media: dvb-frontends/stv0910: release lock on gate_ctrl() failure

Whenever write_reg() fails to open/close the demod's I2C gate, release the
lock to avoid deadlocking situations. If I2c gate open failed, there's no
need to hold a lock, and if close fails, the mutex_unlock() at the end of
the function is never reached, leaving the mutex_lock in locked state,
which in turn will cause potential for deadlocks. Thus, release the lock
on failure.

While we're touching gate_ctrl(), add some explanation about the need for
locking and the shared I2C bus/gate.

Cc: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

diff --git a/drivers/media/dvb-frontends/stv0910.c b/drivers/media/dvb-frontends/stv0910.c
index d1ae955..0d4a6a1 100644 (file)
--- a/drivers/media/dvb-frontends/stv0910.c
+++ b/drivers/media/dvb-frontends/stv0910.c
@@ -1221,17 +1221,32 @@ static int gate_ctrl(struct dvb_frontend *fe, int enable)
        struct stv *state = fe->demodulator_priv;
        u8 i2crpt = state->i2crpt & ~0x86;
 
-       if (enable)
-               mutex_lock(&state->base->i2c_lock);
+       /*
+        * mutex_lock note: Concurrent I2C gate bus accesses must be
+        * prevented (STV0910 = dual demod on a single IC with a single I2C
+        * gate/bus, and two tuners attached), similar to most (if not all)
+        * other I2C host interfaces/busses.
+        *
+        * enable=1 (open I2C gate) will grab the lock
+        * enable=0 (close I2C gate) releases the lock
+        */
 
-       if (enable)
+       if (enable) {
+               mutex_lock(&state->base->i2c_lock);
                i2crpt |= 0x80;
-       else
+       } else {
                i2crpt |= 0x02;
+       }
 
        if (write_reg(state, state->nr ? RSTV0910_P2_I2CRPT :
-                     RSTV0910_P1_I2CRPT, i2crpt) < 0)
+                     RSTV0910_P1_I2CRPT, i2crpt) < 0) {
+               /* don't hold the I2C bus lock on failure */
+               mutex_unlock(&state->base->i2c_lock);
+               dev_err(&state->base->i2c->dev,
+                       "%s() write_reg failure (enable=%d)\n",
+                       __func__, enable);
                return -EIO;
+       }
 
        state->i2crpt = i2crpt;