firmware: arm_scmi: Add atomic mode support to smc transport

Add a Kernel configuration option to enable SCMI SMC transport atomic
mode operation for selected SCMI transactions and leave it as default
disabled.

Substitute mutex usages with busy-waiting and declare smc transport as
.atomic_enabled if such Kernel configuration option is enabled.

Link: https://lore.kernel.org/r/20211220195646.44498-8-cristian.marussi@arm.com
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

diff --git a/drivers/firmware/arm_scmi/Kconfig b/drivers/firmware/arm_scmi/Kconfig
index 638ecec..d429326 100644 (file)
--- a/drivers/firmware/arm_scmi/Kconfig
+++ b/drivers/firmware/arm_scmi/Kconfig
@@ -78,6 +78,20 @@ config ARM_SCMI_TRANSPORT_SMC
          If you want the ARM SCMI PROTOCOL stack to include support for a
          transport based on SMC, answer Y.
 
+config ARM_SCMI_TRANSPORT_SMC_ATOMIC_ENABLE
+       bool "Enable atomic mode support for SCMI SMC transport"
+       depends on ARM_SCMI_TRANSPORT_SMC
+       help
+         Enable support of atomic operation for SCMI SMC based transport.
+
+         If you want the SCMI SMC based transport to operate in atomic
+         mode, avoiding any kind of sleeping behaviour for selected
+         transactions on the TX path, answer Y.
+         Enabling atomic mode operations allows any SCMI driver using this
+         transport to optionally ask for atomic SCMI transactions and operate
+         in atomic context too, at the price of using a number of busy-waiting
+         primitives all over instead. If unsure say N.
+
 config ARM_SCMI_TRANSPORT_VIRTIO
        bool "SCMI transport based on VirtIO"
        depends on VIRTIO=y || VIRTIO=ARM_SCMI_PROTOCOL

diff --git a/drivers/firmware/arm_scmi/smc.c b/drivers/firmware/arm_scmi/smc.c
index df0defd..6c7871a 100644 (file)
--- 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 +25,20 @@
  *
  * @cinfo: SCMI channel info
  * @shmem: Transmit/Receive shared memory area
- * @shmem_lock: Lock to protect access to Tx/Rx shared memory area
+ * @shmem_lock: Lock to protect access to Tx/Rx shared memory area.
+ *             Used when NOT operating in atomic mode.
+ * @inflight: Atomic flag to protect access to Tx/Rx shared memory area.
+ *           Used when operating in atomic mode.
  * @func_id: smc/hvc call function id
  */
 
 struct scmi_smc {
        struct scmi_chan_info *cinfo;
        struct scmi_shared_mem __iomem *shmem;
+       /* Protect access to shmem area */
        struct mutex shmem_lock;
+#define INFLIGHT_NONE  MSG_TOKEN_MAX
+       atomic_t inflight;
        u32 func_id;
 };
 
@@ -54,6 +62,41 @@ static bool smc_chan_available(struct device *dev, int idx)
        return true;
 }
 
+static inline void smc_channel_lock_init(struct scmi_smc *scmi_info)
+{
+       if (IS_ENABLED(CONFIG_ARM_SCMI_TRANSPORT_SMC_ATOMIC_ENABLE))
+               atomic_set(&scmi_info->inflight, INFLIGHT_NONE);
+       else
+               mutex_init(&scmi_info->shmem_lock);
+}
+
+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 inline void
+smc_channel_lock_acquire(struct scmi_smc *scmi_info,
+                        struct scmi_xfer *xfer __maybe_unused)
+{
+       if (IS_ENABLED(CONFIG_ARM_SCMI_TRANSPORT_SMC_ATOMIC_ENABLE))
+               spin_until_cond(smc_xfer_inflight(xfer, &scmi_info->inflight));
+       else
+               mutex_lock(&scmi_info->shmem_lock);
+}
+
+static inline void smc_channel_lock_release(struct scmi_smc *scmi_info)
+{
+       if (IS_ENABLED(CONFIG_ARM_SCMI_TRANSPORT_SMC_ATOMIC_ENABLE))
+               atomic_set(&scmi_info->inflight, INFLIGHT_NONE);
+       else
+               mutex_unlock(&scmi_info->shmem_lock);
+}
+
 static int smc_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
                          bool tx)
 {
@@ -114,7 +157,7 @@ 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);
+       smc_channel_lock_init(scmi_info);
        cinfo->transport_info = scmi_info;
 
        return 0;
@@ -140,10 +183,10 @@ 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);
+       smc_channel_lock_acquire(scmi_info, xfer);
 
        shmem_tx_prepare(scmi_info->shmem, xfer);
 
@@ -151,7 +194,7 @@ static int smc_send_message(struct scmi_chan_info *cinfo,
 
        /* Only SMCCC_RET_NOT_SUPPORTED is valid error code */
        if (res.a0) {
-               mutex_unlock(&scmi_info->shmem_lock);
+               smc_channel_lock_release(scmi_info);
                return -EOPNOTSUPP;
        }
 
@@ -170,7 +213,7 @@ 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);
+       smc_channel_lock_release(scmi_info);
 }
 
 static const struct scmi_transport_ops scmi_smc_ops = {
@@ -196,4 +239,5 @@ const struct scmi_desc scmi_smc_desc = {
         * from the shared memory area.
         */
        .sync_cmds_completed_on_ret = true,
+       .atomic_enabled = IS_ENABLED(CONFIG_ARM_SCMI_TRANSPORT_SMC_ATOMIC_ENABLE),
 };