gains must be used for getting allowed values:

int rtlsdr_get_tuner_gains(rtlsdr_dev_t *dev, int *gains)
{
	/* all gain values are expressed in tenths of a dB */
	const int e4k_gains[] = { -10, 15, 40, 65, 90, 115, 140, 165, 190, 215,
				  240, 290, 340, 420 };
	const int fc0012_gains[] = { -99, -40, 71, 179, 192 };
	const int fc0013_gains[] = { -99, -73, -65, -63, -60, -58, -54, 58, 61,
				       63, 65, 67, 68, 70, 71, 179, 181, 182,
				       184, 186, 188, 191, 197 };
	const int fc2580_gains[] = { 0 /* no gain values */ };
	const int r82xx_gains[] = { 0, 9, 14, 27, 37, 77, 87, 125, 144, 157,
				     166, 197, 207, 229, 254, 280, 297, 328,
				     338, 364, 372, 386, 402, 421, 434, 439,
				     445, 480, 496 }; 
   ....

Describe alternatives you've considered

Additional context