[PATCH] spi: omap2-mcspi: fix dma transfer for vmalloced buffer

[PATCH] spi: omap2-mcspi: fix dma transfer for vmalloced buffer

[Akinobu Mita]

Currently omap2-mcspi cannot handle dma transfer for vmalloced buffer.
I hit this problem when using mtdblock on spi-nor.

This lets the SPI core handle the page mapping for dma transfer buffer.

Signed-off-by: Akinobu Mita <akinobu.mita-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: Mark Brown <broonie-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
---
 drivers/spi/spi-omap2-mcspi.c | 62 ++++++++++++-------------------------------
 1 file changed, 17 insertions(+), 45 deletions(-)

diff --git a/drivers/spi/spi-omap2-mcspi.c b/drivers/spi/spi-omap2-mcspi.c
index 0caa3c8..43a02e3 100644
--- a/drivers/spi/spi-omap2-mcspi.c
+++ b/drivers/spi/spi-omap2-mcspi.c
@@ -423,16 +423,12 @@ static void omap2_mcspi_tx_dma(struct spi_device *spi,
 
 	if (mcspi_dma->dma_tx) {
 		struct dma_async_tx_descriptor *tx;
-		struct scatterlist sg;
 
 		dmaengine_slave_config(mcspi_dma->dma_tx, &cfg);
 
-		sg_init_table(&sg, 1);
-		sg_dma_address(&sg) = xfer->tx_dma;
-		sg_dma_len(&sg) = xfer->len;
-
-		tx = dmaengine_prep_slave_sg(mcspi_dma->dma_tx, &sg, 1,
-		DMA_MEM_TO_DEV, DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
+		tx = dmaengine_prep_slave_sg(mcspi_dma->dma_tx, xfer->tx_sg.sgl,
+					     xfer->tx_sg.nents, DMA_MEM_TO_DEV,
+					     DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
 		if (tx) {
 			tx->callback = omap2_mcspi_tx_callback;
 			tx->callback_param = spi;
@@ -478,20 +474,15 @@ omap2_mcspi_rx_dma(struct spi_device *spi, struct spi_transfer *xfer,
 
 	if (mcspi_dma->dma_rx) {
 		struct dma_async_tx_descriptor *tx;
-		struct scatterlist sg;
 
 		dmaengine_slave_config(mcspi_dma->dma_rx, &cfg);
 
 		if ((l & OMAP2_MCSPI_CHCONF_TURBO) && mcspi->fifo_depth == 0)
 			dma_count -= es;
 
-		sg_init_table(&sg, 1);
-		sg_dma_address(&sg) = xfer->rx_dma;
-		sg_dma_len(&sg) = dma_count;
-
-		tx = dmaengine_prep_slave_sg(mcspi_dma->dma_rx, &sg, 1,
-				DMA_DEV_TO_MEM, DMA_PREP_INTERRUPT |
-				DMA_CTRL_ACK);
+		tx = dmaengine_prep_slave_sg(mcspi_dma->dma_rx, xfer->rx_sg.sgl,
+					     xfer->rx_sg.nents, DMA_DEV_TO_MEM,
+					     DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
 		if (tx) {
 			tx->callback = omap2_mcspi_rx_callback;
 			tx->callback_param = spi;
@@ -505,8 +496,6 @@ omap2_mcspi_rx_dma(struct spi_device *spi, struct spi_transfer *xfer,
 	omap2_mcspi_set_dma_req(spi, 1, 1);
 
 	wait_for_completion(&mcspi_dma->dma_rx_completion);
-	dma_unmap_single(mcspi->dev, xfer->rx_dma, count,
-			 DMA_FROM_DEVICE);
 
 	if (mcspi->fifo_depth > 0)
 		return count;
@@ -619,8 +608,6 @@ omap2_mcspi_txrx_dma(struct spi_device *spi, struct spi_transfer *xfer)
 
 	if (tx != NULL) {
 		wait_for_completion(&mcspi_dma->dma_tx_completion);
-		dma_unmap_single(mcspi->dev, xfer->tx_dma, xfer->len,
-				 DMA_TO_DEVICE);
 
 		if (mcspi->fifo_depth > 0) {
 			irqstat_reg = mcspi->base + OMAP2_MCSPI_IRQSTATUS;
@@ -1087,6 +1074,16 @@ static void omap2_mcspi_cleanup(struct spi_device *spi)
 		gpio_free(spi->cs_gpio);
 }
 
