linux-kernel - [PATCH] i915: Fix HBLANK Expansion Quirk Causing Modeset Failure on Dell WD19TB Dock at 3440x1440@...Hz

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <20240926002533.10153-2-bendhoefs@gmail.com>
Date: Wed, 25 Sep 2024 19:25:34 -0500
From: Benjamin Hoefs <bendhoefs@...il.com>
To: jani.nikula@...ux.intel.com
Cc: rodrigo.vivi@...el.com,
	joonas.lahtinen@...ux.intel.com,
	tursulin@...ulin.net,
	airlied@...il.com,
	simona@...ll.ch,
	intel-gfx@...ts.freedesktop.org,
	intel-xe@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org,
	Benjamin Hoefs <bendhoefs@...il.com>
Subject: [PATCH] i915: Fix HBLANK Expansion Quirk Causing Modeset Failure on Dell WD19TB Dock at 3440x1440@...Hz

Hello,

I am using a Dell WD19TB dock with a 3440x1440 monitor. Using it at
100Hz used to work but recently I tried it again and discovered it no longer
did, specifically the modeset seems to silently fail with no error message in
userspace utilities like kscreen-doctor and xrandr and no output in dmesg.
I found the problematic commit using git bisect to be
55eaef164174480df6827edeac15620f3cbcd52b "Handle the Synaptics HBlank
expansion quirk".

I found the issue to be the hblank_expasion_quirk_needs_dsc function which uses
the following comparison in the current kernel tree:

if (mode_hblank_period_ns(adjusted_mode) > hblank_limit)
	return false;

with hblank_limit being earlier set as

int hblank_limit = is_uhbr_sink ? 500 : 300;

However, my monitor's HBLANK period in the 3440x1440@...Hz mode is
exactly 300 ns as verified by this printk immediately before the
problematic comparison.

printk(KERN_INFO "Hello, kernel world! %i\n",
	mode_hblank_period_ns(adjusted_mode));
[   38.429839] Hello, kernel world! 300

With the attached change the modeset works as expected at 100Hz. Would it be
acceptable to modify the comparison from > to >= here?

I'll do my best to provide any additional details you may need although
that printk and '=' sign is the only kernel code I've written, so my best may
not be great :).

Signed-off-by: Benjamin D. Hoefs <bendhoefs@...il.com>
---
 drivers/gpu/drm/i915/display/intel_dp_mst.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
index 15541932b809..052c5a67df93 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
@@ -446,7 +446,7 @@ hblank_expansion_quirk_needs_dsc(const struct intel_connector *connector,
 	if (is_uhbr_sink && !drm_dp_is_uhbr_rate(limits->max_rate))
 		return false;

-	if (mode_hblank_period_ns(adjusted_mode) > hblank_limit)
+	if (mode_hblank_period_ns(adjusted_mode) >= hblank_limit)
 		return false;

 	return true;
-- 
2.46.2