[PATCH] drm/amd/display: fix division by zero in get_estimated_bw()

[Hari Mishal]

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 <harimish...@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