[PATCH v2] ARM: edma: fix residue race for cyclic

[John Ogness]

When retrieving the residue value for cyclic transfers, the
SRC/DST fields of the active PaRAM are read. However, the AM335x
Technical Reference Manual states:

  11.3.3.6 Parameter Set Updates

  After the TR is read from the PaRAM (and is in the process
  of being submitted to the EDMA3TC), the following fields are
  updated as needed: ... SRC DST

This means SRC/DST is incremented even though the DMA transfer
may not have started yet or is in progress. Thus if the reader
of the residue accesses the DMA buffer too quickly, the CPU is
misinformed about the data that has been successfully processed.

The CCSTAT.ACTV register is a boolean that is set if any TR is
being processed by either the EMDA3CC or EDMA3TC. By polling
this register it is possible to ensure that the residue value
returned is valid for immediate processing. However, since the
DMA engine may be active, polling may never hit a moment where
no TR is being processed. To handle this, the SRC/DST is also
polled to see if it changes. And as a last resort, a max loop
count for the busy waiting exists to avoid an infinite loop.

Signed-off-by: John Ogness <john.ogness at linutronix.de>
---
 v1-v2 changes
 . rebased for next-20151016
 . added multiple exit conditions for busy wait loop

 drivers/dma/edma.c |   40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)
diff --git a/drivers/dma/edma.c b/drivers/dma/edma.c
index 7eefbf1..8d3b3ac 100644
--- a/drivers/dma/edma.c
+++ b/drivers/dma/edma.c
@@ -24,6 +24,8 @@
 #include <linux/platform_device.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
+#include <linux/ratelimit.h>
+#include <linux/printk.h>
 #include <linux/of.h>
 #include <linux/of_dma.h>
 #include <linux/of_irq.h>
@@ -1785,11 +1787,24 @@ static void edma_issue_pending(struct dma_chan *chan)
 	spin_unlock_irqrestore(&echan->vchan.lock, flags);
 }
 
+#define EDMA_CCSTAT_ACTV (1 << 4)
+
+/*
+ * This limit exists to avoid a possible infinite loop when waiting
+ * for confirmation that a particular transfer is completed. However,
+ * large bursts to/from slow devices might actually require many
+ * loops (in which case busy waiting is bad anyway). On an AM335x
+ * transfering 48 bytes from the UART RX-FIFO, as many as 55 loops
+ * have been seen.
+ */
+#define EDMA_MAX_TR_WAIT_LOOPS 10000
+
 static u32 edma_residue(struct edma_desc *edesc)
 {
 	bool dst = edesc->direction == DMA_DEV_TO_MEM;
 	struct edma_pset *pset = edesc->pset;
 	dma_addr_t done, pos;
+	int loop_count = 0;
 	int i;
 
 	/*
@@ -1799,6 +1814,31 @@ static u32 edma_residue(struct edma_desc *edesc)
 	pos = edma_get_position(edesc->echan->ecc, edesc->echan->slot[0], dst);
 
 	/*
+	 * "pos" may represent a transfer request that is still being
+	 * processed by the EDMACC or EDMATC. We will busy wait until
+	 * one of the situations occurs:
+	 *   1. no transfer requests are active
+	 *   2. a different transfer request is being processed
+	 *   3. we hit the loop limit
+	 */
+	while (edma_read(edesc->echan->ecc, EDMA_CCSTAT) & EDMA_CCSTAT_ACTV) {
+		/* check if a different transfer request is active */
+		if (edma_get_position(edesc->echan->ecc,
+				      edesc->echan->slot[0], dst) != pos) {
+			break;
+		}
+
+		loop_count++;
+		if (loop_count == EDMA_MAX_TR_WAIT_LOOPS) {
+			pr_debug_ratelimited("%s: possibly returning "
+					     "invalid residue\n", __func__);
+			break;
+		}
+
+		cpu_relax();
+	}
+
+	/*
 	 * Cyclic is simple. Just subtract pset[0].addr from pos.
 	 *
 	 * We never update edesc->residue in the cyclic case, so we
-- 
1.7.10.4