firmware: arm_scmi: add support for polling based SCMI transfers

It would be useful to have options to perform some SCMI transfers
atomically by polling for the completion flag instead of interrupt
driven. The SCMI specification has option to disable the interrupt and
poll for the completion flag in the shared memory.

This patch adds support for polling based SCMI transfers using that
option. This might be used for uninterrupted/atomic DVFS operations
from the scheduler context.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/firmware/arm_scmi/driver.c

@@ -18,10 +18,12 @@
 #include <linux/export.h>
 #include <linux/io.h>
 #include <linux/kernel.h>
+#include <linux/ktime.h>
 #include <linux/mailbox_client.h>
 #include <linux/module.h>
 #include <linux/of_address.h>
 #include <linux/of_device.h>
+#include <linux/processor.h>
 #include <linux/semaphore.h>
 #include <linux/slab.h>
 
@@ -335,6 +337,30 @@ void scmi_one_xfer_put(const struct scmi_handle *handle, struct scmi_xfer *xfer)
 	spin_unlock_irqrestore(&minfo->xfer_lock, flags);
 }
 
+static bool
+scmi_xfer_poll_done(const struct scmi_info *info, struct scmi_xfer *xfer)
+{
+	struct scmi_shared_mem *mem = info->tx_payload;
+	u16 xfer_id = MSG_XTRACT_TOKEN(le32_to_cpu(mem->msg_header));
+
+	if (xfer->hdr.seq != xfer_id)
+		return false;
+
+	return le32_to_cpu(mem->channel_status) &
+		(SCMI_SHMEM_CHAN_STAT_CHANNEL_ERROR |
+		SCMI_SHMEM_CHAN_STAT_CHANNEL_FREE);
+}
+
+#define SCMI_MAX_POLL_TO_NS	(100 * NSEC_PER_USEC)
+
+static bool scmi_xfer_done_no_timeout(const struct scmi_info *info,
+				      struct scmi_xfer *xfer, ktime_t stop)
+{
+	ktime_t __cur = ktime_get();
+
+	return scmi_xfer_poll_done(info, xfer) || ktime_after(__cur, stop);
+}
+
 /**
  * scmi_do_xfer() - Do one transfer
  *
@@ -361,15 +387,28 @@ int scmi_do_xfer(const struct scmi_handle *handle, struct scmi_xfer *xfer)
 	/* mbox_send_message returns non-negative value on success, so reset */
 	ret = 0;
 
-	/* And we wait for the response. */
-	timeout = msecs_to_jiffies(info->desc->max_rx_timeout_ms);
-	if (!wait_for_completion_timeout(&xfer->done, timeout)) {
-		dev_err(dev, "mbox timed out in resp(caller: %pS)\n",
-			(void *)_RET_IP_);
-		ret = -ETIMEDOUT;
-	} else if (xfer->hdr.status) {
-		ret = scmi_to_linux_errno(xfer->hdr.status);
+	if (xfer->hdr.poll_completion) {
+		ktime_t stop = ktime_add_ns(ktime_get(), SCMI_MAX_POLL_TO_NS);
+
+		spin_until_cond(scmi_xfer_done_no_timeout(info, xfer, stop));
+
+		if (ktime_before(ktime_get(), stop))
+			scmi_fetch_response(xfer, info->tx_payload);
+		else
+			ret = -ETIMEDOUT;
+	} else {
+		/* And we wait for the response. */
+		timeout = msecs_to_jiffies(info->desc->max_rx_timeout_ms);
+		if (!wait_for_completion_timeout(&xfer->done, timeout)) {
+			dev_err(dev, "mbox timed out in resp(caller: %pS)\n",
+				(void *)_RET_IP_);
+			ret = -ETIMEDOUT;
+		}
 	}
+
+	if (!ret && xfer->hdr.status)
+		ret = scmi_to_linux_errno(xfer->hdr.status);
+
 	/*
 	 * NOTE: we might prefer not to need the mailbox ticker to manage the
 	 * transfer queueing since the protocol layer queues things by itself.