[PATCH v1 4/5] drivers: media: bcm2835-unicam: Fix for possible dummy buffer overrun

[Jacopo Mondi]

Hi Naush

On Fri, Nov 22, 2024 at 11:35:59AM +0000, Naushir Patuck wrote:
> Hi Jacopo,

Thanks for the explanation

>
> On Fri, 22 Nov 2024 at 11:20, Jacopo Mondi
> <jacopo.mondi at ideasonboard.com> wrote:
> >
> > Hi Naush
> >
> > On Fri, Nov 22, 2024 at 08:41:51AM +0000, Naushir Patuck wrote:
> > > The Unicam hardware has been observed to cause a buffer overrun when
> > > using the dummy buffer as a circular buffer. The conditions that cause
> > > the overrun are not fully known, but it seems to occur when the memory
> > > bus is heavily loaded.
> > >
> > > To avoid the overrun, program the hardware with a buffer size of 0 when
> > > using the dummy buffer. This will cause overrun into the allocated dummy
> > > buffer, but avoid out of bounds writes.
> > >
> > > Signed-off-by: Naushir Patuck <naush at raspberrypi.com>

Reviewed-by: Jacopo Mondi <jacopo.mondi at ideasonboard.com>

Thanks
  j

> > > ---
> > >  drivers/media/platform/broadcom/bcm2835-unicam.c | 9 ++++++++-
> > >  1 file changed, 8 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/media/platform/broadcom/bcm2835-unicam.c b/drivers/media/platform/broadcom/bcm2835-unicam.c
> > > index 550eb1b064f1..f10064107d54 100644
> > > --- a/drivers/media/platform/broadcom/bcm2835-unicam.c
> > > +++ b/drivers/media/platform/broadcom/bcm2835-unicam.c
> > > @@ -640,7 +640,14 @@ static inline void unicam_reg_write_field(struct unicam_device *unicam, u32 offs
> > >  static void unicam_wr_dma_addr(struct unicam_node *node,
> > >                              struct unicam_buffer *buf)
> > >  {
> > > -     dma_addr_t endaddr = buf->dma_addr + buf->size;
> > > +     /*
> > > +      * Due to a HW bug causing buffer overruns in circular buffer mode under
> > > +      * certain (not yet fully known) conditions, the dummy buffer allocation
> > > +      * is set to a a single page size, but the hardware gets programmed with
> > > +      * a buffer size of 0.
> > > +      */
> > > +     dma_addr_t endaddr = buf->dma_addr +
> > > +                          (buf != &node->dummy_buf ? buf->size : 0);
> >
> > So the DMA engine doesn't actually write any data to dummy_buf
> > anymore ?
> >
> >
> > Does it still need to be allocated at all ? Or can we simply set the
> > dma transfer size to 0 ?
>
> The DMA engine does still write to the buffer, so the allocation needs
> to occur. The zero size programmed into the register is a quirk of the
> HW itself, and is used to ensure the write wrap correctly in the
> buffer.
>
> Naush
>
> >
> > >
> > >       if (node->id == UNICAM_IMAGE_NODE) {
> > >               unicam_reg_write(node->dev, UNICAM_IBSA0, buf->dma_addr);
> > > --
> > > 2.34.1
> > >
> > >
>