[PATCH] iio: ad_sigma_delta: Don't put SPI transfer buffer on the stack

[PATCH] iio: ad_sigma_delta: Don't put SPI transfer buffer on the stack

[Alexandru Ardelean]

From: Lars-Peter Clausen <lars@metafoo.de>

Use a heap allocated memory for the SPI transfer buffer. Using stack memory
will not work on some architectures when using DMA.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
---

Note: this doesn't need a stable/fixes tag. It's more of an enhancement.

 drivers/iio/adc/ad_sigma_delta.c       | 22 +++++++++++-----------
 include/linux/iio/adc/ad_sigma_delta.h |  3 ++-
 2 files changed, 13 insertions(+), 12 deletions(-)

diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
index af6cbc683214..02d9049b9218 100644
--- a/drivers/iio/adc/ad_sigma_delta.c
+++ b/drivers/iio/adc/ad_sigma_delta.c
@@ -58,7 +58,7 @@ EXPORT_SYMBOL_GPL(ad_sd_set_comm);
 int ad_sd_write_reg(struct ad_sigma_delta *sigma_delta, unsigned int reg,
 	unsigned int size, unsigned int val)
 {
-	uint8_t *data = sigma_delta->data;
+	uint8_t *data = sigma_delta->reg_data;
 	struct spi_transfer t = {
 		.tx_buf		= data,
 		.len		= size + 1,
@@ -102,7 +102,7 @@ EXPORT_SYMBOL_GPL(ad_sd_write_reg);
 static int ad_sd_read_reg_raw(struct ad_sigma_delta *sigma_delta,
 	unsigned int reg, unsigned int size, uint8_t *val)
 {
-	uint8_t *data = sigma_delta->data;
+	uint8_t *data = sigma_delta->reg_data;
 	int ret;
 	struct spi_transfer t[] = {
 		{
@@ -148,24 +148,24 @@ int ad_sd_read_reg(struct ad_sigma_delta *sigma_delta,
 {
 	int ret;
 
-	ret = ad_sd_read_reg_raw(sigma_delta, reg, size, sigma_delta->data);
+	ret = ad_sd_read_reg_raw(sigma_delta, reg, size, sigma_delta->reg_data);
 	if (ret < 0)
 		goto out;
 
 	switch (size) {
 	case 4:
-		*val = get_unaligned_be32(sigma_delta->data);
+		*val = get_unaligned_be32(sigma_delta->reg_data);
 		break;
 	case 3:
-		*val = (sigma_delta->data[0] << 16) |
-			(sigma_delta->data[1] << 8) |
-			sigma_delta->data[2];
+		*val = (sigma_delta->reg_data[0] << 16) |
+			(sigma_delta->reg_data[1] << 8) |
+			sigma_delta->reg_data[2];
 		break;
 	case 2:
-		*val = get_unaligned_be16(sigma_delta->data);
+		*val = get_unaligned_be16(sigma_delta->reg_data);
 		break;
 	case 1:
-		*val = sigma_delta->data[0];
+		*val = sigma_delta->reg_data[0];
 		break;
 	default:
 		ret = -EINVAL;
@@ -403,12 +403,12 @@ static irqreturn_t ad_sd_trigger_handler(int irq, void *p)
 	struct iio_poll_func *pf = p;
 	struct iio_dev *indio_dev = pf->indio_dev;
 	struct ad_sigma_delta *sigma_delta = iio_device_get_drvdata(indio_dev);
+	uint8_t *data = sigma_delta->buf_data;
 	unsigned int reg_size;
 	unsigned int data_reg;
-	uint8_t data[16];
 	int ret;
 
-	memset(data, 0x00, 16);
+	memset(sigma_delta->buf_data, 0x00, sizeof(sigma_delta->buf_data));
 
 	reg_size = indio_dev->channels[0].scan_type.realbits +
 			indio_dev->channels[0].scan_type.shift;
diff --git a/include/linux/iio/adc/ad_sigma_delta.h b/include/linux/iio/adc/ad_sigma_delta.h
index 6e9fb1932dde..fb77b2ebe498 100644
--- a/include/linux/iio/adc/ad_sigma_delta.h
+++ b/include/linux/iio/adc/ad_sigma_delta.h
@@ -79,7 +79,8 @@ struct ad_sigma_delta {
 	 * DMA (thus cache coherency maintenance) requires the
 	 * transfer buffers to live in their own cache lines.
 	 */
-	uint8_t				data[4] ____cacheline_aligned;
+	uint8_t				reg_data[4] ____cacheline_aligned;
+	uint8_t				buf_data[16];
 };
 
 static inline int ad_sigma_delta_set_channel(struct ad_sigma_delta *sd,
-- 
2.17.1

Re: [PATCH] iio: ad_sigma_delta: Don't put SPI transfer buffer on the stack

[Jonathan Cameron]

On Tue, 9 Apr 2019 10:58:21 +0300
Alexandru Ardelean <alexandru.ardelean@analog.com> wrote:

> From: Lars-Peter Clausen <lars@metafoo.de>
> 
> Use a heap allocated memory for the SPI transfer buffer. Using stack memory
> will not work on some architectures when using DMA.
> 
> Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
> Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Yikes, surprised this one snuck in here.
> ---
> 
> Note: this doesn't need a stable/fixes tag. It's more of an enhancement.

Talk me through this...  Right now I think this is a bug that is going
to cause random and extremely hard to debug problems.  I've had these
in the past and they are really really nasty as it's random stack corruption.
The nasty is not the fact it's on the stack, but the fact that it isn't
in it's own cacheline.

So my inclination is this definitely needs stable and fixes tag.
What am I missing?