+static bool omap2_mcspi_can_dma(struct spi_master *master,
+				struct spi_device *spi,
+				struct spi_transfer *xfer)
+{
+	if (xfer->len < DMA_MIN_BYTES)
+		return false;
+
+	return true;
+}
+
 static int omap2_mcspi_work_one(struct omap2_mcspi *mcspi,
 		struct spi_device *spi, struct spi_transfer *t)
 {
@@ -1268,32 +1265,6 @@ static int omap2_mcspi_transfer_one(struct spi_master *master,
 		return -EINVAL;
 	}
 
-	if (len < DMA_MIN_BYTES)
-		goto skip_dma_map;
-
-	if (mcspi_dma->dma_tx && tx_buf != NULL) {
-		t->tx_dma = dma_map_single(mcspi->dev, (void *) tx_buf,
-				len, DMA_TO_DEVICE);
-		if (dma_mapping_error(mcspi->dev, t->tx_dma)) {
-			dev_dbg(mcspi->dev, "dma %cX %d bytes error\n",
-					'T', len);
-			return -EINVAL;
-		}
-	}
-	if (mcspi_dma->dma_rx && rx_buf != NULL) {
-		t->rx_dma = dma_map_single(mcspi->dev, rx_buf, t->len,
-				DMA_FROM_DEVICE);
-		if (dma_mapping_error(mcspi->dev, t->rx_dma)) {
-			dev_dbg(mcspi->dev, "dma %cX %d bytes error\n",
-					'R', len);
-			if (tx_buf != NULL)
-				dma_unmap_single(mcspi->dev, t->tx_dma,
-						len, DMA_TO_DEVICE);
-			return -EINVAL;
-		}
-	}
-
-skip_dma_map:
 	return omap2_mcspi_work_one(mcspi, spi, t);
 }
 
@@ -1377,6 +1348,7 @@ static int omap2_mcspi_probe(struct platform_device *pdev)
 	master->transfer_one = omap2_mcspi_transfer_one;
 	master->set_cs = omap2_mcspi_set_cs;
 	master->cleanup = omap2_mcspi_cleanup;
+	master->can_dma = omap2_mcspi_can_dma;
 	master->dev.of_node = node;
 	master->max_speed_hz = OMAP2_MCSPI_MAX_FREQ;
 	master->min_speed_hz = OMAP2_MCSPI_MAX_FREQ >> 15;
-- 
2.5.0

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Applied "spi: omap2-mcspi: fix dma transfer for vmalloced buffer" to the spi tree

[Mark Brown]

The patch

   spi: omap2-mcspi: fix dma transfer for vmalloced buffer

has been applied to the spi tree at

   git://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git 

All being well this means that it will be integrated into the linux-next
tree (usually sometime in the next 24 hours) and sent to Linus during
the next merge window (or sooner if it is a bug fix), however if
problems are discovered then the patch may be dropped or reverted.  

You may get further e-mails resulting from automated or manual testing
and review of the tree, please engage with people reporting problems and
send followup patches addressing any issues that are reported if needed.

If any updates are required or you are submitting further changes they
should be sent as incremental updates against current git, existing
patches will not be replaced.

Please add any relevant lists and maintainers to the CCs when replying
to this mail.

Thanks,
Mark

>From 3525e0aac91c4de5d20b1f22a6c6e2b39db3cc96 Mon Sep 17 00:00:00 2001
From: Akinobu Mita <akinobu.mita-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Date: Tue, 22 Mar 2016 01:00:21 +0900
Subject: [PATCH] spi: omap2-mcspi: fix dma transfer for vmalloced buffer

Currently omap2-mcspi cannot handle dma transfer for vmalloced buffer.
I hit this problem when using mtdblock on spi-nor.

