[v3,3/3] dmaengine: imx-sdma: allocate max 20 bds for one transfer

[v3,3/3] dmaengine: imx-sdma: allocate max 20 bds for one transfer

[Lucas Stach]

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

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

Regards,
Lucas

> Signed-off-by: Robin Gong <yibin.gong@nxp.com>
> ---
>  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 {
> >  	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)
> >  		goto disable_clk_ahb;
>  
> >  	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);
>  
> >  	return 0;
> @@ -1314,6 +1323,12 @@ static struct sdma_desc *sdma_transfer_init(struct sdma_channel *sdmac,
>  {
> >  	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;
> > +	}
> +
> >  	desc = kzalloc((sizeof(*desc)), GFP_NOWAIT);
> >  	if (!desc)
> >  		goto err_out;
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[v3,3/3] dmaengine: imx-sdma: allocate max 20 bds for one transfer

[Robin Gong]

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[v3,3/3] dmaengine: imx-sdma: allocate max 20 bds for one transfer

[Robin Gong]

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 = 20 * 65535 = ~1.28MB.

Signed-off-by: Robin Gong <yibin.gong@nxp.com>
---
 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 {
 	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)
 		goto disable_clk_ahb;
 
 	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);
 
 	return 0;
@@ -1314,6 +1323,12 @@ static struct sdma_desc *sdma_transfer_init(struct sdma_channel *sdmac,
 {
 	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;
+	}
+
 	desc = kzalloc((sizeof(*desc)), GFP_NOWAIT);
 	if (!desc)
 		goto err_out;

[v3,3/3] dmaengine: imx-sdma: allocate max 20 bds for one transfer

[Lucas Stach]

Am Montag, den 23.07.2018, 13:55 +0000 schrieb Robin Gong:
> > -----Original Message-----
> > From: Lucas Stach [mailto:l.stach@pengutronix.de]
> > Sent: 2018年7月23日 18:54
> > To: Robin Gong <yibin.gong@nxp.com>; vkoul@kernel.org;
> > dan.j.williams@intel.com; s.hauer@pengutronix.de; linux@armlinux.or
> > g.uk
> > Cc: dmaengine@vger.kernel.org; dl-linux-imx <linux-imx@nxp.com>;
> > kernel@pengutronix.de; linux-arm-kernel@lists.infradead.org;
> > linux-kernel@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:
> > > 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
> > > = 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.

So something isn't right with this change.

Regards,
Lucas

> > 
> > 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
> > 
> > > Signed-off-by: Robin Gong <yibin.gong@nxp.com>
> > > ---
> > >  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 {
> > > >  	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)
> > > >  		goto disable_clk_ahb;
> > > >  	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);
> > > >  	return 0;
> > > 
> > > @@ -1314,6 +1323,12 @@ static struct sdma_desc
> > > *sdma_transfer_init(struct sdma_channel *sdmac,
> > >  {
> > > >  	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;
> > > > +	}
> > > 
> > > +
> > > >  	desc = kzalloc((sizeof(*desc)), GFP_NOWAIT);
> > > >  	if (!desc)
> > > >  		goto err_out;
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[v3,3/3] dmaengine: imx-sdma: allocate max 20 bds for one transfer

[Robin Gong]

> -----Original Message-----
> From: Lucas Stach [mailto:l.stach@pengutronix.de]
> Sent: 2018年7月24日 17:22
> To: Robin Gong <yibin.gong@nxp.com>; vkoul@kernel.org;
> dan.j.williams@intel.com; s.hauer@pengutronix.de; linux@armlinux.org.uk
> Cc: dmaengine@vger.kernel.org; dl-linux-imx <linux-imx@nxp.com>;
> kernel@pengutronix.de; linux-arm-kernel@lists.infradead.org;
> linux-kernel@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:
> > > -----Original Message-----
> > > From: Lucas Stach [mailto:l.stach@pengutronix.de]
> > > Sent: 2018年7月23日 18:54
> > > To: Robin Gong <yibin.gong@nxp.com>; vkoul@kernel.org;
> > > dan.j.williams@intel.com; s.hauer@pengutronix.de; linux@armlinux.or
> > > g.uk
> > > Cc: dmaengine@vger.kernel.org; dl-linux-imx <linux-imx@nxp.com>;
> > > kernel@pengutronix.de; linux-arm-kernel@lists.infradead.org;
> > > linux-kernel@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:
> > > > 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
> > > > = 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.
> 
> 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.
> 
> Regards,
> Lucas
> 
> > >
> > > 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
> > >
> > > > Signed-off-by: Robin Gong <yibin.gong@nxp.com>
> > > > ---
> > > >  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 {
> > > > >  	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)
> > > > >  		goto disable_clk_ahb;
> > > > >  	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);
> > > > >  	return 0;
> > > >
> > > > @@ -1314,6 +1323,12 @@ static struct sdma_desc
> > > > *sdma_transfer_init(struct sdma_channel *sdmac,
> > > >  {
> > > > >  	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;
> > > > > +	}
> > > >
> > > > +
> > > > >  	desc = kzalloc((sizeof(*desc)), GFP_NOWAIT);
> > > > >  	if (!desc)
> > > > >  		goto err_out;

[v3,3/3] dmaengine: imx-sdma: allocate max 20 bds for one transfer

[Robin Gong]

Hello Lucas,
	Any comment for my reply?

