get_estimated_bw() divides by link->dpia_bw_alloc_config.bw_granularity,
which is zeroed by reset_bw_alloc_struct() and only populated once
DP_TUNNELING_BW_ALLOC_CAP_CHANGED has been handled.

link_dp_dpia_handle_bw_alloc_status(), the DPCD interrupt handler,
calls get_estimated_bw() whenever DP_TUNNELING_ESTIMATED_BW_CHANGED
is set, independently of whether DP_TUNNELING_BW_ALLOC_CAP_CHANGED
has ever fired for that link. A connected USB4/DPIA tunneling device
that reports an estimated-bandwidth change before ever reporting a
capability change drives a division by zero in this IRQ path.

link_dpia_send_bw_alloc_request() already guards the same
bw_granularity division; add the identical guard here rather than
introducing a new pattern.

Fixes: 8e5cfe547bf3 ("drm/amd/display: upstream link_dp_dpia_bw.c")
Assisted-by: gkh_clanker_t1000
Signed-off-by: Hari Mishal <harimishal1@gmail.com>
---
 .../gpu/drm/amd/display/dc/link/protocols/link_dp_dpia_bw.c  | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_dpia_bw.c b/drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_dpia_bw.c
index dd854d992692..f43fc4b78a8d 100644
--- a/drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_dpia_bw.c
+++ b/drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_dpia_bw.c
@@ -103,6 +103,11 @@ static int get_estimated_bw(struct dc_link *link)
 {
 	uint8_t bw_estimated_bw = 0;
 
+	if (link->dpia_bw_alloc_config.bw_granularity == 0) {
+		DC_LOG_ERROR("%s: BW granularity is zero!\n", __func__);
+		return 0;
+	}
+
 	core_link_read_dpcd(
 			link,
 			ESTIMATED_BW,
-- 
2.43.0

Weekly promotion tests found no regression.

This patch is
Reviewed-by: Alex Hung <alex.hung@amd.com>

On 8/22/26 08:57, Hari Mishal wrote:
> [Some people who received this message don't often get email from harimishal1@gmail.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
> 
> get_estimated_bw() divides by link->dpia_bw_alloc_config.bw_granularity,
> which is zeroed by reset_bw_alloc_struct() and only populated once
> DP_TUNNELING_BW_ALLOC_CAP_CHANGED has been handled.
> 
> link_dp_dpia_handle_bw_alloc_status(), the DPCD interrupt handler,
> calls get_estimated_bw() whenever DP_TUNNELING_ESTIMATED_BW_CHANGED
> is set, independently of whether DP_TUNNELING_BW_ALLOC_CAP_CHANGED
> has ever fired for that link. A connected USB4/DPIA tunneling device
> that reports an estimated-bandwidth change before ever reporting a
> capability change drives a division by zero in this IRQ path.
> 
> link_dpia_send_bw_alloc_request() already guards the same
> bw_granularity division; add the identical guard here rather than
> introducing a new pattern.
> 
> Fixes: 8e5cfe547bf3 ("drm/amd/display: upstream link_dp_dpia_bw.c")
> Assisted-by: gkh_clanker_t1000
> Signed-off-by: Hari Mishal <harimishal1@gmail.com>
> ---
>   .../gpu/drm/amd/display/dc/link/protocols/link_dp_dpia_bw.c  | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_dpia_bw.c b/drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_dpia_bw.c
> index dd854d992692..f43fc4b78a8d 100644
> --- a/drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_dpia_bw.c
> +++ b/drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_dpia_bw.c
> @@ -103,6 +103,11 @@ static int get_estimated_bw(struct dc_link *link)
>   {
>          uint8_t bw_estimated_bw = 0;
> 
> +       if (link->dpia_bw_alloc_config.bw_granularity == 0) {
> +               DC_LOG_ERROR("%s: BW granularity is zero!\n", __func__);
> +               return 0;
> +       }
> +
>          core_link_read_dpcd(
>                          link,
>                          ESTIMATED_BW,
> --
> 2.43.0