This lets the SPI core handle the page mapping for dma transfer buffer.

Signed-off-by: Akinobu Mita <akinobu.mita-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Signed-off-by: Mark Brown <broonie-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
---
 drivers/spi/spi-omap2-mcspi.c | 62 ++++++++++++-------------------------------
 1 file changed, 17 insertions(+), 45 deletions(-)

diff --git a/drivers/spi/spi-omap2-mcspi.c b/drivers/spi/spi-omap2-mcspi.c
index 0caa3c8bef46..43a02e377b3b 100644
--- a/drivers/spi/spi-omap2-mcspi.c
+++ b/drivers/spi/spi-omap2-mcspi.c
@@ -423,16 +423,12 @@ static void omap2_mcspi_tx_dma(struct spi_device *spi,
 
 	if (mcspi_dma->dma_tx) {
 		struct dma_async_tx_descriptor *tx;
-		struct scatterlist sg;
 
 		dmaengine_slave_config(mcspi_dma->dma_tx, &cfg);
 
-		sg_init_table(&sg, 1);
-		sg_dma_address(&sg) = xfer->tx_dma;
-		sg_dma_len(&sg) = xfer->len;
-
-		tx = dmaengine_prep_slave_sg(mcspi_dma->dma_tx, &sg, 1,
-		DMA_MEM_TO_DEV, DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
+		tx = dmaengine_prep_slave_sg(mcspi_dma->dma_tx, xfer->tx_sg.sgl,
+					     xfer->tx_sg.nents, DMA_MEM_TO_DEV,
+					     DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
 		if (tx) {
 			tx->callback = omap2_mcspi_tx_callback;
 			tx->callback_param = spi;
@@ -478,20 +474,15 @@ omap2_mcspi_rx_dma(struct spi_device *spi, struct spi_transfer *xfer,
 
 	if (mcspi_dma->dma_rx) {
 		struct dma_async_tx_descriptor *tx;
-		struct scatterlist sg;
 
 		dmaengine_slave_config(mcspi_dma->dma_rx, &cfg);
 
 		if ((l & OMAP2_MCSPI_CHCONF_TURBO) && mcspi->fifo_depth == 0)
 			dma_count -= es;
 
-		sg_init_table(&sg, 1);
-		sg_dma_address(&sg) = xfer->rx_dma;
-		sg_dma_len(&sg) = dma_count;
-
-		tx = dmaengine_prep_slave_sg(mcspi_dma->dma_rx, &sg, 1,
-				DMA_DEV_TO_MEM, DMA_PREP_INTERRUPT |
-				DMA_CTRL_ACK);
+		tx = dmaengine_prep_slave_sg(mcspi_dma->dma_rx, xfer->rx_sg.sgl,
+					     xfer->rx_sg.nents, DMA_DEV_TO_MEM,
+					     DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
 		if (tx) {
 			tx->callback = omap2_mcspi_rx_callback;
 			tx->callback_param = spi;
@@ -505,8 +496,6 @@ omap2_mcspi_rx_dma(struct spi_device *spi, struct spi_transfer *xfer,
 	omap2_mcspi_set_dma_req(spi, 1, 1);
 
 	wait_for_completion(&mcspi_dma->dma_rx_completion);
-	dma_unmap_single(mcspi->dev, xfer->rx_dma, count,
-			 DMA_FROM_DEVICE);
 
 	if (mcspi->fifo_depth > 0)
 		return count;
@@ -619,8 +608,6 @@ omap2_mcspi_txrx_dma(struct spi_device *spi, struct spi_transfer *xfer)
 
 	if (tx != NULL) {
 		wait_for_completion(&mcspi_dma->dma_tx_completion);
-		dma_unmap_single(mcspi->dev, xfer->tx_dma, xfer->len,
-				 DMA_TO_DEVICE);
 
 		if (mcspi->fifo_depth > 0) {
 			irqstat_reg = mcspi->base + OMAP2_MCSPI_IRQSTATUS;
@@ -1087,6 +1074,16 @@ static void omap2_mcspi_cleanup(struct spi_device *spi)
 		gpio_free(spi->cs_gpio);
 }
 