> -----Original Message-----
> From: Robin Gong
> Sent: 2018年7月25日 9:25
> To: 'Lucas Stach' <l.stach@pengutronix.de>; vkoul@kernel.org;
> dan.j.williams@intel.com; s.hauer@pengutronix.de; linux@armlinux.org.uk
> Cc: dmaengine@vger.kernel.org; dl-linux-imx <linux-imx@nxp.com>;
> kernel@pengutronix.de; linux-arm-kernel@lists.infradead.org;
> linux-kernel@vger.kernel.org
> Subject: RE: [PATCH v3 3/3] dmaengine: imx-sdma: allocate max 20 bds for one
> transfer
> 
> > -----Original Message-----
> > From: Lucas Stach [mailto:l.stach@pengutronix.de]
> > Sent: 2018年7月24日 17:22
> > To: Robin Gong <yibin.gong@nxp.com>; vkoul@kernel.org;
> > dan.j.williams@intel.com; s.hauer@pengutronix.de;
> > linux@armlinux.org.uk
> > Cc: dmaengine@vger.kernel.org; dl-linux-imx <linux-imx@nxp.com>;
> > kernel@pengutronix.de; linux-arm-kernel@lists.infradead.org;
> > linux-kernel@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:
> > > > -----Original Message-----
> > > > From: Lucas Stach [mailto:l.stach@pengutronix.de]
> > > > Sent: 2018年7月23日 18:54
> > > > To: Robin Gong <yibin.gong@nxp.com>; vkoul@kernel.org;
> > > > dan.j.williams@intel.com; s.hauer@pengutronix.de;
> > > > linux@armlinux.or g.uk
> > > > Cc: dmaengine@vger.kernel.org; dl-linux-imx <linux-imx@nxp.com>;
> > > > kernel@pengutronix.de; linux-arm-kernel@lists.infradead.org;
> > > > linux-kernel@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:
> > > > > 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
> > > > > = 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.
> >
> > 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.
> >
> > Regards,
> > Lucas
> >
> > > >
> > > > 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
> > > >
> > > > > Signed-off-by: Robin Gong <yibin.gong@nxp.com>
> > > > > ---
> > > > >  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 {
> > > > > >  	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)
> > > > > >  		goto disable_clk_ahb;
> > > > > >  	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);
> > > > > >  	return 0;
> > > > >
> > > > > @@ -1314,6 +1323,12 @@ static struct sdma_desc
> > > > > *sdma_transfer_init(struct sdma_channel *sdmac,
> > > > >  {
> > > > > >  	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;
> > > > > > +	}
> > > > >
> > > > > +
> > > > > >  	desc = kzalloc((sizeof(*desc)), GFP_NOWAIT);
> > > > > >  	if (!desc)
> > > > > >  		goto err_out;

[v3,3/3] dmaengine: imx-sdma: allocate max 20 bds for one transfer

[Lucas Stach]

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

> > -----Original Message-----
> > From: Robin Gong
> > Sent: 2018年7月25日 9:25
> > To: 'Lucas Stach' <l.stach@pengutronix.de>; vkoul@kernel.org;
> > dan.j.williams@intel.com; s.hauer@pengutronix.de; linux@armlinux.or
> > g.uk
> > Cc: dmaengine@vger.kernel.org; dl-linux-imx <linux-imx@nxp.com>;
> > kernel@pengutronix.de; linux-arm-kernel@lists.infradead.org;
> > linux-kernel@vger.kernel.org
> > Subject: RE: [PATCH v3 3/3] dmaengine: imx-sdma: allocate max 20
> > bds for one
> > transfer
> > 
> > > -----Original Message-----
> > > From: Lucas Stach [mailto:l.stach@pengutronix.de]
> > > Sent: 2018年7月24日 17:22
> > > To: Robin Gong <yibin.gong@nxp.com>; vkoul@kernel.org;
> > > dan.j.williams@intel.com; s.hauer@pengutronix.de;
> > > linux@armlinux.org.uk
> > > Cc: dmaengine@vger.kernel.org; dl-linux-imx <linux-imx@nxp.com>;
> > > kernel@pengutronix.de; linux-arm-kernel@lists.infradead.org;
> > > linux-kernel@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:
> > > > > -----Original Message-----
> > > > > From: Lucas Stach [mailto:l.stach@pengutronix.de]
> > > > > Sent: 2018年7月23日 18:54
> > > > > To: Robin Gong <yibin.gong@nxp.com>; vkoul@kernel.org;
> > > > > dan.j.williams@intel.com; s.hauer@pengutronix.de;
> > > > > linux@armlinux.or g.uk
> > > > > Cc: dmaengine@vger.kernel.org; dl-linux-imx <linux-imx@nxp.co
> > > > > m>;
> > > > > kernel@pengutronix.de; linux-arm-kernel@lists.infradead.org;
> > > > > linux-kernel@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:
> > > > > > 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
> > > > > > = 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.
> > > 
> > > 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.
> > > 
> > > Regards,
> > > Lucas
> > > 
> > > > > 
> > > > > 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
> > > > > 
> > > > > > Signed-off-by: Robin Gong <yibin.gong@nxp.com>
> > > > > > ---
> > > > > >  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 {
> > > > > > >  	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)
> > > > > > >  		goto disable_clk_ahb;
> > > > > > >  	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);
> > > > > > >  	return 0;
> > > > > > 
> > > > > > @@ -1314,6 +1323,12 @@ static struct sdma_desc
> > > > > > *sdma_transfer_init(struct sdma_channel *sdmac,
> > > > > >  {
> > > > > > >  	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;
> > > > > > > +	}
> > > > > > 
> > > > > > +
> > > > > > >  	desc = kzalloc((sizeof(*desc)), GFP_NOWAIT);
> > > > > > >  	if (!desc)
> > > > > > >  		goto err_out;
---
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