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

[Naushir Patuck <naush@raspberrypi.com>]

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