Jonathan

> 
>  drivers/iio/adc/ad_sigma_delta.c       | 22 +++++++++++-----------
>  include/linux/iio/adc/ad_sigma_delta.h |  3 ++-
>  2 files changed, 13 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
> index af6cbc683214..02d9049b9218 100644
> --- a/drivers/iio/adc/ad_sigma_delta.c
> +++ b/drivers/iio/adc/ad_sigma_delta.c
> @@ -58,7 +58,7 @@ EXPORT_SYMBOL_GPL(ad_sd_set_comm);
>  int ad_sd_write_reg(struct ad_sigma_delta *sigma_delta, unsigned int reg,
>  	unsigned int size, unsigned int val)
>  {
> -	uint8_t *data = sigma_delta->data;
> +	uint8_t *data = sigma_delta->reg_data;
>  	struct spi_transfer t = {
>  		.tx_buf		= data,
>  		.len		= size + 1,
> @@ -102,7 +102,7 @@ EXPORT_SYMBOL_GPL(ad_sd_write_reg);
>  static int ad_sd_read_reg_raw(struct ad_sigma_delta *sigma_delta,
>  	unsigned int reg, unsigned int size, uint8_t *val)
>  {
> -	uint8_t *data = sigma_delta->data;
> +	uint8_t *data = sigma_delta->reg_data;
>  	int ret;
>  	struct spi_transfer t[] = {
>  		{
> @@ -148,24 +148,24 @@ int ad_sd_read_reg(struct ad_sigma_delta *sigma_delta,
>  {
>  	int ret;
>  
> -	ret = ad_sd_read_reg_raw(sigma_delta, reg, size, sigma_delta->data);
> +	ret = ad_sd_read_reg_raw(sigma_delta, reg, size, sigma_delta->reg_data);
>  	if (ret < 0)
>  		goto out;
>  
>  	switch (size) {
>  	case 4:
> -		*val = get_unaligned_be32(sigma_delta->data);
> +		*val = get_unaligned_be32(sigma_delta->reg_data);
>  		break;
>  	case 3:
> -		*val = (sigma_delta->data[0] << 16) |
> -			(sigma_delta->data[1] << 8) |
> -			sigma_delta->data[2];
> +		*val = (sigma_delta->reg_data[0] << 16) |
> +			(sigma_delta->reg_data[1] << 8) |
> +			sigma_delta->reg_data[2];
>  		break;
>  	case 2:
> -		*val = get_unaligned_be16(sigma_delta->data);
> +		*val = get_unaligned_be16(sigma_delta->reg_data);
>  		break;
>  	case 1:
> -		*val = sigma_delta->data[0];
> +		*val = sigma_delta->reg_data[0];
>  		break;
>  	default:
>  		ret = -EINVAL;
> @@ -403,12 +403,12 @@ static irqreturn_t ad_sd_trigger_handler(int irq, void *p)
>  	struct iio_poll_func *pf = p;
>  	struct iio_dev *indio_dev = pf->indio_dev;
>  	struct ad_sigma_delta *sigma_delta = iio_device_get_drvdata(indio_dev);
> +	uint8_t *data = sigma_delta->buf_data;
>  	unsigned int reg_size;
>  	unsigned int data_reg;
> -	uint8_t data[16];
>  	int ret;
>  
> -	memset(data, 0x00, 16);
> +	memset(sigma_delta->buf_data, 0x00, sizeof(sigma_delta->buf_data));
>  
>  	reg_size = indio_dev->channels[0].scan_type.realbits +
>  			indio_dev->channels[0].scan_type.shift;
> diff --git a/include/linux/iio/adc/ad_sigma_delta.h b/include/linux/iio/adc/ad_sigma_delta.h
> index 6e9fb1932dde..fb77b2ebe498 100644
> --- a/include/linux/iio/adc/ad_sigma_delta.h
> +++ b/include/linux/iio/adc/ad_sigma_delta.h
> @@ -79,7 +79,8 @@ struct ad_sigma_delta {
>  	 * DMA (thus cache coherency maintenance) requires the
>  	 * transfer buffers to live in their own cache lines.
>  	 */
> -	uint8_t				data[4] ____cacheline_aligned;
> +	uint8_t				reg_data[4] ____cacheline_aligned;
> +	uint8_t				buf_data[16];
>  };
>  
>  static inline int ad_sigma_delta_set_channel(struct ad_sigma_delta *sd,

Re: [PATCH] iio: ad_sigma_delta: Don't put SPI transfer buffer on the stack

[Ardelean, Alexandru]

On Sun, 2019-04-14 at 13:45 +0100, Jonathan Cameron wrote:
> [External]
> 
> 
> On Tue, 9 Apr 2019 10:58:21 +0300
> Alexandru Ardelean <alexandru.ardelean@analog.com> wrote:
> 
> > From: Lars-Peter Clausen <lars@metafoo.de>
> > 
> > Use a heap allocated memory for the SPI transfer buffer. Using stack
> > memory
> > will not work on some architectures when using DMA.
> > 
> > Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
> > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
> 
> Yikes, surprised this one snuck in here.
> > ---
> > 
> > Note: this doesn't need a stable/fixes tag. It's more of an
> > enhancement.
> 
> Talk me through this...  Right now I think this is a bug that is going
> to cause random and extremely hard to debug problems.  I've had these
> in the past and they are really really nasty as it's random stack
> corruption.
> The nasty is not the fact it's on the stack, but the fact that it isn't
> in it's own cacheline.
> 
> So my inclination is this definitely needs stable and fixes tag.
> What am I missing?

I guess I'm not that great with discerning stuff (about whether it needs a
stable & fixes tag).
I guess I'm still in an early phase with this one.

My sentiment, is that there haven't been enough support questions about
weird issues with sigma-delta to have categorized this earlier.

But, I guess you're right. This could do with a Fixes/stable tag.

