A lot of code in amdgpu seems to sprinkle in

if (foo != NULL) …

Checks pretty much all over the place, many times in locations where it's clear foo (whatever foo may be) should never be NULL unless we've run into a programming error. This is definitely one of those places, as dm_mst_get_pbn_divider() should never be getting called with a NULL dc_link pointer.

The problem with this code pattern is that many times the places I've seen it used in amdgpu have no real error handling. This is actually quite bad, if we try to avoid the NULL pointer and instead simply skip any code that was expecting a valid pointer - we're already in undefined territory. Subsequent code we execute may have expected sideaffects from the code we skipped that are no longer present, which leads to even more unpredictable behavior then a simple segfault. This could be silent errors or even just another segfault somewhere else.

If we simply segfault though, that's not good either. But unlike the former solution, no subsequent code in the kernel thread will execute - and we will likely even get a clear backtrace from the invalid memory access. Of course, the preferred approach is to simply handle the possibility of both NULL and non-NULL pointers with nice error handling code. However, that's not always desirable or even possible, and in those cases it's likely just better to fail predictably rather than unpredictably.

This code is a nice example of that - if link is NULL, you'll return a PBN divisor of 0. And thus, you've simply traded in your potential segfault for a potential divide by 0 error. This was something I actually managed to hit while working on the legacy MST removal work.

Signed-off-by: Lyude Paul lyude@redhat.com --- drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c | 3 --- 1 file changed, 3 deletions(-)

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c index 1259f2f7a8f9..35c7def8f2bd 100644 --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c @@ -537,9 +537,6 @@ void amdgpu_dm_initialize_dp_connector(struct amdgpu_display_manager *dm,

int dm_mst_get_pbn_divider(struct dc_link *link) { - if (!link) - return 0; - return dc_link_bandwidth_kbps(link, dc_link_get_link_cap(link)) / (8 * 1000 * 54); }