ACK: [SRU][R][PATCH 0/1] drm/i915/lnl+/tc: Fix false disconnect of active DP-alt TC port during long HPD pulse

[Yufeng Gao]

On 8/4/26 10:24, AceLan Kao wrote:
> From: "Chia-Lin Kao (AceLan)" <acelan.kao at canonical.com>
>
> BugLink: https://bugs.launchpad.net/bugs/2143879
>
> [Impact]
> On HP systems (CID: 202512-38233) with Intel Lunar Lake GPU and HP Type-C to HDMI dongle
> (03f0:8a4a), switching to mirror mode via hotkey fails. The system cannot switch mirror
> mode on X11 when using a Type-C to HDMI external monitor.
> The root cause is that on XELPDP+ platforms (Lunar Lake and later), the LTTPR dongle
> fires a long HPD pulse after link training. The tc_phy_hpd_live_status() function reads
> PICAINTERRUPT_ISR to determine DP-alt live status, but the XELPDP_DP_ALT_HOTPLUG_MASK
> bit deasserts during a long HPD pulse. Since intel_tc_port_handles_hpd_glitches() returns
> true for TC dp-alt ports, the 4ms glitch-filter retry loop is bypassed, giving no
> protection against this transient deassertion. The hotplug handler then marks DP-1 as
> disconnected, and the compositor disables eDP-1.
> Error log:
>    xe 0000:00:02.0: [drm:intel_dp_hpd_pulse [xe]] got hpd irq on [ENCODER:270:DDI TC1/PHY TC1] - long
>    xe 0000:00:02.0: [drm:drm_dp_dpcd_access [drm_display_helper]] AUX USBC1/DDI TC1/PHY TC1: Too many retries, giving up. First error: -6
>    xe 0000:00:02.0: [drm:intel_hotplug_detect_connector [xe]] [CONNECTOR:271:DP-1] status updated from connected to disconnected (epoch counter 178->179)
>    [drm:intel_dp_hpd_pulse] got hpd irq on [ENCODER:270:DDI TC1/PHY TC1] - long
>    [drm:intel_tc_port_update_mode] Port D/TC#1: TC port mode reset (disconnected -> tbt-alt)
>    [drm:intel_dp_hpd_pulse] got hpd irq on [ENCODER:270:DDI TC1/PHY TC1] - long
>    [drm:intel_hotplug_detect_connector] [CONNECTOR:271:DP-1] status updated from connected to disconnected
> Affected hardware: HP systems (CID: 202512-38233) with Intel Lunar Lake GPU
> Failure rate: High (reproducible with HP Type-C to HDMI dongle 03f0:8a4a on X11)
>
> [Fix]
> When the port has an active link (link_refcount > 0), trust the PHY ownership register
> (XELPDP_PORT_BUF_CTL1 & XELPDP_TC_PHY_OWNERSHIP) over the transient HPD live status
> in intel_tc_port_connected(). The ownership register is stable and does not toggle
> during HPD pulses. The TC cold power domain is guaranteed to be held when
> link_refcount > 0, as __intel_tc_port_lock() already asserts and verifies this via
> tc_phy_is_owned() before reaching intel_tc_port_connected().
> The patch is currently under review on the Intel DRM mailing list (not yet merged to
> mainline).
> Patch:
> https://lore.kernel.org/lkml/20260305075857.40077-1-acelan.kao@canonical.com/T/#u
>
> [Test Plan]
> 1. Connect HP Type-C to HDMI dongle (03f0:8a4a) to the system.
> 2. Ensure the external monitor is displaying output (extended mode).
> 3. Press the hotkey to switch to mirror mode.
> Without the patch: The external monitor fails to switch to mirror mode. The kernel log
> shows DP-1 connector status updated from connected to disconnected, and the internal
> display may also blank.
> With the patch: Mirror mode switches successfully. The kernel log shows the TC port
> correctly stays in or returns to dp-alt mode:
>    [drm:intel_tc_port_update_mode] Port D/TC#1: TC port mode reset (dp-alt -> disconnected)
>    [drm:intel_tc_port_update_mode] Port D/TC#1: TC port mode reset (disconnected -> dp-alt)
>
> [Where problems could occur]
> It may affect the i915/xe display driver's Type-C port management on Lunar Lake and
> later platforms.
> If the condition `tc_phy_is_owned()` returns a stale or incorrect ownership state while
> link_refcount > 0, the driver could incorrectly report a port as connected when the
> physical link has actually been removed. This would manifest as a phantom connected
> display that cannot be detected as disconnected until the next full port reset, potentially
> causing display configuration errors or a hung compositor.
> Additionally, the early-return path bypasses the existing tc_phy_hpd_live_status() mask
> check. If there is an edge case where link_refcount > 0 but the port mode transitions
> away from TC_PORT_DP_ALT between the refcount check and the ownership check, the port
> status could be misreported.
> The patch is submitted upstream for review but not yet merged.
>
> [Other Info]
> The patch is submitted to the Intel DRM mailing list for review but not yet merged to
> mainline.
> This issue only reproduces with the HP Type-C to HDMI dongle (03f0:8a4a) on X11. Wayland
> and other dongle types (Type-C to DP/VGA, built-in HDMI/Thunderbolt) are not affected.
>
> Chia-Lin Kao (AceLan) (1):
>    UBUNTU: SAUCE: drm/i915/lnl+/tc: Fix false disconnect of active DP-alt
>      TC port during long HPD pulse
>
>   drivers/gpu/drm/i915/display/intel_tc.c | 5 +++++
>   1 file changed, 5 insertions(+)
>
Acked-by: Yufeng Gao <yufeng.gao at canonical.com>