Thanks
Alex

> 
> Jonathan
> 
> > 
> >  drivers/iio/adc/ad_sigma_delta.c       | 22 +++++++++++-----------
> >  include/linux/iio/adc/ad_sigma_delta.h |  3 ++-
> >  2 files changed, 13 insertions(+), 12 deletions(-)
> > 
> > diff --git a/drivers/iio/adc/ad_sigma_delta.c
> > b/drivers/iio/adc/ad_sigma_delta.c
> > index af6cbc683214..02d9049b9218 100644
> > --- a/drivers/iio/adc/ad_sigma_delta.c
> > +++ b/drivers/iio/adc/ad_sigma_delta.c
> > @@ -58,7 +58,7 @@ EXPORT_SYMBOL_GPL(ad_sd_set_comm);
> >  int ad_sd_write_reg(struct ad_sigma_delta *sigma_delta, unsigned int
> > reg,
> >       unsigned int size, unsigned int val)
> >  {
> > -     uint8_t *data = sigma_delta->data;
> > +     uint8_t *data = sigma_delta->reg_data;
> >       struct spi_transfer t = {
> >               .tx_buf         = data,
> >               .len            = size + 1,
> > @@ -102,7 +102,7 @@ EXPORT_SYMBOL_GPL(ad_sd_write_reg);
> >  static int ad_sd_read_reg_raw(struct ad_sigma_delta *sigma_delta,
> >       unsigned int reg, unsigned int size, uint8_t *val)
> >  {
> > -     uint8_t *data = sigma_delta->data;
> > +     uint8_t *data = sigma_delta->reg_data;
> >       int ret;
> >       struct spi_transfer t[] = {
> >               {
> > @@ -148,24 +148,24 @@ int ad_sd_read_reg(struct ad_sigma_delta
> > *sigma_delta,
> >  {
> >       int ret;
> > 
> > -     ret = ad_sd_read_reg_raw(sigma_delta, reg, size, sigma_delta-
> > >data);
> > +     ret = ad_sd_read_reg_raw(sigma_delta, reg, size, sigma_delta-
> > >reg_data);
> >       if (ret < 0)
> >               goto out;
> > 
> >       switch (size) {
> >       case 4:
> > -             *val = get_unaligned_be32(sigma_delta->data);
> > +             *val = get_unaligned_be32(sigma_delta->reg_data);
> >               break;
> >       case 3:
> > -             *val = (sigma_delta->data[0] << 16) |
> > -                     (sigma_delta->data[1] << 8) |
> > -                     sigma_delta->data[2];
> > +             *val = (sigma_delta->reg_data[0] << 16) |
> > +                     (sigma_delta->reg_data[1] << 8) |
> > +                     sigma_delta->reg_data[2];
> >               break;
> >       case 2:
> > -             *val = get_unaligned_be16(sigma_delta->data);
> > +             *val = get_unaligned_be16(sigma_delta->reg_data);
> >               break;
> >       case 1:
> > -             *val = sigma_delta->data[0];
> > +             *val = sigma_delta->reg_data[0];
> >               break;
> >       default:
> >               ret = -EINVAL;
> > @@ -403,12 +403,12 @@ static irqreturn_t ad_sd_trigger_handler(int irq,
> > void *p)
> >       struct iio_poll_func *pf = p;
> >       struct iio_dev *indio_dev = pf->indio_dev;
> >       struct ad_sigma_delta *sigma_delta =
> > iio_device_get_drvdata(indio_dev);
> > +     uint8_t *data = sigma_delta->buf_data;
> >       unsigned int reg_size;
> >       unsigned int data_reg;
> > -     uint8_t data[16];
> >       int ret;
> > 
> > -     memset(data, 0x00, 16);
> > +     memset(sigma_delta->buf_data, 0x00, sizeof(sigma_delta-
> > >buf_data));
> > 
> >       reg_size = indio_dev->channels[0].scan_type.realbits +
> >                       indio_dev->channels[0].scan_type.shift;
> > diff --git a/include/linux/iio/adc/ad_sigma_delta.h
> > b/include/linux/iio/adc/ad_sigma_delta.h
> > index 6e9fb1932dde..fb77b2ebe498 100644
> > --- a/include/linux/iio/adc/ad_sigma_delta.h
> > +++ b/include/linux/iio/adc/ad_sigma_delta.h
> > @@ -79,7 +79,8 @@ struct ad_sigma_delta {
> >        * DMA (thus cache coherency maintenance) requires the
> >        * transfer buffers to live in their own cache lines.
> >        */
> > -     uint8_t                         data[4] ____cacheline_aligned;
> > +     uint8_t                         reg_data[4]
> > ____cacheline_aligned;
> > +     uint8_t                         buf_data[16];
> >  };
> > 
> >  static inline int ad_sigma_delta_set_channel(struct ad_sigma_delta
> > *sd,
> 
> 

[PATCH V2] iio: ad_sigma_delta: Don't put SPI transfer buffer on the stack

[Alexandru Ardelean]

From: Lars-Peter Clausen <lars@metafoo.de>

Use a heap allocated memory for the SPI transfer buffer. Using stack memory
will not work on some architectures when using DMA.

This also avoids potential stack corruption, as it makes sure that the
buffer is in it's own cacheline and (obviously) not on the stack.

Fixes: af3008485ea03 ("iio:adc: Add common code for ADI Sigma Delta devices")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
---
Changelog v1 -> v2:
* added ____cacheline_aligned to `buf_data`
* extended comment about cacheline
* added fixes tag

 drivers/iio/adc/ad_sigma_delta.c       | 22 +++++++++++-----------
 include/linux/iio/adc/ad_sigma_delta.h |  3 ++-
 2 files changed, 13 insertions(+), 12 deletions(-)

diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
index af6cbc683214..02d9049b9218 100644
--- a/drivers/iio/adc/ad_sigma_delta.c
+++ b/drivers/iio/adc/ad_sigma_delta.c
@@ -58,7 +58,7 @@ EXPORT_SYMBOL_GPL(ad_sd_set_comm);
 int ad_sd_write_reg(struct ad_sigma_delta *sigma_delta, unsigned int reg,
 	unsigned int size, unsigned int val)
 {
-	uint8_t *data = sigma_delta->data;
+	uint8_t *data = sigma_delta->reg_data;
 	struct spi_transfer t = {
 		.tx_buf		= data,
 		.len		= size + 1,
@@ -102,7 +102,7 @@ EXPORT_SYMBOL_GPL(ad_sd_write_reg);
 static int ad_sd_read_reg_raw(struct ad_sigma_delta *sigma_delta,
 	unsigned int reg, unsigned int size, uint8_t *val)
 {
-	uint8_t *data = sigma_delta->data;
+	uint8_t *data = sigma_delta->reg_data;
 	int ret;
 	struct spi_transfer t[] = {
 		{
@@ -148,24 +148,24 @@ int ad_sd_read_reg(struct ad_sigma_delta *sigma_delta,
 {
 	int ret;
 
