[RFC] ASoC: fsl-ssi: Missing DMA-Request workaround

[mpa@pengutronix.de (Markus Pargmann)]

There is a hardware issue when using capture and playback at the same
time. In the tested scenario, the capture stream was started slightly
before the playback stream. The first time the application starts after
a powerup, there is a high probability that the SSI unit does not send a
request to the SDMA unit. The playback does not work. I tested this on
i.MX53 with SSI unit 2 kernel 3.10.10.

Here is a register dump of the situation:
SDMA
0x63fb0000
00000000: c0040000 00000000 0000001e 00000000
00000010: 00000001 ffffffff 00000000 e03efa41
00000020: 00000000 00000000 00000000 00000000
00000030: 00000064 00000000 00000003 00000000
00000040: 00000000 00000000 00000000 00006000
00000050: 00000000 00000000 00000001 00004050
00000060: 01208000 00000000 00000000 00000000 /* 0x60: EVT_MIRROR showing all received DMA requests */
           ^ This should be a 3 instead. Bits 24 and 25 are the DMA requests for
             RX and TX. Bit 25 is not set, DMA request 25 is missing.

SSI
0x50014000
00000000: 00000000 00000000 0000005f 00000000
00000010: 00000457 000050c1 01f80f00 0000028d /* 0x14: ISR Interrupt status register */
                          ^
	     At 0x14 we can clearly see the last bit is set (0x1) which
	     is TFE0, Transmit FIFO 0 empty. So it should automatically
	     request data from the DMA engine, but it doesn't.

00000020: 0000028d 0004e000 00040000 000010dd
00000030: 00001f1f 00000000 00000000 00000000
00000040: 00000000 00000000 00000000 00000000
00000050: 00000000 00000000 ........
STATS (/sys/bus/platform/devices/50014000.ssi/statistics)
rfrc=0
tfrc=0
roe1=0
roe0=0
tue1=0
tue0=1094475

I could solve this issue by filling the transmit FIFO with data. After
that, the SDMA unit receives DMA request 25 again and audio playback
works.

This patch adds TUE0 interrupt handler which fills FIFO0 with arbitrary data.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
---

Hi,

does anyone have an idea why the SSI unit does not request DMA by its own at
the beginning? Or is it a hardware errata that is not listed yet?

Regards,

Markus Pargmann


 sound/soc/fsl/fsl_ssi.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/sound/soc/fsl/fsl_ssi.c b/sound/soc/fsl/fsl_ssi.c
index 6b81d0c..8faf2b5 100644
--- a/sound/soc/fsl/fsl_ssi.c
+++ b/sound/soc/fsl/fsl_ssi.c
@@ -269,9 +269,14 @@ static irqreturn_t fsl_ssi_isr(int irq, void *dev_id)
 	}
 
 	if (sisr & CCSR_SSI_SISR_TUE0) {
+		int i;
+
 		ssi_private->stats.tue0++;
 		sisr2 |= CCSR_SSI_SISR_TUE0;
 		ret = IRQ_HANDLED;
+
+		for (i = 0; i != ssi_private->fifo_depth; ++i)
+			write_ssi(0x0, &ssi->stx0);
 	}
 
 	if (sisr & CCSR_SSI_SISR_TFS) {
-- 
1.8.4.2