-----Original Message-----
From: Lucas Stach [mailto:l.stach at pengutronix.de]
Sent: 2018?7?23? 18:54
To: Robin Gong <redacted>; vkoul at kernel.org;
dan.j.williams at intel.com; s.hauer at pengutronix.de; linux at armlinux.org.uk
Cc: dmaengine at vger.kernel.org; dl-linux-imx <redacted>;
kernel at pengutronix.de; linux-arm-kernel at lists.infradead.org;
linux-kernel at vger.kernel.org
Subject: Re: [PATCH v3 3/3] dmaengine: imx-sdma: allocate max 20 bds for one
transfer

Am Dienstag, den 24.07.2018, 01:46 +0800 schrieb Robin Gong:

quoted

If multi-bds used in one transfer, all bds should be consisten
memory.To easily follow it, enlarge the dma pool size into 20 bds, and
it will report error if the number of bds is over than 20. For
dmatest, the max count for single transfer is NUM_BD *

SDMA_BD_MAX_CNT

quoted

= 20 * 65535 = ~1.28MB.

Both the commit message and the comment need a lot more care to actually
tell what this commit is trying to achieve. Currently I don't follow at all. What
does "consisten" mean? Do you mean BDs should be contiguous in memory?

Yes, BDs should be contiguous  one by one in memory.

What do you gain by over-allocating each BD by a factor of 20?

I guess dma_pool_alloc will return error in such case, and then cause dma setup
transfer failure.

Regards,
Lucas

quoted

Signed-off-by: Robin Gong <redacted>
---
?drivers/dma/imx-sdma.c | 17 ++++++++++++++++-
?1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/drivers/dma/imx-sdma.c b/drivers/dma/imx-sdma.c index

b4ec2d2..5973489 100644

--- a/drivers/dma/imx-sdma.c
+++ b/drivers/dma/imx-sdma.c

@@ -298,6 +298,15 @@ struct sdma_context_data {

quoted

?	u32??scratch7;

?} __attribute__ ((packed));

+/*
+ * All bds in one transfer should be consitent on SDMA. To easily
+follow it,just
+ * set the dma pool size as the enough bds. For example, in dmatest
+case, the
+ * max 20 bds means the max for single transfer is NUM_BD *
+SDMA_BD_MAX_CNT = 20
+ * * 65535 = ~1.28MB. 20 bds supposed to be enough basically.If it's
+still not
+ * enough in some specific cases, enlarge it here.Warning message
+would also
+ * appear if the bd numbers is over than 20.
+ */
+#define NUM_BD 20

?struct sdma_engine;

@@ -1273,7 +1282,7 @@ static int sdma_alloc_chan_resources(struct

dma_chan *chan)

quoted

?		goto disable_clk_ahb;

quoted

?	sdmac->bd_pool = dma_pool_create("bd_pool", chan->device->dev,
-				sizeof(struct sdma_buffer_descriptor),
+				NUM_BD * sizeof(struct sdma_buffer_descriptor),
?				32, 0);

quoted

?	return 0;

@@ -1314,6 +1323,12 @@ static struct sdma_desc

*sdma_transfer_init(struct sdma_channel *sdmac,
?{

quoted

?	struct sdma_desc *desc;

quoted

+	if (bds > NUM_BD) {
+		dev_err(sdmac->sdma->dev, "%d bds exceed the max %d\n",
+			bds, NUM_BD);
+		goto err_out;
+	}

+

quoted

?	desc = kzalloc((sizeof(*desc)), GFP_NOWAIT);
?	if (!desc)
?		goto err_out;