-	ret = ad_sd_read_reg_raw(sigma_delta, reg, size, sigma_delta->data);
+	ret = ad_sd_read_reg_raw(sigma_delta, reg, size, sigma_delta->reg_data);
 	if (ret < 0)
 		goto out;
 
 	switch (size) {
 	case 4:
-		*val = get_unaligned_be32(sigma_delta->data);
+		*val = get_unaligned_be32(sigma_delta->reg_data);
 		break;
 	case 3:
-		*val = (sigma_delta->data[0] << 16) |
-			(sigma_delta->data[1] << 8) |
-			sigma_delta->data[2];
+		*val = (sigma_delta->reg_data[0] << 16) |
+			(sigma_delta->reg_data[1] << 8) |
+			sigma_delta->reg_data[2];
 		break;
 	case 2:
-		*val = get_unaligned_be16(sigma_delta->data);
+		*val = get_unaligned_be16(sigma_delta->reg_data);
 		break;
 	case 1:
-		*val = sigma_delta->data[0];
+		*val = sigma_delta->reg_data[0];
 		break;
 	default:
 		ret = -EINVAL;
@@ -403,12 +403,12 @@ static irqreturn_t ad_sd_trigger_handler(int irq, void *p)
 	struct iio_poll_func *pf = p;
 	struct iio_dev *indio_dev = pf->indio_dev;
 	struct ad_sigma_delta *sigma_delta = iio_device_get_drvdata(indio_dev);
+	uint8_t *data = sigma_delta->buf_data;
 	unsigned int reg_size;
 	unsigned int data_reg;
-	uint8_t data[16];
 	int ret;
 
-	memset(data, 0x00, 16);
+	memset(sigma_delta->buf_data, 0x00, sizeof(sigma_delta->buf_data));
 
 	reg_size = indio_dev->channels[0].scan_type.realbits +
 			indio_dev->channels[0].scan_type.shift;
diff --git a/include/linux/iio/adc/ad_sigma_delta.h b/include/linux/iio/adc/ad_sigma_delta.h
index 6e9fb1932dde..d95216ce8a94 100644
--- a/include/linux/iio/adc/ad_sigma_delta.h
+++ b/include/linux/iio/adc/ad_sigma_delta.h
@@ -79,7 +79,8 @@ struct ad_sigma_delta {
 	 * DMA (thus cache coherency maintenance) requires the
 	 * transfer buffers to live in their own cache lines.
 	 */
-	uint8_t				data[4] ____cacheline_aligned;
+	uint8_t				reg_data[4] ____cacheline_aligned;
+	uint8_t				buf_data[16] ____cacheline_aligned;
 };
 
 static inline int ad_sigma_delta_set_channel(struct ad_sigma_delta *sd,
-- 
2.17.1

Re: [PATCH V2] iio: ad_sigma_delta: Don't put SPI transfer buffer on the stack

[Jonathan Cameron]

On Tue, 16 Apr 2019 11:19:16 +0300
Alexandru Ardelean <alexandru.ardelean@analog.com> wrote:

> From: Lars-Peter Clausen <lars@metafoo.de>
> 
> Use a heap allocated memory for the SPI transfer buffer. Using stack memory
> will not work on some architectures when using DMA.
> 
> This also avoids potential stack corruption, as it makes sure that the
> buffer is in it's own cacheline and (obviously) not on the stack.
> 
> Fixes: af3008485ea03 ("iio:adc: Add common code for ADI Sigma Delta devices")
> Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
> Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Hi Alex,

Thanks for the update.

I'm unclear why we can't just use one large buffer for both cases?
i.e put the reg_data usecases in the buf_data buffer (renamed).

Jonathan
> ---
> Changelog v1 -> v2:
> * added ____cacheline_aligned to `buf_data`
> * extended comment about cacheline
> * added fixes tag
> 
>  drivers/iio/adc/ad_sigma_delta.c       | 22 +++++++++++-----------
>  include/linux/iio/adc/ad_sigma_delta.h |  3 ++-
>  2 files changed, 13 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
> index af6cbc683214..02d9049b9218 100644
> --- a/drivers/iio/adc/ad_sigma_delta.c
> +++ b/drivers/iio/adc/ad_sigma_delta.c
> @@ -58,7 +58,7 @@ EXPORT_SYMBOL_GPL(ad_sd_set_comm);
>  int ad_sd_write_reg(struct ad_sigma_delta *sigma_delta, unsigned int reg,
>  	unsigned int size, unsigned int val)
>  {
> -	uint8_t *data = sigma_delta->data;
> +	uint8_t *data = sigma_delta->reg_data;
>  	struct spi_transfer t = {
>  		.tx_buf		= data,
>  		.len		= size + 1,
> @@ -102,7 +102,7 @@ EXPORT_SYMBOL_GPL(ad_sd_write_reg);
>  static int ad_sd_read_reg_raw(struct ad_sigma_delta *sigma_delta,
>  	unsigned int reg, unsigned int size, uint8_t *val)
>  {
> -	uint8_t *data = sigma_delta->data;
> +	uint8_t *data = sigma_delta->reg_data;
>  	int ret;
>  	struct spi_transfer t[] = {
>  		{
> @@ -148,24 +148,24 @@ int ad_sd_read_reg(struct ad_sigma_delta *sigma_delta,
>  {
>  	int ret;
>  
> -	ret = ad_sd_read_reg_raw(sigma_delta, reg, size, sigma_delta->data);
> +	ret = ad_sd_read_reg_raw(sigma_delta, reg, size, sigma_delta->reg_data);
>  	if (ret < 0)
>  		goto out;
>  
>  	switch (size) {
>  	case 4:
> -		*val = get_unaligned_be32(sigma_delta->data);
> +		*val = get_unaligned_be32(sigma_delta->reg_data);
>  		break;
>  	case 3:
> -		*val = (sigma_delta->data[0] << 16) |
> -			(sigma_delta->data[1] << 8) |
> -			sigma_delta->data[2];
> +		*val = (sigma_delta->reg_data[0] << 16) |
> +			(sigma_delta->reg_data[1] << 8) |
> +			sigma_delta->reg_data[2];
>  		break;
>  	case 2:
> -		*val = get_unaligned_be16(sigma_delta->data);
> +		*val = get_unaligned_be16(sigma_delta->reg_data);
>  		break;
>  	case 1:
> -		*val = sigma_delta->data[0];
> +		*val = sigma_delta->reg_data[0];
>  		break;
>  	default:
>  		ret = -EINVAL;
> @@ -403,12 +403,12 @@ static irqreturn_t ad_sd_trigger_handler(int irq, void *p)
>  	struct iio_poll_func *pf = p;
>  	struct iio_dev *indio_dev = pf->indio_dev;
>  	struct ad_sigma_delta *sigma_delta = iio_device_get_drvdata(indio_dev);
> +	uint8_t *data = sigma_delta->buf_data;
>  	unsigned int reg_size;
>  	unsigned int data_reg;
> -	uint8_t data[16];
>  	int ret;
>  
> -	memset(data, 0x00, 16);
> +	memset(sigma_delta->buf_data, 0x00, sizeof(sigma_delta->buf_data));
>  
>  	reg_size = indio_dev->channels[0].scan_type.realbits +
>  			indio_dev->channels[0].scan_type.shift;
> diff --git a/include/linux/iio/adc/ad_sigma_delta.h b/include/linux/iio/adc/ad_sigma_delta.h
> index 6e9fb1932dde..d95216ce8a94 100644
> --- a/include/linux/iio/adc/ad_sigma_delta.h
> +++ b/include/linux/iio/adc/ad_sigma_delta.h
> @@ -79,7 +79,8 @@ struct ad_sigma_delta {
>  	 * DMA (thus cache coherency maintenance) requires the
>  	 * transfer buffers to live in their own cache lines.
>  	 */
> -	uint8_t				data[4] ____cacheline_aligned;
> +	uint8_t				reg_data[4] ____cacheline_aligned;
> +	uint8_t				buf_data[16] ____cacheline_aligned;
The usual assumption is that a device is safe against itself, so we can't
get cacheline corruption in a single action from the device to itself.
This does make me wonder why we have two buffers like this at all though!

