Hi Robin,

Am Montag, den 06.08.2018, 08:04 +0000 schrieb Robin Gong:

Hello Lucas,
	Any comment for my reply?

So I've looked at this again and sadly I need to NACK this patch. It is
a total API abuse of the dma_pool API and even the patch introducing
the dma_pool usage in this driver is wrong and should be reverted.

The SDMA need contiguous buffer descriptors for each channel, something
the dma_pool abstraction isn't able to provide. So either the dma_pool
implementation needs to be extended to support this use-case, or you
can't use this at all in the sdma driver. Adding hacks, which are
abusing the API, to cram a dma_pool into the sdma driver is not a valid
way to implement things for upstream.

Regards,
Lucas

quoted

-----Original Message-----
From: Robin Gong
Sent: 2018?7?25? 9:25
To: 'Lucas Stach' <l.stach@pengutronix.de>; vkoul at kernel.org;
dan.j.williams at intel.com; s.hauer at pengutronix.de; linux at armlinux.or
g.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

quoted

-----Original Message-----
From: Lucas Stach [mailto:l.stach at pengutronix.de]
Sent: 2018?7?24? 17:22
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 Montag, den 23.07.2018, 13:55 +0000 schrieb Robin Gong:

quoted

quoted

-----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.or g.uk
Cc: dmaengine at vger.kernel.org; dl-linux-imx <linux-imx@nxp.co
m>;
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.

Okay, but this isn't what the code change does. By increasing the
size
parameter of the dma pool you just allocate 20 times as much
memory as
needed for each BD. So actually the BDs end up being very non-
contiguous in memory as there are now holes of 19 BD sizes
between the

start of each BD.
Please notice only allocate bds memory from dma pool one time even
in multi
bds.
That's different with the common use case that allocate memory from
dma
pool everytime for every bd. Why do this is to make sure all bd
memory is
contiguous for single transfer whatever single bd or multi-bds,
since two call
dma_pool_alloc() can't promise the address is contiguous especially
for multi
thread case such as dmatest 'threads_per_chan = 5'. You can change
to '
norandom=true ' and ' test_buf_size = 163840' in dmatest.c to look
what issue
coming without this patch.

quoted

So something isn't right with this change.

I think this patch is the easy way to resolve the bd contiguous
issue, but the
cost is to allocate more dma pool memory which may not used.

quoted

Regards,
Lucas

quoted

quoted

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.

quoted

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;
?	sdmac->bd_pool = dma_pool_create("bd_pool",
chan-

quoted

device->dev,

-				sizeof(struct
sdma_buffer_descriptor),
+				NUM_BD * sizeof(struct
sdma_buffer_descriptor),
?				32, 0);
?	return 0;

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

*sdma_transfer_init(struct sdma_channel *sdmac,
?{

quoted

?	struct sdma_desc *desc;
+	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;