-Wenum-enum-conversion in drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_dpia_bw.c

[nathanchance]

After commit 5f3bce13266e ("drm/amd/display: Request usb4 bw for mst streams") in -next, I see:

drivers/gpu/drm/amd/amdgpu/../display/dc/link/protocols/link_dp_dpia_bw.c:548:24: error: arithmetic between different enumeration types ('const enum dc_link_rate' and 'const enum dc_lane_count') [-Werror,-Wenum-enum-conversion]
  548 |                         link_cap->link_rate * link_cap->lane_count * LINK_RATE_REF_FREQ_IN_KHZ * 8;
      |                         ~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~
1 error generated.

These enums are defined in drivers/gpu/drm/amd/display/dc/dc_dp_types.h.

Adding casts would make this disappear:

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 4ef1a6a1d129..a49d67c60cf0 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
@@ -545,7 +545,7 @@ int link_dp_dpia_get_dp_overhead_in_dp_tunneling(struct dc_link *link)
                const struct dc_link_settings *link_cap =
                        dc_link_get_link_cap(link);
                uint32_t link_bw_in_kbps =
-                       link_cap->link_rate * link_cap->lane_count * LINK_RATE_REF_FREQ_IN_KHZ * 8;
+                       (uint32_t)link_cap->link_rate * (uint32_t)link_cap->lane_count * LINK_RATE_REF_FREQ_IN_KHZ * 8;
                link_mst_overhead = (link_bw_in_kbps / 64) + ((link_bw_in_kbps % 64) ? 1 : 0);
        }

Another option would be turning these enums into macros, although I assume that is done for type safety.

Lastly, it appears this warning (-Wenum-enum-conversion) was added to clang as part of some work for C++20, which may not be relevant here? Do we want to unconditionally disable -Wenum-enum-conversion for the kernel in that case?

[nathanchance]

Arnd pointed out that this issue has triggered once before: https://git.kernel.org/linus/985b1f596f9ed56f42b8c2280005f943e1434c06

I think the casts are reasonable given that, so I sent the above diff (with slightly different formatting): https://lore.kernel.org/20240110-amdgpu-display-enum-enum-conversion-v1-1-953ae94fe15e@kernel.org/

[nathanchance]

Accepted: https://gitlab.freedesktop.org/agd5f/linux/-/commit/d7643fe6fb76edb1f2f1497bf5e8b8f4774b5129

[nathanchance]

Merged into mainline: https://git.kernel.org/linus/d7643fe6fb76edb1f2f1497bf5e8b8f4774b5129