>  };
>  
>  static inline int ad_sigma_delta_set_channel(struct ad_sigma_delta *sd,

Re: [PATCH V2] iio: ad_sigma_delta: Don't put SPI transfer buffer on the stack

[Ardelean, Alexandru]

On Mon, 2019-04-22 at 09:56 +0100, Jonathan Cameron wrote:
> [External]
> 
> 
> On Tue, 16 Apr 2019 11:19:16 +0300
> Alexandru Ardelean <alexandru.ardelean@analog.com> wrote:
> 
> > From: Lars-Peter Clausen <lars@metafoo.de>
> > 
> > Use a heap allocated memory for the SPI transfer buffer. Using stack
> > memory
> > will not work on some architectures when using DMA.
> > 
> > This also avoids potential stack corruption, as it makes sure that the
> > buffer is in it's own cacheline and (obviously) not on the stack.
> > 
> > Fixes: af3008485ea03 ("iio:adc: Add common code for ADI Sigma Delta
> > devices")
> > Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
> > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
> 
> Hi Alex,
> 
> Thanks for the update.
> 
> I'm unclear why we can't just use one large buffer for both cases?
> i.e put the reg_data usecases in the buf_data buffer (renamed).
> 

That's a good point.
I didn't think much of it.


Looking through the code, the only argument for doing it like this
(separate buffer) is that the change is less complicated to look at, since
it makes no assumptions about offset in the buffer.

At a later point in time, this `buf_data` will be replaced with a kzalloc-
ed buffer [when adding sequencer support for SigmaDelta].

It may only make sense to have this change [whether 2 buffers or a single
buffer] for stable, to fix the cacheline issue.

I'll take a closer look at this.
It should work well to just increase the existing buffer and re-use that.


> Jonathan
> > ---
> > Changelog v1 -> v2:
> > * added ____cacheline_aligned to `buf_data`
> > * extended comment about cacheline
> > * added fixes tag
> > 
> >  drivers/iio/adc/ad_sigma_delta.c       | 22 +++++++++++-----------
> >  include/linux/iio/adc/ad_sigma_delta.h |  3 ++-
> >  2 files changed, 13 insertions(+), 12 deletions(-)
> > 
> > diff --git a/drivers/iio/adc/ad_sigma_delta.c
> > b/drivers/iio/adc/ad_sigma_delta.c
> > index af6cbc683214..02d9049b9218 100644
> > --- a/drivers/iio/adc/ad_sigma_delta.c
> > +++ b/drivers/iio/adc/ad_sigma_delta.c
> > @@ -58,7 +58,7 @@ EXPORT_SYMBOL_GPL(ad_sd_set_comm);
> >  int ad_sd_write_reg(struct ad_sigma_delta *sigma_delta, unsigned int
> > reg,
> >       unsigned int size, unsigned int val)
> >  {
> > -     uint8_t *data = sigma_delta->data;
> > +     uint8_t *data = sigma_delta->reg_data;
> >       struct spi_transfer t = {
> >               .tx_buf         = data,
> >               .len            = size + 1,
> > @@ -102,7 +102,7 @@ EXPORT_SYMBOL_GPL(ad_sd_write_reg);
> >  static int ad_sd_read_reg_raw(struct ad_sigma_delta *sigma_delta,
> >       unsigned int reg, unsigned int size, uint8_t *val)
> >  {
> > -     uint8_t *data = sigma_delta->data;
> > +     uint8_t *data = sigma_delta->reg_data;
> >       int ret;
> >       struct spi_transfer t[] = {
> >               {
> > @@ -148,24 +148,24 @@ int ad_sd_read_reg(struct ad_sigma_delta
> > *sigma_delta,
> >  {
> >       int ret;
> > 
> > -     ret = ad_sd_read_reg_raw(sigma_delta, reg, size, sigma_delta-
> > >data);
> > +     ret = ad_sd_read_reg_raw(sigma_delta, reg, size, sigma_delta-
> > >reg_data);
> >       if (ret < 0)
> >               goto out;
> > 
> >       switch (size) {
> >       case 4:
> > -             *val = get_unaligned_be32(sigma_delta->data);
> > +             *val = get_unaligned_be32(sigma_delta->reg_data);
> >               break;
> >       case 3:
> > -             *val = (sigma_delta->data[0] << 16) |
> > -                     (sigma_delta->data[1] << 8) |
> > -                     sigma_delta->data[2];
> > +             *val = (sigma_delta->reg_data[0] << 16) |
> > +                     (sigma_delta->reg_data[1] << 8) |
> > +                     sigma_delta->reg_data[2];
> >               break;
> >       case 2:
> > -             *val = get_unaligned_be16(sigma_delta->data);
> > +             *val = get_unaligned_be16(sigma_delta->reg_data);
> >               break;
> >       case 1:
> > -             *val = sigma_delta->data[0];
> > +             *val = sigma_delta->reg_data[0];
> >               break;
> >       default:
> >               ret = -EINVAL;
> > @@ -403,12 +403,12 @@ static irqreturn_t ad_sd_trigger_handler(int irq,
> > void *p)
> >       struct iio_poll_func *pf = p;
> >       struct iio_dev *indio_dev = pf->indio_dev;
> >       struct ad_sigma_delta *sigma_delta =
> > iio_device_get_drvdata(indio_dev);
> > +     uint8_t *data = sigma_delta->buf_data;
> >       unsigned int reg_size;
> >       unsigned int data_reg;
> > -     uint8_t data[16];
> >       int ret;
> > 
> > -     memset(data, 0x00, 16);
> > +     memset(sigma_delta->buf_data, 0x00, sizeof(sigma_delta-
> > >buf_data));
> > 
> >       reg_size = indio_dev->channels[0].scan_type.realbits +
> >                       indio_dev->channels[0].scan_type.shift;
> > diff --git a/include/linux/iio/adc/ad_sigma_delta.h
> > b/include/linux/iio/adc/ad_sigma_delta.h
> > index 6e9fb1932dde..d95216ce8a94 100644
> > --- a/include/linux/iio/adc/ad_sigma_delta.h
> > +++ b/include/linux/iio/adc/ad_sigma_delta.h
> > @@ -79,7 +79,8 @@ struct ad_sigma_delta {
> >        * DMA (thus cache coherency maintenance) requires the
> >        * transfer buffers to live in their own cache lines.
> >        */
> > -     uint8_t                         data[4] ____cacheline_aligned;
> > +     uint8_t                         reg_data[4]
> > ____cacheline_aligned;
> > +     uint8_t                         buf_data[16]
> > ____cacheline_aligned;
> 
> The usual assumption is that a device is safe against itself, so we can't
> get cacheline corruption in a single action from the device to itself.
> This does make me wonder why we have two buffers like this at all though!
> >  };
> > 
> >  static inline int ad_sigma_delta_set_channel(struct ad_sigma_delta
> > *sd,
> 
> 

