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Thu, 10 Sep 2026 04:49:55 +0000 Message-ID: <7ee1a166-bab9-497c-adf4-2872c2f6668f@intel.com> Date: Thu, 10 Sep 2026 10:19:46 +0530 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 11/12] drm/i915/dip: Enable Common SDP Transmission line To: CC: , References: <20260908062224.2897987-1-ankit.k.nautiyal@intel.com> <20260908062224.2897987-12-ankit.k.nautiyal@intel.com> <20260908071133.82E681F00A3D@smtp.kernel.org> Content-Language: en-US From: "Nautiyal, Ankit K" In-Reply-To: <20260908071133.82E681F00A3D@smtp.kernel.org> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MA0PR01CA0086.INDPRD01.PROD.OUTLOOK.COM (2603:1096:a01:ae::11) To DS0PR11MB7558.namprd11.prod.outlook.com (2603:10b6:8:148::11) Precedence: bulk X-Mailing-List: sashiko-reviews@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH7PR11MB7571:EE_|CY8PR11MB6843:EE_ X-MS-Office365-Filtering-Correlation-Id: fa99b510-88b3-4c3e-4c69-08df0ef6f47a X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|366016|23010399003|1800799024|6133799003|4143699003|10067099003|11063799006|56012099006|22082099003|18002099003; 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Sashiko AI review found 2 potential issue(s) to consider: > - [High] Integer underflow when computing cmn_sdp_tl with guardband=0 causes a kernel warning, and a valid transmission line of 0 is incorrectly treated as 'disabled'. > - [Medium] Per-transcoder SDP transmission line programming is incorrectly placed in port-level enable/disable paths, breaking DP MST secondary streams. > -- > > commit dcefae9ad5800e7d0f8f1d55a58dfb400880ed28 > Author: Ankit Nautiyal > > drm/i915/dip: Enable Common SDP Transmission line > > This commit enables programming of the common SDP transmission line on > platforms that support it. It computes and programs the common base > transmission line and per-SDP stagger values from the crtc state during > modeset, and disables the feature on pipe disable. > >> diff --git a/drivers/gpu/drm/i915/display/intel_ddi.c b/drivers/gpu/drm/i915/display/intel_ddi.c >> --- a/drivers/gpu/drm/i915/display/intel_ddi.c >> +++ b/drivers/gpu/drm/i915/display/intel_ddi.c >> @@ -2737,6 +2737,8 @@ static void mtl_ddi_pre_enable_dp(struct intel_atomic_state *state, >> /* 6.o Configure and enable FEC if needed */ >> intel_ddi_enable_fec(encoder, crtc_state); >> >> + intel_dip_cmn_sdp_transmission_line_enable(crtc_state); >> + > [Severity: Medium] > Since mtl_ddi_pre_enable_dp() acts at the port level and only executes for > the first stream in a DP MST topology, will secondary streams' transcoders > miss having their SDP transmission line initialized? > > It seems MST secondary streams will bypass this hook entirely. Furthermore, > mst_stream_compute_config_late() in intel_dp_mst.c doesn't appear to call > intel_dip_cmn_sdp_tl_compute_config_late(), meaning the values aren't > computed for MST streams. VRR is not set for MST yet so we are covered. > >> /* 7.a 128b/132b SST. */ >> if (!is_mst && intel_dp_is_uhbr(crtc_state)) { >> /* VCPID 1, start slot 0 for 128b/132b, tu slots */ > [ ... ] >> @@ -3124,6 +3126,7 @@ static void intel_ddi_buf_disable(struct intel_encoder *encoder, >> DP_TP_CTL_ENABLE, 0); >> } >> >> + intel_dip_cmn_sdp_transmission_line_disable(crtc_state); > [Severity: Medium] > Similarly, since this port-level disable hook only executes when the last > stream is disabled, does this leave the transmission line registers active > for other transcoders in an MST setup when they are individually disabled? > >> intel_ddi_disable_fec(encoder, crtc_state); >> >> if (DISPLAY_VER(display) < 14) >> diff --git a/drivers/gpu/drm/i915/display/intel_dip.c b/drivers/gpu/drm/i915/display/intel_dip.c >> --- a/drivers/gpu/drm/i915/display/intel_dip.c >> +++ b/drivers/gpu/drm/i915/display/intel_dip.c > [ ... ] >> +static >> +void intel_dip_cmn_sdp_tl_compute_config_late(struct intel_crtc_state *crtc_state) >> +{ > [ ... ] >> + if (as_sdp) >> + crtc_state->dip.cmn_sdp_tl = crtc_state->dip.emp_as_sdp_tl; >> + else >> + crtc_state->dip.cmn_sdp_tl = crtc_state->vrr.guardband - 1; > [Severity: High] > If crtc_state->vrr.guardband is 0 (for example, when VRR is not used), will > this subtraction underflow and set the u16 cmn_sdp_tl to 65535? > > If it underflows to 65535, pps_stagger evaluates to a negative value > (0 - 65535) in intel_dip_cmn_sdp_transmission_line_enable(). Will this trigger > the drm_WARN_ON(display->drm, pps_stagger < 0) and abort enabling the > transmission line? > > Also, if guardband evaluates to 1, cmn_sdp_tl will be 0. Does the > !crtc_state->dip.cmn_sdp_tl check in intel_dip_cmn_sdp_transmission_line_enable() > and disable() incorrectly treat a valid transmission line value of 0 as a > sentinel for 'feature disabled', silently skipping the hardware enablement? guardband can't be 0 or 1 here. DP always computes it via the optimized guardband path (intel_vrr_compute_optimized_guardband()), which calls intel_dp_sdp_min_guardband(crtc_state, true) with assume_all_enabled = true. That guarantees a floor of at least 8 (GMP's contribution), regardless of which SDPs are actually active, so no underflow is possible. That said, this does need to change once CMN_SDP_TL is set: with CMN_SDP_TL enabled, the per-packet minimum SDP guardband requirement no longer applies. I'll set a 5-line floor for the guardband in that case, 4 lines to cover the packet types plus 1 extra. Thanks & Regards, Ankit >