[PATCH RESEND 06/10] via-cuda: Avoid TREQ race condition

[Finn Thain]

When a read transaction completes, one of several things will happen:
a new transfer is started by the driver, a new transfer request
is raised by the Cuda (i.e. TREQ asserted), or both happen at once.

When both happen at once, there is a race condition between the TREQ test
in the read_done state and the same test in cuda_start(). Moreover, the
former test uses a stale TREQ value.

Theoretically, this can result in the undesirable outcome that the
interrupt handler completes with the state machine 'idle' when it should
instead start the next transaction.

Avoid this race by calling cuda_start() first and then confirming that it
succeeded. If not, test the current TREQ value before entering the
'reading' state.

Tested-by: Stan Johnson <userm57 at yahoo.com>
Signed-off-by: Finn Thain <fthain at telegraphics.com.au>
---
 drivers/macintosh/via-cuda.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/macintosh/via-cuda.c b/drivers/macintosh/via-cuda.c
index ae3da6b..3212695 100644
--- a/drivers/macintosh/via-cuda.c
+++ b/drivers/macintosh/via-cuda.c
@@ -605,12 +605,11 @@ cuda_interrupt(int irq, void *arg)
 	    memcpy(ibuf, cuda_rbuf, ibuf_len);
 	}
 	reply_ptr = cuda_rbuf;
-	if (TREQ_asserted(status)) {
+	cuda_state = idle;
+	cuda_start();
+	if (cuda_state == idle && TREQ_asserted(in_8(&via[B]))) {
 	    assert_TIP();
 	    cuda_state = reading;
-	} else {
-	    cuda_state = idle;
-	    cuda_start();
 	}
 	break;
 
-- 
2.10.2