+static bool omap2_mcspi_can_dma(struct spi_master *master,
+				struct spi_device *spi,
+				struct spi_transfer *xfer)
+{
+	if (xfer->len < DMA_MIN_BYTES)
+		return false;
+
+	return true;
+}
+
 static int omap2_mcspi_work_one(struct omap2_mcspi *mcspi,
 		struct spi_device *spi, struct spi_transfer *t)
 {
@@ -1268,32 +1265,6 @@ static int omap2_mcspi_transfer_one(struct spi_master *master,
 		return -EINVAL;
 	}
 
-	if (len < DMA_MIN_BYTES)
-		goto skip_dma_map;
-
-	if (mcspi_dma->dma_tx && tx_buf != NULL) {
-		t->tx_dma = dma_map_single(mcspi->dev, (void *) tx_buf,
-				len, DMA_TO_DEVICE);
-		if (dma_mapping_error(mcspi->dev, t->tx_dma)) {
-			dev_dbg(mcspi->dev, "dma %cX %d bytes error\n",
-					'T', len);
-			return -EINVAL;
-		}
-	}
-	if (mcspi_dma->dma_rx && rx_buf != NULL) {
-		t->rx_dma = dma_map_single(mcspi->dev, rx_buf, t->len,
-				DMA_FROM_DEVICE);
-		if (dma_mapping_error(mcspi->dev, t->rx_dma)) {
-			dev_dbg(mcspi->dev, "dma %cX %d bytes error\n",
-					'R', len);
-			if (tx_buf != NULL)
-				dma_unmap_single(mcspi->dev, t->tx_dma,
-						len, DMA_TO_DEVICE);
-			return -EINVAL;
-		}
-	}
-
-skip_dma_map:
 	return omap2_mcspi_work_one(mcspi, spi, t);
 }
 
@@ -1377,6 +1348,7 @@ static int omap2_mcspi_probe(struct platform_device *pdev)
 	master->transfer_one = omap2_mcspi_transfer_one;
 	master->set_cs = omap2_mcspi_set_cs;
 	master->cleanup = omap2_mcspi_cleanup;
+	master->can_dma = omap2_mcspi_can_dma;
 	master->dev.of_node = node;
 	master->max_speed_hz = OMAP2_MCSPI_MAX_FREQ;
 	master->min_speed_hz = OMAP2_MCSPI_MAX_FREQ >> 15;
-- 
2.7.0

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Re: [PATCH] spi: omap2-mcspi: fix dma transfer for vmalloced buffer

[Akinobu Mita]

2016-03-22 1:00 GMT+09:00 Akinobu Mita <akinobu.mita-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>:
> Currently omap2-mcspi cannot handle dma transfer for vmalloced buffer.
> I hit this problem when using mtdblock on spi-nor.
>
> This lets the SPI core handle the page mapping for dma transfer buffer.
>
> Signed-off-by: Akinobu Mita <akinobu.mita-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> Cc: Mark Brown <broonie-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> ---
>  drivers/spi/spi-omap2-mcspi.c | 62 ++++++++++++-------------------------------
>  1 file changed, 17 insertions(+), 45 deletions(-)
>
> diff --git a/drivers/spi/spi-omap2-mcspi.c b/drivers/spi/spi-omap2-mcspi.c
> index 0caa3c8..43a02e3 100644
> --- a/drivers/spi/spi-omap2-mcspi.c
> +++ b/drivers/spi/spi-omap2-mcspi.c
> @@ -423,16 +423,12 @@ static void omap2_mcspi_tx_dma(struct spi_device *spi,
>
>         if (mcspi_dma->dma_tx) {
>                 struct dma_async_tx_descriptor *tx;
> -               struct scatterlist sg;
>
>                 dmaengine_slave_config(mcspi_dma->dma_tx, &cfg);
>
> -               sg_init_table(&sg, 1);
> -               sg_dma_address(&sg) = xfer->tx_dma;
> -               sg_dma_len(&sg) = xfer->len;
> -
> -               tx = dmaengine_prep_slave_sg(mcspi_dma->dma_tx, &sg, 1,
> -               DMA_MEM_TO_DEV, DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
> +               tx = dmaengine_prep_slave_sg(mcspi_dma->dma_tx, xfer->tx_sg.sgl,
> +                                            xfer->tx_sg.nents, DMA_MEM_TO_DEV,
> +                                            DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
>                 if (tx) {
>                         tx->callback = omap2_mcspi_tx_callback;
>                         tx->callback_param = spi;
> @@ -478,20 +474,15 @@ omap2_mcspi_rx_dma(struct spi_device *spi, struct spi_transfer *xfer,
>
>         if (mcspi_dma->dma_rx) {
>                 struct dma_async_tx_descriptor *tx;
> -               struct scatterlist sg;
>
>                 dmaengine_slave_config(mcspi_dma->dma_rx, &cfg);
>
>                 if ((l & OMAP2_MCSPI_CHCONF_TURBO) && mcspi->fifo_depth == 0)
>                         dma_count -= es;
>

