[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
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

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

[Lars-Peter Clausen]

On 11/12/20 10:10 AM, Alexandru Ardelean 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 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);

Younger me didn't know what he was doing, this is wrong. We need the 
extra space for the padding and timestamp.

We also can't put the beginning of the buffer at an 4 byte offset since 
it needs to be 8 byte aligned for the timestamp.

>   
>   	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,

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

[Alexandru Ardelean]

On Thu, Nov 12, 2020 at 11:55 AM Lars-Peter Clausen <lars@metafoo.de> wrote:
>
> On 11/12/20 10:10 AM, Alexandru Ardelean 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 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);
>
> Younger me didn't know what he was doing, this is wrong. We need the
> extra space for the padding and timestamp.
>
> We also can't put the beginning of the buffer at an 4 byte offset since
> it needs to be 8 byte aligned for the timestamp.

I'll correct this.
I was re-spinning this out of some old patches and discussions on this
that I have.
So, then this becomes 24 bytes? Or 16?

Something like:
uint8_t                         data[24] ____cacheline_aligned;

uint8_t *data = &sigma_delta->data[8];


>
> >
> >       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,
>
>

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

[Lars-Peter Clausen]

On 11/12/20 11:14 AM, Alexandru Ardelean wrote:
> On Thu, Nov 12, 2020 at 11:55 AM Lars-Peter Clausen <lars@metafoo.de> wrote:
>> On 11/12/20 10:10 AM, Alexandru Ardelean 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 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);
>> Younger me didn't know what he was doing, this is wrong. We need the
>> extra space for the padding and timestamp.
>>
>> We also can't put the beginning of the buffer at an 4 byte offset since
>> it needs to be 8 byte aligned for the timestamp.
> I'll correct this.
> I was re-spinning this out of some old patches and discussions on this
> that I have.
> So, then this becomes 24 bytes? Or 16?
>
> Something like:
> uint8_t                         data[24] ____cacheline_aligned;
>
> uint8_t *data = &sigma_delta->data[8];

Yes.

[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,  
> >  

[PATCH v3] 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.

The 'data' buffer is split into 'tx_buf' and 'rx_buf', to make a clearer
separation of which part of the buffer should be used for TX & RX.

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       | 18 ++++++++----------
 include/linux/iio/adc/ad_sigma_delta.h |  6 +++++-
 2 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
index 86039e9ecaca..3a6f239d4acc 100644
--- a/drivers/iio/adc/ad_sigma_delta.c
+++ b/drivers/iio/adc/ad_sigma_delta.c
@@ -57,7 +57,7 @@ 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->tx_buf;
 	struct spi_transfer t = {
 		.tx_buf		= data,
 		.len		= size + 1,
@@ -99,7 +99,7 @@ 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->tx_buf;
 	int ret;
 	struct spi_transfer t[] = {
 		{
@@ -146,22 +146,22 @@ 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->rx_buf);
 	if (ret < 0)
 		goto out;
 
 	switch (size) {
 	case 4:
-		*val = get_unaligned_be32(sigma_delta->data);
+		*val = get_unaligned_be32(sigma_delta->rx_buf);
 		break;
 	case 3:
-		*val = get_unaligned_be24(&sigma_delta->data[0]);
+		*val = get_unaligned_be24(sigma_delta->rx_buf);
 		break;
 	case 2:
-		*val = get_unaligned_be16(sigma_delta->data);
+		*val = get_unaligned_be16(sigma_delta->rx_buf);
 		break;
 	case 1:
-		*val = sigma_delta->data[0];
+		*val = sigma_delta->rx_buf[0];
 		break;
 	default:
 		ret = -EINVAL;
@@ -395,11 +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->rx_buf;
 	unsigned int reg_size;
 	unsigned int data_reg;
-	uint8_t data[16];
-
-	memset(data, 0x00, 16);
 
