coresight: cti: Prevent negative values of enable count

Writing 0 to the enable control repeatedly results in a negative value
for enable_req_count. After this, writing 1 to the enable control
appears to not work until the count returns to positive.

Change it so that it's impossible for enable_req_count to be < 0.
Return an error to indicate that the disable request was invalid.

Fixes: 835d722 ("coresight: cti: Initial CoreSight CTI Driver")
Tested-by: Jinlong Mao <quic_jinlmao@quicinc.com>
Signed-off-by: James Clark <james.clark@arm.com>
Reviewed-by: Mike Leach <mike.leach@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20230110110736.2709917-2-james.clark@arm.com

drivers/hwtracing/coresight/coresight-cti-core.c

@@ -151,9 +151,16 @@ static int cti_disable_hw(struct cti_drvdata *drvdata)
 {
 	struct cti_config *config = &drvdata->config;
 	struct coresight_device *csdev = drvdata->csdev;
+	int ret = 0;
 
 	spin_lock(&drvdata->spinlock);
 
+	/* don't allow negative refcounts, return an error */
+	if (!atomic_read(&drvdata->config.enable_req_count)) {
+		ret = -EINVAL;
+		goto cti_not_disabled;
+	}
+
 	/* check refcount - disable on 0 */
 	if (atomic_dec_return(&drvdata->config.enable_req_count) > 0)
 		goto cti_not_disabled;
@@ -171,12 +178,12 @@ static int cti_disable_hw(struct cti_drvdata *drvdata)
 	coresight_disclaim_device_unlocked(csdev);
 	CS_LOCK(drvdata->base);
 	spin_unlock(&drvdata->spinlock);
-	return 0;
+	return ret;
 
 	/* not disabled this call */
 cti_not_disabled:
 	spin_unlock(&drvdata->spinlock);
-	return 0;
+	return ret;
 }
 
 void cti_write_single_reg(struct cti_drvdata *drvdata, int offset, u32 value)