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@xxxxxxxxxx> wrote:
> > On 11/12/20 10:10 AM, Alexandru Ardelean wrote:
> > > From: Lars-Peter Clausen <lars@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx>
> > > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@xxxxxxxxxx>
> > > ---
> > >    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.