[PATCH] soundwire: cadence: Clear message complete before signaling waiting thread

[Richard Fitzgerald <rf@opensource.cirrus.com>]

Clear the CDNS_MCP_INT_RX_WL interrupt before signaling completion.

This is to prevent the potential race where:
- The main thread is scheduled immediately the completion is signaled,
   and starts a new message
- The RX_WL IRQ for this new message happens before sdw_cdns_irq() has
  been re-scheduled.
- When sdw_cdns_irq() is re-scheduled it clears the new RX_WL interrupt.

MAIN THREAD                        |  IRQ THREAD
                                   |
  _cdns_xfer_msg()                 |
  {                                |
     write data to FIFO            |
     wait_for_completion_timeout() |
     <BLOCKED>                     |                       <---- RX_WL IRQ
                                   | sdw_cdns_irq()
                                   | {
                                   |    signal completion
                          <== RESCHEDULE <==
  Handle message completion        |
  }                                |
                                   |
Start new message                  |
  _cdns_xfer_msg()                 |
  {                                |
     write data to FIFO            |
     wait_for_completion_timeout() |
     <BLOCKED>                     |                       <---- RX_WL IRQ
                          ==> RESCHEDULE ==>
                                   |    // New RX_WL IRQ is cleared before
                                   |    // it has been handled.
                                   |    clear CDNS_MCP_INTSTAT

                                   |    return IRQ_HANDLED;
                                   | }

Before this change, this error message was sometimes seen on kernels
that have large amounts of debugging enabled:

   SCP Msg trf timed out

This error indicates that the completion has not been signalled after
500ms.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Fixes: 956baa1992f9 ("soundwire: cdns: Add sdw_master_ops and IO transfer support")
Reported-by: Norman Bintang <normanbt@google.com>
Closes: https://issuetracker.google.com/issues/477099834
---
 drivers/soundwire/cadence_master.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/soundwire/cadence_master.c b/drivers/soundwire/cadence_master.c
index f245c3ffb9e9..b8b62735c893 100644
--- a/drivers/soundwire/cadence_master.c
+++ b/drivers/soundwire/cadence_master.c
@@ -933,6 +933,14 @@ irqreturn_t sdw_cdns_irq(int irq, void *dev_id)
 
 		cdns_read_response(cdns);
 
+		/*
+		 * Clear interrupt before signalling the completion to avoid
+		 * a race between this thread and the main thread starting
+		 * another TX.
+		 */
+		cdns_writel(cdns, CDNS_MCP_INTSTAT, CDNS_MCP_INT_RX_WL);
+		int_status &= ~CDNS_MCP_INT_RX_WL;
+
 		if (defer && defer->msg) {
 			cdns_fill_msg_resp(cdns, defer->msg,
 					   defer->length, 0);
-- 
2.47.3