[PATCH v4 03/19] firmware: qcom: scm: Fix missing smp_load_acquire()

[Mukesh Ojha <mukesh.ojha@oss.qualcomm.com>]

__scm is published in qcom_scm_probe() with smp_store_release()
but qcom_scm_set_download_mode() reads it directly without
smp_load_acquire(), creating a potential ordering violation where a
CPU could observe a stale or partially initialised __scm pointer.

Use smp_load_acquire() to acquire __scm in the
qcom_scm_set_download_mode() function.

Signed-off-by: Mukesh Ojha <mukesh.ojha@oss.qualcomm.com>
---
 drivers/firmware/qcom/qcom_scm.c | 25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/drivers/firmware/qcom/qcom_scm.c b/drivers/firmware/qcom/qcom_scm.c
index 6b601a4b89db..b78da40a4d60 100644
--- a/drivers/firmware/qcom/qcom_scm.c
+++ b/drivers/firmware/qcom/qcom_scm.c
@@ -551,23 +551,23 @@ static int qcom_scm_io_rmw(phys_addr_t addr, unsigned int mask, unsigned int val
 	return qcom_scm_io_writel(addr, new);
 }
 
-static void qcom_scm_set_download_mode(u32 dload_mode)
+static void qcom_scm_set_download_mode(struct qcom_scm *scm, u32 dload_mode)
 {
 	int ret = 0;
 
-	if (__scm->dload_mode_addr) {
-		ret = qcom_scm_io_rmw(__scm->dload_mode_addr, QCOM_DLOAD_MASK,
+	if (scm->dload_mode_addr) {
+		ret = qcom_scm_io_rmw(scm->dload_mode_addr, QCOM_DLOAD_MASK,
 				      FIELD_PREP(QCOM_DLOAD_MASK, dload_mode));
-	} else if (__qcom_scm_is_call_available(__scm->dev, QCOM_SCM_SVC_BOOT,
+	} else if (__qcom_scm_is_call_available(scm->dev, QCOM_SCM_SVC_BOOT,
 						QCOM_SCM_BOOT_SET_DLOAD_MODE)) {
-		ret = __qcom_scm_set_dload_mode(__scm->dev, !!dload_mode);
+		ret = __qcom_scm_set_dload_mode(scm->dev, !!dload_mode);
 	} else if (dload_mode) {
-		dev_err(__scm->dev,
+		dev_err(scm->dev,
 			"No available mechanism for setting download mode\n");
 	}
 
 	if (ret)
-		dev_err(__scm->dev, "failed to set download mode: %d\n", ret);
+		dev_err(scm->dev, "failed to set download mode: %d\n", ret);
 }
 
 /**
@@ -2705,6 +2705,7 @@ static int get_download_mode(char *buffer, const struct kernel_param *kp)
 
 static int set_download_mode(const char *val, const struct kernel_param *kp)
 {
+	struct qcom_scm *scm;
 	bool tmp;
 	int ret;
 
@@ -2720,8 +2721,10 @@ static int set_download_mode(const char *val, const struct kernel_param *kp)
 	}
 
 	download_mode = ret;
-	if (__scm)
-		qcom_scm_set_download_mode(download_mode);
+	/* Pairs with smp_store_release() in qcom_scm_probe(). */
+	scm = smp_load_acquire(&__scm);
+	if (scm)
+		qcom_scm_set_download_mode(scm, download_mode);
 
 	return 0;
 }
@@ -2842,7 +2845,7 @@ static int qcom_scm_probe(struct platform_device *pdev)
 	 * will cause the boot stages to enter download mode, unless
 	 * disabled below by a clean shutdown/reboot.
 	 */
-	qcom_scm_set_download_mode(download_mode);
+	qcom_scm_set_download_mode(scm, download_mode);
 
 	/*
 	 * Disable SDI if indicated by DT that it is enabled by default.
@@ -2875,7 +2878,7 @@ static int qcom_scm_probe(struct platform_device *pdev)
 static void qcom_scm_shutdown(struct platform_device *pdev)
 {
 	/* Clean shutdown, disable download mode to allow normal restart */
-	qcom_scm_set_download_mode(QCOM_DLOAD_NODUMP);
+	qcom_scm_set_download_mode(__scm, QCOM_DLOAD_NODUMP);
 }
 
 static const struct of_device_id qcom_scm_dt_match[] = {
-- 
2.53.0