The actual transfer length can be smaller than xfer->len by above line.
And the last word is filled after dma transfer completion.

So this patch is broken.  Could you revert remove this from your
development tree?
I don't find a way how to fix it without copy and pasting a lot of
line from spi core.

> -               sg_init_table(&sg, 1);
> -               sg_dma_address(&sg) = xfer->rx_dma;
> -               sg_dma_len(&sg) = dma_count;
> -
> -               tx = dmaengine_prep_slave_sg(mcspi_dma->dma_rx, &sg, 1,
> -                               DMA_DEV_TO_MEM, DMA_PREP_INTERRUPT |
> -                               DMA_CTRL_ACK);
> +               tx = dmaengine_prep_slave_sg(mcspi_dma->dma_rx, xfer->rx_sg.sgl,
> +                                            xfer->rx_sg.nents, DMA_DEV_TO_MEM,
> +                                            DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
>                 if (tx) {
>                         tx->callback = omap2_mcspi_rx_callback;
>                         tx->callback_param = spi;
> @@ -505,8 +496,6 @@ omap2_mcspi_rx_dma(struct spi_device *spi, struct spi_transfer *xfer,
>         omap2_mcspi_set_dma_req(spi, 1, 1);
>
>         wait_for_completion(&mcspi_dma->dma_rx_completion);
> -       dma_unmap_single(mcspi->dev, xfer->rx_dma, count,
> -                        DMA_FROM_DEVICE);
>
>         if (mcspi->fifo_depth > 0)
>                 return count;
> @@ -619,8 +608,6 @@ omap2_mcspi_txrx_dma(struct spi_device *spi, struct spi_transfer *xfer)
>
>         if (tx != NULL) {
>                 wait_for_completion(&mcspi_dma->dma_tx_completion);
> -               dma_unmap_single(mcspi->dev, xfer->tx_dma, xfer->len,
> -                                DMA_TO_DEVICE);
>
>                 if (mcspi->fifo_depth > 0) {
>                         irqstat_reg = mcspi->base + OMAP2_MCSPI_IRQSTATUS;
> @@ -1087,6 +1074,16 @@ static void omap2_mcspi_cleanup(struct spi_device *spi)
>                 gpio_free(spi->cs_gpio);
>  }
>
> +static bool omap2_mcspi_can_dma(struct spi_master *master,
> +                               struct spi_device *spi,
> +                               struct spi_transfer *xfer)
> +{
> +       if (xfer->len < DMA_MIN_BYTES)
> +               return false;
> +
> +       return true;
> +}
> +
>  static int omap2_mcspi_work_one(struct omap2_mcspi *mcspi,
>                 struct spi_device *spi, struct spi_transfer *t)
>  {
> @@ -1268,32 +1265,6 @@ static int omap2_mcspi_transfer_one(struct spi_master *master,
>                 return -EINVAL;
>         }
>
> -       if (len < DMA_MIN_BYTES)
> -               goto skip_dma_map;
> -
> -       if (mcspi_dma->dma_tx && tx_buf != NULL) {
> -               t->tx_dma = dma_map_single(mcspi->dev, (void *) tx_buf,
> -                               len, DMA_TO_DEVICE);
> -               if (dma_mapping_error(mcspi->dev, t->tx_dma)) {
> -                       dev_dbg(mcspi->dev, "dma %cX %d bytes error\n",
> -                                       'T', len);
> -                       return -EINVAL;
> -               }
> -       }
> -       if (mcspi_dma->dma_rx && rx_buf != NULL) {
> -               t->rx_dma = dma_map_single(mcspi->dev, rx_buf, t->len,
> -                               DMA_FROM_DEVICE);
> -               if (dma_mapping_error(mcspi->dev, t->rx_dma)) {
> -                       dev_dbg(mcspi->dev, "dma %cX %d bytes error\n",
> -                                       'R', len);
> -                       if (tx_buf != NULL)
> -                               dma_unmap_single(mcspi->dev, t->tx_dma,
> -                                               len, DMA_TO_DEVICE);
> -                       return -EINVAL;
> -               }
> -       }
> -
> -skip_dma_map:
>         return omap2_mcspi_work_one(mcspi, spi, t);
>  }
>
> @@ -1377,6 +1348,7 @@ static int omap2_mcspi_probe(struct platform_device *pdev)
>         master->transfer_one = omap2_mcspi_transfer_one;
>         master->set_cs = omap2_mcspi_set_cs;
>         master->cleanup = omap2_mcspi_cleanup;
> +       master->can_dma = omap2_mcspi_can_dma;
>         master->dev.of_node = node;
>         master->max_speed_hz = OMAP2_MCSPI_MAX_FREQ;
>         master->min_speed_hz = OMAP2_MCSPI_MAX_FREQ >> 15;
> --
> 2.5.0
>
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Re: [PATCH] spi: omap2-mcspi: fix dma transfer for vmalloced buffer