[PATCH] iio: ad_sigma_delta: Don't put SPI transfer buffer on the stack

[Alexandru Ardelean]

From: Lars-Peter Clausen <lars@metafoo.de>

Use a heap allocated memory for the SPI transfer buffer. Using stack memory
can corrupt stack memory when using DMA on some systems.

This change adds 4 bytes at the end of the current DMA buffer, which will
be used by the trigger handler.
This is required because the first 4 bytes are reserved for register data.

Fixes: af3008485ea03 ("iio:adc: Add common code for ADI Sigma Delta devices")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
---
 drivers/iio/adc/ad_sigma_delta.c       | 4 ++--
 include/linux/iio/adc/ad_sigma_delta.h | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
index 86039e9ecaca..33297f26508a 100644
--- a/drivers/iio/adc/ad_sigma_delta.c
+++ b/drivers/iio/adc/ad_sigma_delta.c
@@ -395,11 +395,11 @@ static irqreturn_t ad_sd_trigger_handler(int irq, void *p)
 	struct iio_poll_func *pf = p;
 	struct iio_dev *indio_dev = pf->indio_dev;
 	struct ad_sigma_delta *sigma_delta = iio_device_get_drvdata(indio_dev);
+	uint8_t *data = &sigma_delta->data[4];
 	unsigned int reg_size;
 	unsigned int data_reg;
-	uint8_t data[16];
 
-	memset(data, 0x00, 16);
+	memset(data, 0x00, 4);
 
 	reg_size = indio_dev->channels[0].scan_type.realbits +
 			indio_dev->channels[0].scan_type.shift;
diff --git a/include/linux/iio/adc/ad_sigma_delta.h b/include/linux/iio/adc/ad_sigma_delta.h
index a3a838dcf8e4..ac4ac4752c62 100644
--- a/include/linux/iio/adc/ad_sigma_delta.h
+++ b/include/linux/iio/adc/ad_sigma_delta.h
@@ -80,7 +80,7 @@ struct ad_sigma_delta {
 	 * DMA (thus cache coherency maintenance) requires the
 	 * transfer buffers to live in their own cache lines.
 	 */
-	uint8_t				data[4] ____cacheline_aligned;
+	uint8_t				data[8] ____cacheline_aligned;
 };
 
 static inline int ad_sigma_delta_set_channel(struct ad_sigma_delta *sd,
-- 
2.17.1

[PATCH v2] iio: ad_sigma_delta: Don't put SPI transfer buffer on the stack

[Alexandru Ardelean]

From: Lars-Peter Clausen <lars@metafoo.de>

Use a heap allocated memory for the SPI transfer buffer. Using stack memory
can corrupt stack memory when using DMA on some systems.

This change moves the buffer from the stack of the trigger handler call to
the heap of the buffer of the state struct. The size increases takes into
account the alignment for the timestamp, which is 8 bytes.
So the buffer is put at an offset of 8 bytes.

Fixes: af3008485ea03 ("iio:adc: Add common code for ADI Sigma Delta devices")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
---

Changelog v1 -> v2:
* bumped the buffer on state struct to 24 bytes
* increased the offset to 8 bytes to account for the timestamp alignment

 drivers/iio/adc/ad_sigma_delta.c       | 2 +-
 include/linux/iio/adc/ad_sigma_delta.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
index 86039e9ecaca..9f730c9d6aaa 100644
--- a/drivers/iio/adc/ad_sigma_delta.c
+++ b/drivers/iio/adc/ad_sigma_delta.c
@@ -395,9 +395,9 @@ static irqreturn_t ad_sd_trigger_handler(int irq, void *p)
 	struct iio_poll_func *pf = p;
 	struct iio_dev *indio_dev = pf->indio_dev;
 	struct ad_sigma_delta *sigma_delta = iio_device_get_drvdata(indio_dev);
+	uint8_t *data = &sigma_delta->data[8];
 	unsigned int reg_size;
 	unsigned int data_reg;
-	uint8_t data[16];
 
 	memset(data, 0x00, 16);
 
diff --git a/include/linux/iio/adc/ad_sigma_delta.h b/include/linux/iio/adc/ad_sigma_delta.h
index a3a838dcf8e4..8fb74755f873 100644
--- a/include/linux/iio/adc/ad_sigma_delta.h
+++ b/include/linux/iio/adc/ad_sigma_delta.h
@@ -80,7 +80,7 @@ struct ad_sigma_delta {
 	 * DMA (thus cache coherency maintenance) requires the
 	 * transfer buffers to live in their own cache lines.
 	 */
-	uint8_t				data[4] ____cacheline_aligned;
+	uint8_t				data[24] ____cacheline_aligned;
 };
 
 static inline int ad_sigma_delta_set_channel(struct ad_sigma_delta *sd,
-- 
2.27.0

