linux-kernel - [PATCH AUTOSEL 6.15 025/110] drm/amd/display: Avoid divide by zero by initializing dummy pitch to 1

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Message-Id: <20250601232435.3507697-25-sashal@kernel.org>
Date: Sun,  1 Jun 2025 19:23:07 -0400
From: Sasha Levin <sashal@...nel.org>
To: patches@...ts.linux.dev,
	stable@...r.kernel.org
Cc: Mario Limonciello <mario.limonciello@....com>,
	Alex Hung <alex.hung@....com>,
	Zaeem Mohamed <zaeem.mohamed@....com>,
	Mark Broadworth <mark.broadworth@....com>,
	Alex Deucher <alexander.deucher@....com>,
	Sasha Levin <sashal@...nel.org>,
	austin.zheng@....com,
	jun.lei@....com,
	harry.wentland@....com,
	sunpeng.li@....com,
	christian.koenig@....com,
	airlied@...il.com,
	simona@...ll.ch,
	amd-gfx@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH AUTOSEL 6.15 025/110] drm/amd/display: Avoid divide by zero by initializing dummy pitch to 1

From: Mario Limonciello <mario.limonciello@....com>

[ Upstream commit 7e40f64896e8e3dca471e287672db5ace12ea0be ]

[Why]
If the dummy values in `populate_dummy_dml_surface_cfg()` aren't updated
then they can lead to a divide by zero in downstream callers like
CalculateVMAndRowBytes()

[How]
Initialize dummy value to a value to avoid divide by zero.

Reviewed-by: Alex Hung <alex.hung@....com>
Signed-off-by: Mario Limonciello <mario.limonciello@....com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@....com>
Tested-by: Mark Broadworth <mark.broadworth@....com>
Signed-off-by: Alex Deucher <alexander.deucher@....com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---

**YES** This commit should be backported to stable kernel trees.
**Analysis:** This is a classic divide-by-zero prevention fix that
follows the exact same pattern as all the similar commits marked "YES"
in the historical examples. The commit: 1. **Fixes a critical bug**:
Prevents division by zero errors that could cause kernel crashes or
undefined behavior 2. **Extremely minimal and safe change**: Only
changes `out->PitchC[location] = 0;` to `out->PitchC[location] = 1;` on
line 900 of
`drivers/gpu/drm/amd/display/dc/dml2/dml2_translation_helper.c` 3.
**Well-documented issue**: The commit message clearly explains that
dummy values in `populate_dummy_dml_surface_cfg()` can lead to divide by
zero in downstream callers like `CalculateVMAndRowBytes()` 4. **Follows
established pattern**: The fix is identical to Similar Commits #1, #2,
and #5 which were all marked "YES" for backporting The code change is in
the `populate_dummy_dml_surface_cfg()` function where dummy/placeholder
values are initialized. Setting `PitchC[location]` to 1 instead of 0
ensures that any downstream code performing calculations using this
value as a denominator won't encounter division by zero errors. **Key
evidence supporting backporting:** - **AMD Display subsystem**: This is
a critical graphics subsystem where crashes can severely impact user
experience - **Crash prevention**: Division by zero can cause kernel
panics - **Zero risk of regression**: Changing a dummy value from 0 to 1
has no functional impact other than preventing crashes - **Small,
contained fix**: Single line change in one function - **Clear
precedent**: Multiple similar commits fixing divide-by-zero in AMD
display code have been backported This fix directly addresses a
potential stability issue with minimal risk, making it an ideal
candidate for stable tree backporting under the kernel's stable tree
rules.

 drivers/gpu/drm/amd/display/dc/dml2/dml2_translation_helper.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/display/dc/dml2/dml2_translation_helper.c b/drivers/gpu/drm/amd/display/dc/dml2/dml2_translation_helper.c
index ab6baf2698012..5de775fd8fcee 100644
--- a/drivers/gpu/drm/amd/display/dc/dml2/dml2_translation_helper.c
+++ b/drivers/gpu/drm/amd/display/dc/dml2/dml2_translation_helper.c
@@ -896,7 +896,7 @@ static void populate_dummy_dml_surface_cfg(struct dml_surface_cfg_st *out, unsig
 	out->SurfaceWidthC[location] = in->timing.h_addressable;
 	out->SurfaceHeightC[location] = in->timing.v_addressable;
 	out->PitchY[location] = ((out->SurfaceWidthY[location] + 127) / 128) * 128;
-	out->PitchC[location] = 0;
+	out->PitchC[location] = 1;
 	out->DCCEnable[location] = false;
 	out->DCCMetaPitchY[location] = 0;
 	out->DCCMetaPitchC[location] = 0;
-- 
2.39.5