counter: 104-quad-8: Fix race condition between FLAG and CNTR reads

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;
hard-coded maximum count values are replaced by a LS7267_CNTR_MAX define
for consistency and clarity.

Fixes: 28e5d3b ("iio: 104-quad-8: Add IIO support for the ACCES 104-QUAD-8")
Cc: <stable@vger.kernel.org> # 6.1.x
Cc: <stable@vger.kernel.org> # 6.2.x
Link: https://lore.kernel.org/r/20230312231554.134858-1-william.gray@linaro.org/
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>

Author: William Breathitt Gray

drivers/counter/104-quad-8.c

@@ -97,10 +97,6 @@ struct quad8 {
 	struct quad8_reg __iomem *reg;
 };
 
-/* 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 */
@@ -133,6 +129,9 @@ struct quad8 {
 #define QUAD8_CMR_QUADRATURE_X2 0x10
 #define QUAD8_CMR_QUADRATURE_X4 0x18
 
+/* Each Counter is 24 bits wide */
+#define LS7267_CNTR_MAX GENMASK(23, 0)
+
 static int quad8_signal_read(struct counter_device *counter,
 			     struct counter_signal *signal,
 			     enum counter_signal_level *level)
@@ -156,18 +155,10 @@ static int quad8_count_read(struct counter_device *counter,
 {
 	struct quad8 *const priv = counter_priv(counter);
 	struct channel_reg __iomem *const chan = priv->reg->channel + count->id;
-	unsigned int flags;
-	unsigned int borrow;
-	unsigned int carry;
 	unsigned long irqflags;
 	int i;
 
-	flags = ioread8(&chan->control);
-	borrow = flags & QUAD8_FLAG_BT;
-	carry = !!(flags & QUAD8_FLAG_CT);
-
-	/* Borrow XOR Carry effectively doubles count range */
-	*val = (unsigned long)(borrow ^ carry) << 24;
+	*val = 0;
 
 	spin_lock_irqsave(&priv->lock, irqflags);
 
@@ -191,8 +182,7 @@ static int quad8_count_write(struct counter_device *counter,
 	unsigned long irqflags;
 	int i;
 
-	/* Only 24-bit values are supported */
-	if (val > 0xFFFFFF)
+	if (val > LS7267_CNTR_MAX)
 		return -ERANGE;
 
 	spin_lock_irqsave(&priv->lock, irqflags);
@@ -806,8 +796,7 @@ static int quad8_count_preset_write(struct counter_device *counter,
 	struct quad8 *const priv = counter_priv(counter);
 	unsigned long irqflags;
 
-	/* Only 24-bit values are supported */
-	if (preset > 0xFFFFFF)
+	if (preset > LS7267_CNTR_MAX)
 		return -ERANGE;
 
 	spin_lock_irqsave(&priv->lock, irqflags);
@@ -834,8 +823,7 @@ static int quad8_count_ceiling_read(struct counter_device *counter,
 		*ceiling = priv->preset[count->id];
 		break;
 	default:
-		/* By default 0x1FFFFFF (25 bits unsigned) is maximum count */
-		*ceiling = 0x1FFFFFF;
+		*ceiling = LS7267_CNTR_MAX;
 		break;
 	}
 
@@ -850,8 +838,7 @@ static int quad8_count_ceiling_write(struct counter_device *counter,
 	struct quad8 *const priv = counter_priv(counter);
 	unsigned long irqflags;
 
-	/* Only 24-bit values are supported */
-	if (ceiling > 0xFFFFFF)
+	if (ceiling > LS7267_CNTR_MAX)
 		return -ERANGE;
 
 	spin_lock_irqsave(&priv->lock, irqflags);