[PATCH 2/2] dmaengine: bcm2835: Avoid splitting periods into very small chunks

[Matthias Reichl]

The current cyclic DMA period splitting implementation can generate
very small chunks at the end of each period. For example a 65536 byte
period will be split into a 65532 byte chunk and a 4 byte chunk on
the "lite" DMA channels.

This increases pressure on the RAM controller as the DMA controller
needs to fetch two control blocks from RAM in quick succession and
could potentially cause latency issues if the RAM is tied up by other
devices.

We can easily avoid these situations by distributing the remaining
length evenly between the last-but-one and the last chunk, making
sure that split chunks will be at least half the maximum length the
DMA controller can handle.

This patch checks if the last chunk would be less than half of
the maximum DMA length and if yes distributes the max len+4...max_len*1.5
bytes evenly between the last 2 chunks. This results in chunk sizes
between max_len/2 and max_len*0.75 bytes.

Signed-off-by: Matthias Reichl <hias@xxxxxxxxx>
Signed-off-by: Martin Sperl <kernel@xxxxxxxxxxxxxxxx>
Tested-by: Clive Messer <clive.messer@xxxxxxxxxxxxxxxxxxxxxx>
---
 drivers/dma/bcm2835-dma.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/dma/bcm2835-dma.c b/drivers/dma/bcm2835-dma.c
index 344bcf92..36b998d 100644
--- a/drivers/dma/bcm2835-dma.c
+++ b/drivers/dma/bcm2835-dma.c
@@ -252,6 +252,20 @@ static void bcm2835_dma_create_cb_set_length(
 
 	/* have we filled in period_length yet? */
 	if (*total_len + control_block->length < period_len) {
+		/*
+		 * If the next control block is the last in the period
+		 * and it's length would be less than half of max_len
+		 * change it so that both control blocks are (almost)
+		 * equally long. This avoids generating very short
+		 * control blocks (worst case would be 4 bytes) which
+		 * might be problematic. We also have to make sure the
+		 * new length is a multiple of 4 bytes.
+		 */
+		if (*total_len + control_block->length + max_len / 2 >
+		    period_len) {
+			control_block->length =
+				DIV_ROUND_UP(period_len - *total_len, 8) * 4;
+		}
 		/* update number of bytes in this period so far */
 		*total_len += control_block->length;
 		return;
-- 
2.1.4