[PATCH 3.2 009/200] [media] dib8000: make 32 bits read atomic

[Ben Hutchings]

3.2.56-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Mauro Carvalho Chehab <m.chehab@xxxxxxxxxxx>

commit 5ac64ba12aca3bef18e61c866583155a3bbf81c4 upstream.

As the dvb-frontend kthread can be called anytime, it can race
with some get status ioctl. So, it seems better to avoid one to
race with the other while reading a 32 bits register.
I can't see any other reason for having a mutex there at I2C, except
to provide such kind of protection, as the I2C core already has a
mutex to protect I2C transfers.

Note: instead of this approach, it could eventually remove the dib8000
specific mutex for it, and either group the 4 ops into one xfer or
to manually control the I2C mutex. The main advantage of the current
approach is that the changes are smaller and more puntual.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@xxxxxxxxxxx>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@xxxxxxxxxxx>
Acked-by: Patrick Boettcher <pboettcher@xxxxxxxxxxxxxx>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/media/dvb/frontends/dib8000.c | 33 +++++++++++++++++++++++++--------
 1 file changed, 25 insertions(+), 8 deletions(-)

--- a/drivers/media/dvb/frontends/dib8000.c
+++ b/drivers/media/dvb/frontends/dib8000.c
@@ -114,15 +114,10 @@ static u16 dib8000_i2c_read16(struct i2c
 	return ret;
 }
 
-static u16 dib8000_read_word(struct dib8000_state *state, u16 reg)
+static u16 __dib8000_read_word(struct dib8000_state *state, u16 reg)
 {
 	u16 ret;
 
-	if (mutex_lock_interruptible(&state->i2c_buffer_lock) < 0) {
-		dprintk("could not acquire lock");
-		return 0;
-	}
-
 	state->i2c_write_buffer[0] = reg >> 8;
 	state->i2c_write_buffer[1] = reg & 0xff;
 
@@ -140,6 +135,21 @@ static u16 dib8000_read_word(struct dib8
 		dprintk("i2c read error on %d", reg);
 
 	ret = (state->i2c_read_buffer[0] << 8) | state->i2c_read_buffer[1];
+
+	return ret;
+}
+
+static u16 dib8000_read_word(struct dib8000_state *state, u16 reg)
+{
+	u16 ret;
+
+	if (mutex_lock_interruptible(&state->i2c_buffer_lock) < 0) {
+		dprintk("could not acquire lock");
+		return 0;
+	}
+
+	ret = __dib8000_read_word(state, reg);
+
 	mutex_unlock(&state->i2c_buffer_lock);
 
 	return ret;
@@ -149,8 +159,15 @@ static u32 dib8000_read32(struct dib8000
 {
 	u16 rw[2];
 
-	rw[0] = dib8000_read_word(state, reg + 0);
-	rw[1] = dib8000_read_word(state, reg + 1);
+	if (mutex_lock_interruptible(&state->i2c_buffer_lock) < 0) {
+		dprintk("could not acquire lock");
+		return 0;
+	}
+
+	rw[0] = __dib8000_read_word(state, reg + 0);
+	rw[1] = __dib8000_read_word(state, reg + 1);
+
+	mutex_unlock(&state->i2c_buffer_lock);
 
 	return ((rw[0] << 16) | (rw[1]));
 }

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