Re: [PATCH v2] iio: ad_sigma_delta: Don't put SPI transfer buffer on the stack

[Jonathan Cameron]

On Fri, 13 Nov 2020 11:40:59 +0200
Alexandru Ardelean <alexandru.ardelean@analog.com> wrote:

> From: Lars-Peter Clausen <lars@metafoo.de>
> 
> Use a heap allocated memory for the SPI transfer buffer. Using stack memory
> can corrupt stack memory when using DMA on some systems.
> 
> This change moves the buffer from the stack of the trigger handler call to
> the heap of the buffer of the state struct. The size increases takes into
> account the alignment for the timestamp, which is 8 bytes.
> So the buffer is put at an offset of 8 bytes.
> 
> Fixes: af3008485ea03 ("iio:adc: Add common code for ADI Sigma Delta devices")
> Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
> Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>

There are neater options for solving this problem - see inline.

In particular I don't think you have a problem with setting the
rx and tx buffers to use the same memory.

> ---
> 
> Changelog v1 -> v2:
> * bumped the buffer on state struct to 24 bytes
> * increased the offset to 8 bytes to account for the timestamp alignment
> 
>  drivers/iio/adc/ad_sigma_delta.c       | 2 +-
>  include/linux/iio/adc/ad_sigma_delta.h | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
> index 86039e9ecaca..9f730c9d6aaa 100644
> --- a/drivers/iio/adc/ad_sigma_delta.c
> +++ b/drivers/iio/adc/ad_sigma_delta.c
> @@ -395,9 +395,9 @@ static irqreturn_t ad_sd_trigger_handler(int irq, void *p)
>  	struct iio_poll_func *pf = p;
>  	struct iio_dev *indio_dev = pf->indio_dev;
>  	struct ad_sigma_delta *sigma_delta = iio_device_get_drvdata(indio_dev);
> +	uint8_t *data = &sigma_delta->data[8];
>  	unsigned int reg_size;
>  	unsigned int data_reg;
> -	uint8_t data[16];
>  
>  	memset(data, 0x00, 16);
>  
> diff --git a/include/linux/iio/adc/ad_sigma_delta.h b/include/linux/iio/adc/ad_sigma_delta.h
> index a3a838dcf8e4..8fb74755f873 100644
> --- a/include/linux/iio/adc/ad_sigma_delta.h
> +++ b/include/linux/iio/adc/ad_sigma_delta.h
> @@ -80,7 +80,7 @@ struct ad_sigma_delta {
>  	 * DMA (thus cache coherency maintenance) requires the
>  	 * transfer buffers to live in their own cache lines.
>  	 */

If you do end up with something like this, it needs a clear explanation of 'why'
the size is 24 bytes.  No good just having it in the patch description.

> -	uint8_t				data[4] ____cacheline_aligned;
> +	uint8_t				data[24] ____cacheline_aligned;

This is downright confusing.  I'd just split the buffer into tx and rx
parts.   The first (doesn't matter which) needs to be marked __cacheline_aligned.
If the rx is second mark it __aligned(8) to force that to be appropriate for
the timestamp.

Or... (I haven't checked thoroughly for this from point of view of how it is used
in the drivers) use the same buffer for tx and rx.  That is supposed to be safe for
SPI drivers though wonderfully there is a ? after the statement of that in
include/linux/spi.h.  I think that is just pointing out that microwire doesn't
support duplex rather than saying it's invalid in general...



>  };
>  
>  static inline int ad_sigma_delta_set_channel(struct ad_sigma_delta *sd,

Re: [PATCH v2] iio: ad_sigma_delta: Don't put SPI transfer buffer on the stack

[Alexandru Ardelean]

On Sat, Nov 14, 2020 at 6:20 PM Jonathan Cameron <jic23@kernel.org> wrote:
>
> On Fri, 13 Nov 2020 11:40:59 +0200
> Alexandru Ardelean <alexandru.ardelean@analog.com> wrote:
>
> > From: Lars-Peter Clausen <lars@metafoo.de>
> >
> > Use a heap allocated memory for the SPI transfer buffer. Using stack memory
> > can corrupt stack memory when using DMA on some systems.
> >
> > This change moves the buffer from the stack of the trigger handler call to
> > the heap of the buffer of the state struct. The size increases takes into
> > account the alignment for the timestamp, which is 8 bytes.
> > So the buffer is put at an offset of 8 bytes.
> >
> > Fixes: af3008485ea03 ("iio:adc: Add common code for ADI Sigma Delta devices")
> > Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
> > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
>
> There are neater options for solving this problem - see inline.
>
> In particular I don't think you have a problem with setting the
> rx and tx buffers to use the same memory.
>
> > ---
> >
> > Changelog v1 -> v2:
> > * bumped the buffer on state struct to 24 bytes
> > * increased the offset to 8 bytes to account for the timestamp alignment
> >
> >  drivers/iio/adc/ad_sigma_delta.c       | 2 +-
> >  include/linux/iio/adc/ad_sigma_delta.h | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
> > index 86039e9ecaca..9f730c9d6aaa 100644
> > --- a/drivers/iio/adc/ad_sigma_delta.c
> > +++ b/drivers/iio/adc/ad_sigma_delta.c
> > @@ -395,9 +395,9 @@ static irqreturn_t ad_sd_trigger_handler(int irq, void *p)
> >       struct iio_poll_func *pf = p;
> >       struct iio_dev *indio_dev = pf->indio_dev;
> >       struct ad_sigma_delta *sigma_delta = iio_device_get_drvdata(indio_dev);
> > +     uint8_t *data = &sigma_delta->data[8];
> >       unsigned int reg_size;
> >       unsigned int data_reg;
> > -     uint8_t data[16];
> >
> >       memset(data, 0x00, 16);
> >
> > diff --git a/include/linux/iio/adc/ad_sigma_delta.h b/include/linux/iio/adc/ad_sigma_delta.h
> > index a3a838dcf8e4..8fb74755f873 100644
> > --- a/include/linux/iio/adc/ad_sigma_delta.h
> > +++ b/include/linux/iio/adc/ad_sigma_delta.h
> > @@ -80,7 +80,7 @@ struct ad_sigma_delta {
> >        * DMA (thus cache coherency maintenance) requires the
> >        * transfer buffers to live in their own cache lines.
> >        */
>
> If you do end up with something like this, it needs a clear explanation of 'why'
> the size is 24 bytes.  No good just having it in the patch description.
>
> > -     uint8_t                         data[4] ____cacheline_aligned;
> > +     uint8_t                         data[24] ____cacheline_aligned;
>
> This is downright confusing.  I'd just split the buffer into tx and rx
> parts.   The first (doesn't matter which) needs to be marked __cacheline_aligned.
> If the rx is second mark it __aligned(8) to force that to be appropriate for
> the timestamp.
>
> Or... (I haven't checked thoroughly for this from point of view of how it is used
> in the drivers) use the same buffer for tx and rx.  That is supposed to be safe for
> SPI drivers though wonderfully there is a ? after the statement of that in
> include/linux/spi.h.  I think that is just pointing out that microwire doesn't
> support duplex rather than saying it's invalid in general...

