[RFC PATCH v2 8/8] firmware: arm_scmi: Make smc transport atomic

[Cristian Marussi <cristian.marussi@arm.com>]

Substitute mutex usages with busy-waiting and declare smc transport as
.atomic_capable.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
---
 drivers/firmware/arm_scmi/smc.c | 28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/drivers/firmware/arm_scmi/smc.c b/drivers/firmware/arm_scmi/smc.c
index cab1d0d50d8e..e33985a02068 100644
--- a/drivers/firmware/arm_scmi/smc.c
+++ b/drivers/firmware/arm_scmi/smc.c
@@ -7,6 +7,7 @@
  */
 
 #include <linux/arm-smccc.h>
+#include <linux/atomic.h>
 #include <linux/device.h>
 #include <linux/err.h>
 #include <linux/interrupt.h>
@@ -14,6 +15,7 @@
 #include <linux/of.h>
 #include <linux/of_address.h>
 #include <linux/of_irq.h>
+#include <linux/processor.h>
 #include <linux/slab.h>
 
 #include "common.h"
@@ -23,14 +25,15 @@
  *
  * @cinfo: SCMI channel info
  * @shmem: Transmit/Receive shared memory area
- * @shmem_lock: Lock to protect access to Tx/Rx shared memory area
+ * @inflight: Atomic flag to protect access to Tx/Rx shared memory area
  * @func_id: smc/hvc call function id
  */
 
 struct scmi_smc {
 	struct scmi_chan_info *cinfo;
 	struct scmi_shared_mem __iomem *shmem;
-	struct mutex shmem_lock;
+#define INFLIGHT_NONE	MSG_TOKEN_MAX
+	atomic_t inflight;
 	u32 func_id;
 };
 
@@ -114,7 +117,7 @@ static int smc_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
 
 	scmi_info->func_id = func_id;
 	scmi_info->cinfo = cinfo;
-	mutex_init(&scmi_info->shmem_lock);
+	atomic_set(&scmi_info->inflight, INFLIGHT_NONE);
 	cinfo->transport_info = scmi_info;
 
 	return 0;
@@ -133,6 +136,15 @@ static int smc_chan_free(int id, void *p, void *data)
 	return 0;
 }
 
+static bool smc_xfer_inflight(struct scmi_xfer *xfer, atomic_t *inflight)
+{
+	int ret;
+
+	ret = atomic_cmpxchg(inflight, INFLIGHT_NONE, xfer->hdr.seq);
+
+	return ret == INFLIGHT_NONE;
+}
+
 static int smc_send_message(struct scmi_chan_info *cinfo,
 			    struct scmi_xfer *xfer)
 {
@@ -140,16 +152,17 @@ static int smc_send_message(struct scmi_chan_info *cinfo,
 	struct arm_smccc_res res;
 
 	/*
-	 * Channel lock will be released only once response has been
+	 * Channel will be released only once response has been
 	 * surely fully retrieved, so after .mark_txdone()
 	 */
-	mutex_lock(&scmi_info->shmem_lock);
+	spin_until_cond(smc_xfer_inflight(xfer, &scmi_info->inflight));
+
 	shmem_tx_prepare(scmi_info->shmem, xfer);
 
 	arm_smccc_1_1_invoke(scmi_info->func_id, 0, 0, 0, 0, 0, 0, 0, &res);
 	/* Only SMCCC_RET_NOT_SUPPORTED is valid error code */
 	if (res.a0) {
-		mutex_unlock(&scmi_info->shmem_lock);
+		atomic_set(&scmi_info->inflight, INFLIGHT_NONE);
 		return -EOPNOTSUPP;
 	}
 
@@ -168,7 +181,7 @@ static void smc_mark_txdone(struct scmi_chan_info *cinfo, int ret)
 {
 	struct scmi_smc *scmi_info = cinfo->transport_info;
 
-	mutex_unlock(&scmi_info->shmem_lock);
+	atomic_set(&scmi_info->inflight, INFLIGHT_NONE);
 }
 
 static bool
@@ -194,4 +207,5 @@ const struct scmi_desc scmi_smc_desc = {
 	.max_rx_timeout_ms = 30,
 	.max_msg = 20,
 	.max_msg_size = 128,
+	.atomic_capable = true,
 };
-- 
2.17.1