media: vidtv: implement a demodulator driver

Implement a I2C demodulator driver, simulating support for DVB-T, DVB-C
and DVB-S.

This demodulator will periodically check the signal quality against a table
and drop the TS lock if it drops below a threshold value, regaining it in
the event that the signal improves.
Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/test-drivers/vidtv/Makefile

 # SPDX-License-Identifier: GPL-2.0
 
 dvb-vidtv-tuner-objs := vidtv_tuner.o
+dvb-vidtv-demod-objs := vidtv_demod.o
 
-obj-$(CONFIG_DVB_VIDTV)	+= dvb-vidtv-tuner.o
+obj-$(CONFIG_DVB_VIDTV)	+= dvb-vidtv-tuner.o dvb-vidtv-demod.o

drivers/media/test-drivers/vidtv/vidtv_demod.h

+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * The Virtual DTV test driver serves as a reference DVB driver and helps
+ * validate the existing APIs in the media subsystem. It can also aid
+ * developers working on userspace applications.
+ *
+ * Copyright (C) 2020 Daniel W. S. Almeida
+ * Based on the example driver written by Emard <emard@softhome.net>
+ */
+
+#ifndef VIDTV_DEMOD_H
+#define VIDTV_DEMOD_H
+
+#include <linux/dvb/frontend.h>
+#include <media/dvb_frontend.h>
+
+/**
+ * struct vidtv_demod_cnr_to_qual_s - Map CNR values to a given combination of
+ * modulation and fec_inner
+ * @modulation: see enum fe_modulation
+ * @fec: see enum fe_fec_rate
+ *
+ * This struct matches values for 'good' and 'ok' CNRs given the combination
+ * of modulation and fec_inner in use. We might simulate some noise if the
+ * signal quality is not too good.
+ *
+ * The values were taken from libdvbv5.
+ */
+struct vidtv_demod_cnr_to_qual_s {
+	u32 modulation;
+	u32 fec;
+	u32 cnr_ok;
+	u32 cnr_good;
+};
+
+/**
+ * struct vidtv_demod_config - Configuration used to init the demod
+ * @drop_tslock_prob_on_low_snr: probability of losing the lock due to low snr
+ * @recover_tslock_prob_on_good_snr: probability of recovering when the signal
+ * improves
+ *
+ * The configuration used to init the demodulator module, usually filled
+ * by a bridge driver. For vidtv, this is filled by vidtv_bridge before the
+ * demodulator module is probed.
+ */
+struct vidtv_demod_config {
+	u8 drop_tslock_prob_on_low_snr;
+	u8 recover_tslock_prob_on_good_snr;
+};
+
+/**
+ * struct vidtv_demod_state - The demodulator state
+ * @frontend: The frontend structure allocated by the demod.
+ * @config: The config used to init the demod.
+ * @poll_snr: The task responsible for periodically checking the simulated
+ * signal quality, eventually dropping or reacquiring the TS lock.
+ * @status: the demod status.
+ * @cold_start: Whether the demod has not been init yet.
+ * @poll_snr_thread_running: Whether the task responsible for periodically
+ * checking the simulated signal quality is running.
+ * @poll_snr_thread_restart: Whether we should restart the poll_snr task.
+ */
+struct vidtv_demod_state {
+	struct dvb_frontend frontend;
+	struct vidtv_demod_config config;
+	struct delayed_work poll_snr;
+	enum fe_status status;
+	u16 tuner_cnr;
+	bool cold_start;
+	bool poll_snr_thread_running;
+	bool poll_snr_thread_restart;
+};
+#endif // VIDTV_DEMOD_H