[RFC PATCH 03/10] firmware: arm_scmi: Add configurable polling mode for transports

[Cristian Marussi]

SCMI communications along TX channels can optionally be provided of a
completion interrupt; when such interrupt is not available, command
transactions should rely on polling, where the SCMI core takes care to
repeatedly evaluates the transport-specific .poll_done() function to
determine if a request was completed or timed out.

Such mechanism is already present and working on a single transfer base:
SCMI protocols can indeed enable hdr.poll_completion on specific commands
ahead of each transfer and cause that transaction to be handled with
polling.

Introduce a couple of flags to be able to enforce such polling behaviour
globally at will:

 - scmi_desc.force_polling: to statically switch the whole transport to
 			    polling mode.

 - scmi_chan_info.needs_polling: to switch a single channel dynamically to
 				 polling mode if at runtime is determined
				 that no completion interrupt was available
				 for such channel.

Signed-off-by: Cristian Marussi <cristian.marussi at arm.com>
---
 drivers/firmware/arm_scmi/common.h | 10 ++++++++++
 drivers/firmware/arm_scmi/driver.c | 17 +++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/drivers/firmware/arm_scmi/common.h b/drivers/firmware/arm_scmi/common.h
index 8685619d38f9..8f4e6ebfc0ef 100644
--- a/drivers/firmware/arm_scmi/common.h
+++ b/drivers/firmware/arm_scmi/common.h
@@ -272,11 +272,18 @@ void scmi_protocol_release(const struct scmi_handle *handle, u8 protocol_id);
  * @dev: Reference to device in the SCMI hierarchy corresponding to this
  *	 channel
  * @handle: Pointer to SCMI entity handle
+ * @needs_polling: Flag to indicate that this channel has no completion
+ *		   interrupt mechanism, so it needs SCMI core to poll, using
+ *		   .poll_done(), to determine when a command has completed.
+ *		   This can be dynamically set by transports at run-time inside
+ *		   their provided .chan_setup() when they determine no
+ *		   completion interrupt is available.
  * @transport_info: Transport layer related information
  */
 struct scmi_chan_info {
 	struct device *dev;
 	struct scmi_handle *handle;
+	bool needs_polling;
 	void *transport_info;
 };
 
@@ -322,12 +329,15 @@ struct scmi_device *scmi_child_dev_find(struct device *parent,
  * @max_msg: Maximum number of messages that can be pending
  *	simultaneously in the system
  * @max_msg_size: Maximum size of data per message that can be handled.
+ * @force_polling: Flag to force this whole transport to use SCMI core polling
+ *		   mechanism instead of completion interrupts even if available.
  */
 struct scmi_desc {
 	const struct scmi_transport_ops *ops;
 	int max_rx_timeout_ms;
 	int max_msg;
 	int max_msg_size;
+	bool force_polling;
 };
 
 extern const struct scmi_desc scmi_mailbox_desc;
diff --git a/drivers/firmware/arm_scmi/driver.c b/drivers/firmware/arm_scmi/driver.c
index 759ae4a23e74..c11ff49f6b62 100644
--- a/drivers/firmware/arm_scmi/driver.c
+++ b/drivers/firmware/arm_scmi/driver.c
@@ -319,6 +319,15 @@ static void scmi_handle_response(struct scmi_chan_info *cinfo,
 	}
 
 	xfer = &minfo->xfer_block[xfer_id];
+
+	/* Discard unexpected messages when polling is active. */
+	if (msg_type != MSG_TYPE_DELAYED_RESP && xfer->hdr.poll_completion) {
+		WARN_ON_ONCE(1);
+		dev_dbg(dev,
+			"Completion IRQ received but using polling. Ignore.\n");
+		return;
+	}
+
 	/*
 	 * Even if a response was indeed expected on this slot at this point,
 	 * a buggy platform could wrongly reply feeding us an unexpected
@@ -443,6 +452,9 @@ static int do_xfer(const struct scmi_protocol_handle *ph,
 	if (unlikely(!cinfo))
 		return -EINVAL;
 
+	if (info->desc->force_polling || cinfo->needs_polling)
+		xfer->hdr.poll_completion = true;
+
 	trace_scmi_xfer_begin(xfer->transfer_id, xfer->hdr.id,
 			      xfer->hdr.protocol_id, xfer->hdr.seq,
 			      xfer->hdr.poll_completion);
@@ -1102,6 +1114,11 @@ static int scmi_chan_setup(struct scmi_info *info, struct device *dev,
 	if (ret)
 		return ret;
 
+	if (tx && (cinfo->needs_polling || info->desc->force_polling))
+		dev_info(dev,
+			 "Enabled polling mode for TX channel - prot_id:%d\n",
+			 prot_id);
+
 idr_alloc:
 	ret = idr_alloc(idr, cinfo, prot_id, prot_id + 1, GFP_KERNEL);
 	if (ret != prot_id) {
-- 
2.17.1