[PATCH 5.4 v3 4/6] counter: 104-quad-8: Fix race condition between FLAG and CNTR reads

[William Breathitt Gray <william.gray@linaro.org>]

The Counter (CNTR) register is 24 bits wide, but we can have an
effective 25-bit count value by setting bit 24 to the XOR of the Borrow
flag and Carry flag. The flags can be read from the FLAG register, but a
race condition exists: the Borrow flag and Carry flag are instantaneous
and could change by the time the count value is read from the CNTR
register.

Since the race condition could result in an incorrect 25-bit count
value, remove support for 25-bit count values from this driver.

Fixes: 28e5d3bb0325 ("iio: 104-quad-8: Add IIO support for the ACCES 104-QUAD-8")
Cc: <stable@vger.kernel.org> # 5.4.x
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
---
 drivers/counter/104-quad-8.c | 20 +++-----------------
 1 file changed, 3 insertions(+), 17 deletions(-)

diff --git a/drivers/counter/104-quad-8.c b/drivers/counter/104-quad-8.c
index f261a57af..48de69f58 100644
--- a/drivers/counter/104-quad-8.c
+++ b/drivers/counter/104-quad-8.c
@@ -57,10 +57,6 @@ struct quad8_iio {
 
 #define QUAD8_REG_CHAN_OP 0x11
 #define QUAD8_REG_INDEX_INPUT_LEVELS 0x16
-/* Borrow Toggle flip-flop */
-#define QUAD8_FLAG_BT BIT(0)
-/* Carry Toggle flip-flop */
-#define QUAD8_FLAG_CT BIT(1)
 /* Error flag */
 #define QUAD8_FLAG_E BIT(4)
 /* Up/Down flag */
@@ -639,19 +635,9 @@ static int quad8_count_read(struct counter_device *counter,
 {
 	struct quad8_iio *const priv = counter->priv;
 	const int base_offset = priv->base + 2 * count->id;
-	unsigned int flags;
-	unsigned int borrow;
-	unsigned int carry;
-	unsigned long position;
+	unsigned long position = 0;
 	int i;
 
-	flags = inb(base_offset + 1);
-	borrow = flags & QUAD8_FLAG_BT;
-	carry = !!(flags & QUAD8_FLAG_CT);
-
-	/* Borrow XOR Carry effectively doubles count range */
-	position = (unsigned long)(borrow ^ carry) << 24;
-
 	mutex_lock(&priv->lock);
 
 	/* Reset Byte Pointer; transfer Counter to Output Latch */
@@ -1204,8 +1190,8 @@ static ssize_t quad8_count_ceiling_read(struct counter_device *counter,
 
 	mutex_unlock(&priv->lock);
 
-	/* By default 0x1FFFFFF (25 bits unsigned) is maximum count */
-	return sprintf(buf, "33554431\n");
+	/* By default 0xFFFFFF (24 bits unsigned) is maximum count */
+	return sprintf(buf, "16777215\n");
 }
 
 static ssize_t quad8_count_ceiling_write(struct counter_device *counter,

base-commit: 126ee8982bfcae6111f0fd157ee5cdd0ec7f5ca5
-- 
2.39.2