firmware: arm_scmi: Add configurable polling mode for transports

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 evaluate the transport-specific .poll_done() function, if
available, to determine if and when a request was fully 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.no_completion_irq: to switch a single channel dynamically
   to polling mode if, at runtime, is determined that no completion
   interrupt was available for such channel.

Link: https://lore.kernel.org/r/20211220195646.44498-2-cristian.marussi@arm.com
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/firmware/arm_scmi/common.h

@@ -339,11 +339,16 @@ 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
+ * @no_completion_irq: Flag to indicate that this channel has no completion
+ *		       interrupt mechanism for synchronous commands.
+ *		       This can be dynamically set by transports at run-time
+ *		       inside their provided .chan_setup().
  * @transport_info: Transport layer related information
  */
 struct scmi_chan_info {
 	struct device *dev;
 	struct scmi_handle *handle;
+	bool no_completion_irq;
 	void *transport_info;
 };
 
@@ -402,6 +407,8 @@ struct scmi_device *scmi_child_dev_find(struct device *parent,
  *	be pending simultaneously in the system. May be overridden by the
  *	get_max_msg op.
  * @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 {
 	int (*transport_init)(void);
@@ -410,6 +417,7 @@ struct scmi_desc {
 	int max_rx_timeout_ms;
 	int max_msg;
 	int max_msg_size;
+	const bool force_polling;
 };
 
 #ifdef CONFIG_ARM_SCMI_TRANSPORT_MAILBOX

drivers/firmware/arm_scmi/driver.c

@@ -609,6 +609,24 @@ static inline void scmi_clear_channel(struct scmi_info *info,
 		info->desc->ops->clear_channel(cinfo);
 }
 
+static inline bool is_polling_required(struct scmi_chan_info *cinfo,
+				       struct scmi_info *info)
+{
+	return cinfo->no_completion_irq || info->desc->force_polling;
+}
+
+static inline bool is_transport_polling_capable(struct scmi_info *info)
+{
+	return info->desc->ops->poll_done;
+}
+
+static inline bool is_polling_enabled(struct scmi_chan_info *cinfo,
+				      struct scmi_info *info)
+{
+	return is_polling_required(cinfo, info) &&
+		is_transport_polling_capable(info);
+}
+
 static void scmi_handle_notification(struct scmi_chan_info *cinfo,
 				     u32 msg_hdr, void *priv)
 {
@@ -817,6 +835,7 @@ static int do_xfer(const struct scmi_protocol_handle *ph,
 	struct device *dev = info->dev;
 	struct scmi_chan_info *cinfo;
 
+	/* Check for polling request on custom command xfers at first */
 	if (xfer->hdr.poll_completion && !info->desc->ops->poll_done) {
 		dev_warn_once(dev,
 			      "Polling mode is not supported by transport.\n");
@@ -827,6 +846,10 @@ static int do_xfer(const struct scmi_protocol_handle *ph,
 	if (unlikely(!cinfo))
 		return -EINVAL;
 
+	/* True ONLY if also supported by transport. */
+	if (is_polling_enabled(cinfo, info))
+		xfer->hdr.poll_completion = true;
+
 	/*
 	 * Initialise protocol id now from protocol handle to avoid it being
 	 * overridden by mistake (or malice) by the protocol code mangling with
@@ -1527,6 +1550,16 @@ static int scmi_chan_setup(struct scmi_info *info, struct device *dev,
 	if (ret)
 		return ret;
 
+	if (tx && is_polling_required(cinfo, info)) {
+		if (is_transport_polling_capable(info))
+			dev_info(dev,
+				 "Enabled polling mode TX channel - prot_id:%d\n",
+				 prot_id);
+		else
+			dev_warn(dev,
+				 "Polling mode NOT supported by transport.\n");
+	}
+
 idr_alloc:
 	ret = idr_alloc(idr, cinfo, prot_id, prot_id + 1, GFP_KERNEL);
 	if (ret != prot_id) {