[RFC] ARM: edma: unconditionally ack the error interrupt

[bigeasy@linutronix.de (Sebastian Andrzej Siewior)]

With 8250-dma, 8250-omap and am335x I observe the following:

- start a RX transfer which will finish once the FIFO has enough data
- The TX side starts a large TX transfer, say 1244 bytes. It takes approx
  102ms for the transfer to complete
- cancel the RX transfer & start the RX transfer a few times
- the TX transfer completes. dma_irq_handler() notices this and
  schedules the completion callback
- dma_ccerr_handler() is invoked. It returns IRQ_NONE because all four
  checked registers return 0.
- the last irq handler is repeated a few times until the irq core shuts it
  down.

I see the above describes pattern also without dma_ccerr_handler()
beeing invoked. But if it is invoked, it always _after_ the dma interrupt
handler for the TX handler was handled. If I disable TX-DMA for the UART
then I don't see this dma_ccerr_handler() at all.

Testing longer I see two addition scenarios of dma_ccerr_handler():
- EDMA_EMR0 reports 0x04000000 (the channel used by TX-UART).
  edma_callback() reports "looks like slot is null". Looks harmless.
- EDMA_EMR0 reports the same values but later in the loop where a match
  search again EDMA_EMR0 reports 0 and so thing is done.

Since it looks like the EDMA_EMR0 is loosing its content before the
dma_ccerr_handler() is invoked, I suggest to unconditionally ack the
interrupt so the irq core does not shut it down.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 arch/arm/common/edma.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/arm/common/edma.c b/arch/arm/common/edma.c
index 88099175fc56..b31f3b7b3851 100644
--- a/arch/arm/common/edma.c
+++ b/arch/arm/common/edma.c
@@ -432,8 +432,10 @@ static irqreturn_t dma_ccerr_handler(int irq, void *data)
 	if ((edma_read_array(ctlr, EDMA_EMR, 0) == 0) &&
 	    (edma_read_array(ctlr, EDMA_EMR, 1) == 0) &&
 	    (edma_read(ctlr, EDMA_QEMR) == 0) &&
-	    (edma_read(ctlr, EDMA_CCERR) == 0))
+	    (edma_read(ctlr, EDMA_CCERR) == 0)) {
+		edma_write(ctlr, EDMA_EEVAL, 1);
 		return IRQ_NONE;
+	}
 
 	while (1) {
 		int j = -1;
-- 
2.1.0