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

Naushir Patuck
2024-11-22T11:36:36Z

Hi Jacopo,

On Fri, 22 Nov 2024 at 11:20, Jacopo Mondi
<jacopo.mondi@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@raspberrypi.com>
> > ---
> > 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
> >
> >