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

[Jacopo Mondi]

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>
> ---
>  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 ?

>
>  	if (node->id == UNICAM_IMAGE_NODE) {
>  		unicam_reg_write(node->dev, UNICAM_IBSA0, buf->dma_addr);
> --
> 2.34.1
>
>