[PATCH/RFC] dmaengine: rcar-dmac: Use CAE/CAIR instead of error IRQ

[Magnus Damm <magnus.damm@gmail.com>]

From: Magnus Damm <damm+renesas@opensource.se>

While using SYS-DMAC together with the IPMMU it became evident
that the shared error interrupt hooked up by rcar-dmac.c never
got invoked but instead the per-channel CAE bit got set which
in turn may generate a per-channel interrupt if CAIE is set.

This patch removes the shared interrupt handler and instead
simply enables CAIE and makes sure CAE gets cleared by the
interrupt handler. Without enabling CAIE and clearing CAE the
DMA transfer blocks forever in case of error.

During normal operation it is hard to trigger error interrupts,
but I have managed to trigger the SYS-DMAC error by using local
IPMMU debug code that tracks active page table entries using
the AF bit. This debug code results in rather high latencies
which in turn makes the SYS-DMAC generate error interrupts.

Tested on r8a7795 Salvator-X. Not for upstream merge - needs
more discussion and testing on other SoCs.

Not-Yet-Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
---

 drivers/dma/sh/rcar-dmac.c |   60 ++------------------------------------------
 1 file changed, 3 insertions(+), 57 deletions(-)

--- 0001/drivers/dma/sh/rcar-dmac.c
+++ work/drivers/dma/sh/rcar-dmac.c	2016-01-14 18:16:23.040513000 +0900
@@ -808,23 +808,6 @@ static void rcar_dmac_stop(struct rcar_d
 	rcar_dmac_write(dmac, RCAR_DMAOR, 0);
 }
 
-static void rcar_dmac_abort(struct rcar_dmac *dmac)
-{
-	unsigned int i;
-
-	/* Stop all channels. */
-	for (i = 0; i < dmac->n_channels; ++i) {
-		struct rcar_dmac_chan *chan = &dmac->channels[i];
-
-		/* Stop and reinitialize the channel. */
-		spin_lock(&chan->lock);
-		rcar_dmac_chan_halt(chan);
-		spin_unlock(&chan->lock);
-
-		rcar_dmac_chan_reinit(chan);
-	}
-}
-
 /* -----------------------------------------------------------------------------
  * Descriptors preparation
  */
@@ -864,7 +847,7 @@ static void rcar_dmac_chan_configure_des
 	}
 
 	desc->xfer_shift = ilog2(xfer_size);
-	desc->chcr = chcr | chcr_ts[desc->xfer_shift];
+	desc->chcr = chcr | chcr_ts[desc->xfer_shift] | RCAR_DMACHCR_CAIE;
 }
 
 /*
@@ -1427,6 +1410,8 @@ static irqreturn_t rcar_dmac_isr_channel
 	spin_lock(&chan->lock);
 
 	chcr = rcar_dmac_chan_read(chan, RCAR_DMACHCR);
+	if (chcr & RCAR_DMACHCR_CAE)
+		mask |= RCAR_DMACHCR_CAE;
 	if (chcr & RCAR_DMACHCR_TE)
 		mask |= RCAR_DMACHCR_DE;
 	rcar_dmac_chan_write(chan, RCAR_DMACHCR, chcr & ~mask);
@@ -1497,24 +1482,6 @@ static irqreturn_t rcar_dmac_isr_channel
 	return IRQ_HANDLED;
 }
 
-static irqreturn_t rcar_dmac_isr_error(int irq, void *data)
-{
-	struct rcar_dmac *dmac = data;
-
-	if (!(rcar_dmac_read(dmac, RCAR_DMAOR) & RCAR_DMAOR_AE))
-		return IRQ_NONE;
-
-	/*
-	 * An unrecoverable error occurred on an unknown channel. Halt the DMAC,
-	 * abort transfers on all channels, and reinitialize the DMAC.
-	 */
-	rcar_dmac_stop(dmac);
-	rcar_dmac_abort(dmac);
-	rcar_dmac_init(dmac);
-
-	return IRQ_HANDLED;
-}
-
 /* -----------------------------------------------------------------------------
  * OF xlate and channel filter
  */
@@ -1693,8 +1660,6 @@ static int rcar_dmac_probe(struct platfo
 	struct rcar_dmac *dmac;
 	struct resource *mem;
 	unsigned int i;
-	char *irqname;
-	int irq;
 	int ret;
 
 	dmac = devm_kzalloc(&pdev->dev, sizeof(*dmac), GFP_KERNEL);
@@ -1732,25 +1697,6 @@ static int rcar_dmac_probe(struct platfo
 	if (IS_ERR(dmac->iomem))
 		return PTR_ERR(dmac->iomem);
 
-	irq = platform_get_irq_byname(pdev, "error");
-	if (irq < 0) {
-		dev_err(&pdev->dev, "no error IRQ specified\n");
-		return -ENODEV;
-	}
-
-	irqname = devm_kasprintf(dmac->dev, GFP_KERNEL, "%s:error",
-				 dev_name(dmac->dev));
-	if (!irqname)
-		return -ENOMEM;
-
-	ret = devm_request_irq(&pdev->dev, irq, rcar_dmac_isr_error, 0,
-			       irqname, dmac);
-	if (ret) {
-		dev_err(&pdev->dev, "failed to request IRQ %u (%d)\n",
-			irq, ret);
-		return ret;
-	}
-
 	/* Enable runtime PM and initialize the device. */
 	pm_runtime_enable(&pdev->dev);
 	ret = pm_runtime_get_sync(&pdev->dev);