linux-kernel - [PATCH] dmaengine: omap-dma: Enable packed accesses for cyclic transfers

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Date:	Mon, 14 Sep 2015 15:31:05 +0300
From:	Peter Ujfalusi <peter.ujfalusi@...com>
To:	<vinod.koul@...el.com>
CC:	<dmaengine@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-omap@...r.kernel.org>, <linux@....linux.org.uk>,
	<misael.lopez@...com>
Subject: [PATCH] dmaengine: omap-dma: Enable packed accesses for cyclic transfers

From: Misael Lopez Cruz <misael.lopez@...com>

The L3 throughput can be higher than expected when packed access
is not enabled.  The ratio depends on the number of bytes in a
transaction and the EMIF interface width.

The throughput was measured for the following settings/cases:

* Case 1: Burst size of 64 bytes, packed access disabled
* Case 2: Burst size of 64 bytes, packed access enabled
* Case 3: Burst disabled, packed access disabled

Throughput measurements were done during McASP-based audio
playback on the Jacinto6 EVM using the omapconf tool [1]:
$ omapconf trace bw -m sdma_rd

 ---------------------------------------------------------
                                  Throughput (MB/s)
  Audio parameters            Case 1    Case 2    Case 3
 ---------------------------------------------------------
  44.1kHz, 16-bits, stereo      1.41      0.18      1.41
  44.1kHz, 32-bits, stereo      1.41      0.35      1.41
  44.1kHz, 16-bits, 4-chan      2.82      0.35      2.82
  44.1kHz, 16-bits, 6-chan      4.23      0.53      4.23
  44.1kHz, 16-bits, 8-chan      5.64      0.71      5.64
 ---------------------------------------------------------

>From above measurements, case 2 is the only one that delivers
the expected throughput for the given audio parameters.  For
that reason, the packed accesses are now enabled.

It's worth to mention that packed accesses cannot be enabled
for all addressing modes. In cyclic transfers, it can be
enabled in the source for MEM_TO_DEV and in dest for DEV_TO_MEM,
as they use post-increment mode which supports packed accesses.

Peter Ujfalusi:
>From the TRM regarding to this:
"NOTE: Except in the constant addressing mode, the source or
destination must be specified as packed for burst transactions
to occur."

So w/o the packed setting the burst on the MEM side was not
enabled, this explains the numbers.

[1] https://github.com/omapconf/omapconf

Signed-off-by: Misael Lopez Cruz <misael.lopez@...com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@...com>
---
 drivers/dma/omap-dma.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/dma/omap-dma.c b/drivers/dma/omap-dma.c
index 249445c8a4c6..1dfc71c90123 100644
--- a/drivers/dma/omap-dma.c
+++ b/drivers/dma/omap-dma.c
@@ -935,8 +935,12 @@ static struct dma_async_tx_descriptor *omap_dma_prep_dma_cyclic(
 		else
 			d->ccr |= CCR_SYNC_ELEMENT;
 
-		if (dir == DMA_DEV_TO_MEM)
+		if (dir == DMA_DEV_TO_MEM) {
 			d->ccr |= CCR_TRIGGER_SRC;
+			d->csdp |= CSDP_DST_PACKED;
+		} else {
+			d->csdp |= CSDP_SRC_PACKED;
+		}
 
 		d->cicr |= CICR_MISALIGNED_ERR_IE | CICR_TRANS_ERR_IE;
 
-- 
2.5.2

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