 	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..efc2413abaf4 100644
--- a/include/linux/iio/adc/ad_sigma_delta.h
+++ b/include/linux/iio/adc/ad_sigma_delta.h
@@ -79,8 +79,12 @@ struct ad_sigma_delta {
 	/*
 	 * DMA (thus cache coherency maintenance) requires the
 	 * transfer buffers to live in their own cache lines.
+	 * 'tx_buf' is up to 32 bits.
+	 * 'rx_buf' is up to 32 bits per sample + 64 bit timestamp,
+	 * rounded to 16 bytes to take into account padding.
 	 */
-	uint8_t				data[4] ____cacheline_aligned;
+	uint8_t				tx_buf[4] ____cacheline_aligned;
+	uint8_t				rx_buf[16] __aligned(8);
 };
 
 static inline int ad_sigma_delta_set_channel(struct ad_sigma_delta *sd,
-- 
2.17.1

Re: [PATCH v3] iio: ad_sigma_delta: Don't put SPI transfer buffer on the stack

[Jonathan Cameron]

On Tue, 24 Nov 2020 14:38:07 +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.
> 
> The 'data' buffer is split into 'tx_buf' and 'rx_buf', to make a clearer
> separation of which part of the buffer should be used for TX & RX.
> 
> 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>
Given where we are in the cycle and the fact this has been there a long time
I've queued it up for the next merge window.  Also marked it for stable so
it will get backported in the medium term.

Applied to the togreg branch of iio.git and pushed out as testing for
the autobuilders to do their usual magic.

Thanks,

Jonathan

> ---
>  drivers/iio/adc/ad_sigma_delta.c       | 18 ++++++++----------
>  include/linux/iio/adc/ad_sigma_delta.h |  6 +++++-
>  2 files changed, 13 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
> index 86039e9ecaca..3a6f239d4acc 100644
> --- a/drivers/iio/adc/ad_sigma_delta.c
> +++ b/drivers/iio/adc/ad_sigma_delta.c
> @@ -57,7 +57,7 @@ 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->tx_buf;
>  	struct spi_transfer t = {
>  		.tx_buf		= data,
>  		.len		= size + 1,
> @@ -99,7 +99,7 @@ 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->tx_buf;
>  	int ret;
>  	struct spi_transfer t[] = {
>  		{
> @@ -146,22 +146,22 @@ 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->rx_buf);
>  	if (ret < 0)
>  		goto out;
>  
>  	switch (size) {
>  	case 4:
> -		*val = get_unaligned_be32(sigma_delta->data);
> +		*val = get_unaligned_be32(sigma_delta->rx_buf);
>  		break;
>  	case 3:
> -		*val = get_unaligned_be24(&sigma_delta->data[0]);
> +		*val = get_unaligned_be24(sigma_delta->rx_buf);
>  		break;
>  	case 2:
> -		*val = get_unaligned_be16(sigma_delta->data);
> +		*val = get_unaligned_be16(sigma_delta->rx_buf);
>  		break;
>  	case 1:
> -		*val = sigma_delta->data[0];
> +		*val = sigma_delta->rx_buf[0];
>  		break;
>  	default:
>  		ret = -EINVAL;
> @@ -395,11 +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->rx_buf;
>  	unsigned int reg_size;
>  	unsigned int data_reg;
> -	uint8_t data[16];
> -
> -	memset(data, 0x00, 16);
>  
>  	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..efc2413abaf4 100644
> --- a/include/linux/iio/adc/ad_sigma_delta.h
> +++ b/include/linux/iio/adc/ad_sigma_delta.h
> @@ -79,8 +79,12 @@ struct ad_sigma_delta {
>  	/*
>  	 * DMA (thus cache coherency maintenance) requires the
>  	 * transfer buffers to live in their own cache lines.
> +	 * 'tx_buf' is up to 32 bits.
> +	 * 'rx_buf' is up to 32 bits per sample + 64 bit timestamp,
> +	 * rounded to 16 bytes to take into account padding.
>  	 */
> -	uint8_t				data[4] ____cacheline_aligned;
> +	uint8_t				tx_buf[4] ____cacheline_aligned;
> +	uint8_t				rx_buf[16] __aligned(8);
>  };
>  
>  static inline int ad_sigma_delta_set_channel(struct ad_sigma_delta *sd,