[PATCH 2/3] platform/x86: intel_scu_ipc: Check status upon timeout in ipc_wait_for_interrupt()

[Stephen Boyd <swboyd@chromium.org>]

It's possible for the completion in ipc_wait_for_interrupt() to timeout,
simply because the interrupt was delayed in being processed. A timeout
in itself is not an error. This driver should check the status register
upon a timeout to ensure that scheduling or interrupt processing delays
don't affect the outcome of the IPC return value.

 CPU0                                                   SCU
 ----                                                   ---
 ipc_wait_for_interrupt()
  wait_for_completion_timeout(&scu->cmd_complete)
  [TIMEOUT]                                             status[IPC_BUSY]=0

Fix this problem by reading the status bit in all cases, regardless of
the timeout. If the completion times out, we'll assume the problem was
that the IPC_BUSY bit was still set, but if the status bit is cleared in
the meantime we know that we hit some scheduling delay and we should
just check the error bit.

Cc: Prashant Malani <pmalani@chromium.org>
Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Fixes: ed12f295bfd5 ("ipc: Added support for IPC interrupt mode")
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
---
 drivers/platform/x86/intel_scu_ipc.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/drivers/platform/x86/intel_scu_ipc.c b/drivers/platform/x86/intel_scu_ipc.c
index 5a37becc65aa..2a21153e3bf3 100644
--- a/drivers/platform/x86/intel_scu_ipc.c
+++ b/drivers/platform/x86/intel_scu_ipc.c
@@ -246,16 +246,19 @@ static inline int busy_loop(struct intel_scu_ipc_dev *scu)
 /* Wait till ipc ioc interrupt is received or timeout in 10 HZ */
 static inline int ipc_wait_for_interrupt(struct intel_scu_ipc_dev *scu)
 {
-	int status;
+	unsigned long time_left;
+	u8 status;
+	int err = 0;
 
-	if (!wait_for_completion_timeout(&scu->cmd_complete, IPC_TIMEOUT))
-		return -ETIMEDOUT;
+	time_left = wait_for_completion_timeout(&scu->cmd_complete, IPC_TIMEOUT);
+	if (!time_left)
+		err = -ETIMEDOUT;
 
 	status = ipc_read_status(scu);
-	if (status & IPC_STATUS_ERR)
-		return -EIO;
+	if (!(status & IPC_STATUS_BUSY))
+		err = (status & IPC_STATUS_ERR) ? -EIO : 0;
 
-	return 0;
+	return err;
 }
 
 static int intel_scu_ipc_check_status(struct intel_scu_ipc_dev *scu)
-- 
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