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From:   Lyude Paul <lyude@redhat.com>
To:     amd-gfx@freedesktop.org
Cc:     Harry Wentland <harry.wentland@amd.com>,
        Leo Li <sunpeng.li@amd.com>,
        Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>,
        Alex Deucher <alexander.deucher@amd.com>,
        =?UTF-8?q?Christian=20K=C3=B6nig?= <christian.koenig@amd.com>,
        "Pan, Xinhui" <Xinhui.Pan@amd.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>, Hersen Wu <hersenwu@amd.com>,
        Roman Li <Roman.Li@amd.com>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Fangzhi Zuo <Jerry.Zuo@amd.com>,
        Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>,
        amd-gfx@lists.freedesktop.org (open list:AMD DISPLAY CORE),
        dri-devel@lists.freedesktop.org (open list:DRM DRIVERS),
        linux-kernel@vger.kernel.org (open list)
Subject: [PATCH 3/3] drm/amdgpu/dm: Drop != NULL check in dm_mst_get_pbn_divider()
Date:   Thu,  2 Jun 2022 17:00:56 -0400
Message-Id: <20220602210056.73316-4-lyude@redhat.com>
In-Reply-To: <20220602210056.73316-1-lyude@redhat.com>
References: <20220602210056.73316-1-lyude@redhat.com>
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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);
 }
-- 
2.35.3