media: m88ds3103: serialize reset messages in m88ds3103_set_frontend

Ref: https://bugzilla.kernel.org/show_bug.cgi?id=199323

Users are experiencing problems with the DVBSky S960/S960C USB devices
since the following commit:

9d659ae1: ("locking/mutex: Add lock handoff to avoid starvation")

The device malfunctions after running for an indeterminable period of
time, and the problem can only be cleared by rebooting the machine.

It is possible to encourage the problem to surface by blocking the
signal to the LNB.

Further debugging revealed the cause of the problem.

In the following capture:
- thread #1325 is running m88ds3103_set_frontend
- thread #42 is running ts2020_stat_work

a> [1325] usb 1-1: dvb_usb_v2_generic_io: >>> 08 68 02 07 80
   [1325] usb 1-1: dvb_usb_v2_generic_io: <<< 08
   [42] usb 1-1: dvb_usb_v2_generic_io: >>> 09 01 01 68 3f
   [42] usb 1-1: dvb_usb_v2_generic_io: <<< 08 ff
   [42] usb 1-1: dvb_usb_v2_generic_io: >>> 08 68 02 03 11
   [42] usb 1-1: dvb_usb_v2_generic_io: <<< 07
   [42] usb 1-1: dvb_usb_v2_generic_io: >>> 09 01 01 60 3d
   [42] usb 1-1: dvb_usb_v2_generic_io: <<< 07 ff
b> [1325] usb 1-1: dvb_usb_v2_generic_io: >>> 08 68 02 07 00
   [1325] usb 1-1: dvb_usb_v2_generic_io: <<< 07
   [42] usb 1-1: dvb_usb_v2_generic_io: >>> 08 68 02 03 11
   [42] usb 1-1: dvb_usb_v2_generic_io: <<< 07
   [42] usb 1-1: dvb_usb_v2_generic_io: >>> 09 01 01 60 21
   [42] usb 1-1: dvb_usb_v2_generic_io: <<< 07 ff
   [42] usb 1-1: dvb_usb_v2_generic_io: >>> 08 68 02 03 11
   [42] usb 1-1: dvb_usb_v2_generic_io: <<< 07
   [42] usb 1-1: dvb_usb_v2_generic_io: >>> 09 01 01 60 66
   [42] usb 1-1: dvb_usb_v2_generic_io: <<< 07 ff
   [1325] usb 1-1: dvb_usb_v2_generic_io: >>> 08 68 02 03 11
   [1325] usb 1-1: dvb_usb_v2_generic_io: <<< 07
   [1325] usb 1-1: dvb_usb_v2_generic_io: >>> 08 60 02 10 0b
   [1325] usb 1-1: dvb_usb_v2_generic_io: <<< 07

Two i2c messages are sent to perform a reset in m88ds3103_set_frontend:

  a. 0x07, 0x80
  b. 0x07, 0x00

However, as shown in the capture, the regmap mutex is being handed over
to another thread (ts2020_stat_work) in between these two messages.

>From here, the device responds to every i2c message with an 07 message,
and will only return to normal operation following a power cycle.

Use regmap_multi_reg_write to group the two reset messages, ensuring
both are processed before the regmap mutex is unlocked.
Signed-off-by: James Hutchinson <jahutchinson99@googlemail.com>
Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/dvb-frontends/m88ds3103.c

@@ -309,6 +309,9 @@ static int m88ds3103_set_frontend(struct dvb_frontend *fe)
 	u16 u16tmp;
 	u32 tuner_frequency_khz, target_mclk;
 	s32 s32tmp;
+	static const struct reg_sequence reset_buf[] = {
+		{0x07, 0x80}, {0x07, 0x00}
+	};
 
 	dev_dbg(&client->dev,
 		"delivery_system=%d modulation=%d frequency=%u symbol_rate=%d inversion=%d pilot=%d rolloff=%d\n",
@@ -321,11 +324,7 @@ static int m88ds3103_set_frontend(struct dvb_frontend *fe)
 	}
 
 	/* reset */
-	ret = regmap_write(dev->regmap, 0x07, 0x80);
-	if (ret)
-		goto err;
-
-	ret = regmap_write(dev->regmap, 0x07, 0x00);
+	ret = regmap_multi_reg_write(dev->regmap, reset_buf, 2);
 	if (ret)
 		goto err;