I'm a bit paranoid to use the same buffer for RX & TX [in general].
It sounds like this could hide some bugs in some weird DMA implementations.
The DMA implementations could be fine on their own, but they wouldn't
expect that TX & RX buffers point to the same place.

>
>
>
> >  };
> >
> >  static inline int ad_sigma_delta_set_channel(struct ad_sigma_delta *sd,
>

Re: [PATCH v2] iio: ad_sigma_delta: Don't put SPI transfer buffer on the stack

[Jonathan Cameron]

On Thu, 19 Nov 2020 10:30:02 +0200
Alexandru Ardelean <ardeleanalex@gmail.com> wrote:

> On Sat, Nov 14, 2020 at 6:20 PM Jonathan Cameron <jic23@kernel.org> wrote:
> >
> > On Fri, 13 Nov 2020 11:40:59 +0200
> > Alexandru Ardelean <alexandru.ardelean@analog.com> wrote:
> >  
> > > From: Lars-Peter Clausen <lars@metafoo.de>
> > >
> > > Use a heap allocated memory for the SPI transfer buffer. Using stack memory
> > > can corrupt stack memory when using DMA on some systems.
> > >
> > > This change moves the buffer from the stack of the trigger handler call to
> > > the heap of the buffer of the state struct. The size increases takes into
> > > account the alignment for the timestamp, which is 8 bytes.
> > > So the buffer is put at an offset of 8 bytes.
> > >
> > > Fixes: af3008485ea03 ("iio:adc: Add common code for ADI Sigma Delta devices")
> > > Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
> > > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>  
> >
> > There are neater options for solving this problem - see inline.
> >
> > In particular I don't think you have a problem with setting the
> > rx and tx buffers to use the same memory.
> >  
> > > ---
> > >
> > > Changelog v1 -> v2:
> > > * bumped the buffer on state struct to 24 bytes
> > > * increased the offset to 8 bytes to account for the timestamp alignment
> > >
> > >  drivers/iio/adc/ad_sigma_delta.c       | 2 +-
> > >  include/linux/iio/adc/ad_sigma_delta.h | 2 +-
> > >  2 files changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
> > > index 86039e9ecaca..9f730c9d6aaa 100644
> > > --- a/drivers/iio/adc/ad_sigma_delta.c
> > > +++ b/drivers/iio/adc/ad_sigma_delta.c
> > > @@ -395,9 +395,9 @@ static irqreturn_t ad_sd_trigger_handler(int irq, void *p)
> > >       struct iio_poll_func *pf = p;
> > >       struct iio_dev *indio_dev = pf->indio_dev;
> > >       struct ad_sigma_delta *sigma_delta = iio_device_get_drvdata(indio_dev);
> > > +     uint8_t *data = &sigma_delta->data[8];
> > >       unsigned int reg_size;
> > >       unsigned int data_reg;
> > > -     uint8_t data[16];
> > >
> > >       memset(data, 0x00, 16);
> > >
> > > diff --git a/include/linux/iio/adc/ad_sigma_delta.h b/include/linux/iio/adc/ad_sigma_delta.h
> > > index a3a838dcf8e4..8fb74755f873 100644
> > > --- a/include/linux/iio/adc/ad_sigma_delta.h
> > > +++ b/include/linux/iio/adc/ad_sigma_delta.h
> > > @@ -80,7 +80,7 @@ struct ad_sigma_delta {
> > >        * DMA (thus cache coherency maintenance) requires the
> > >        * transfer buffers to live in their own cache lines.
> > >        */  
> >
> > If you do end up with something like this, it needs a clear explanation of 'why'
> > the size is 24 bytes.  No good just having it in the patch description.
> >  
> > > -     uint8_t                         data[4] ____cacheline_aligned;
> > > +     uint8_t                         data[24] ____cacheline_aligned;  
> >
> > This is downright confusing.  I'd just split the buffer into tx and rx
> > parts.   The first (doesn't matter which) needs to be marked __cacheline_aligned.
> > If the rx is second mark it __aligned(8) to force that to be appropriate for
> > the timestamp.
> >
> > Or... (I haven't checked thoroughly for this from point of view of how it is used
> > in the drivers) use the same buffer for tx and rx.  That is supposed to be safe for
> > SPI drivers though wonderfully there is a ? after the statement of that in
> > include/linux/spi.h.  I think that is just pointing out that microwire doesn't
> > support duplex rather than saying it's invalid in general...  
> 
> I'm a bit paranoid to use the same buffer for RX & TX [in general].
> It sounds like this could hide some bugs in some weird DMA implementations.
> The DMA implementations could be fine on their own, but they wouldn't
> expect that TX & RX buffers point to the same place.

Whilst in theory it should either be fine, or the hardware should use
a bounce buffer if it's not, I can understand your paranoia so
I'm fine with separate buffers.

Jonathan

> 
> >
> >
> >  
> > >  };
> > >
> > >  static inline int ad_sigma_delta_set_channel(struct ad_sigma_delta *sd,  
> >