[Mark Brown]

[-- Attachment #1: Type: text/plain, Size: 847 bytes --]

On Thu, Apr 07, 2016 at 09:17:29PM +0900, Akinobu Mita wrote:
> 2016-03-22 1:00 GMT+09:00 Akinobu Mita <akinobu.mita-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>:

> >                 if ((l & OMAP2_MCSPI_CHCONF_TURBO) && mcspi->fifo_depth == 0)
> >                         dma_count -= es;

> The actual transfer length can be smaller than xfer->len by above line.
> And the last word is filled after dma transfer completion.

> So this patch is broken.  Could you revert remove this from your
> development tree?
> I don't find a way how to fix it without copy and pasting a lot of
> line from spi core.

Can you please send a patch doing the revert and explaining why we can't
do this in the changelog?  That might help someone else looking at this
to figure it out in future, or at least stop them trying the same thing
and running into problems.

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Re: [PATCH] spi: omap2-mcspi: fix dma transfer for vmalloced buffer

[Akinobu Mita]

2016-04-08 2:57 GMT+09:00 Mark Brown <broonie-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>:
> On Thu, Apr 07, 2016 at 09:17:29PM +0900, Akinobu Mita wrote:
>> 2016-03-22 1:00 GMT+09:00 Akinobu Mita <akinobu.mita-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>:
>
>> >                 if ((l & OMAP2_MCSPI_CHCONF_TURBO) && mcspi->fifo_depth == 0)
>> >                         dma_count -= es;
>
>> The actual transfer length can be smaller than xfer->len by above line.
>> And the last word is filled after dma transfer completion.
>
>> So this patch is broken.  Could you revert remove this from your
>> development tree?
>> I don't find a way how to fix it without copy and pasting a lot of
>> line from spi core.
>
> Can you please send a patch doing the revert and explaining why we can't
> do this in the changelog?  That might help someone else looking at this
> to figure it out in future, or at least stop them trying the same thing
> and running into problems.

I'm going to come up with a fix that detects xfer->len != dma_count
and remap rx_sg in omap2_mcspi_rx_dma().  But I plan to spend
enough time tesing it, so feel free to apply revert that I sent a while ago.
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