From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6CA9DC433EF for ; Tue, 26 Apr 2022 21:17:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1355321AbiDZVUv (ORCPT ); Tue, 26 Apr 2022 17:20:51 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45760 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1355225AbiDZVUu (ORCPT ); Tue, 26 Apr 2022 17:20:50 -0400 Received: from mail-ej1-x630.google.com (mail-ej1-x630.google.com [IPv6:2a00:1450:4864:20::630]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1D5A7C7EA6 for ; Tue, 26 Apr 2022 14:17:41 -0700 (PDT) Received: by mail-ej1-x630.google.com with SMTP id m20so17854374ejj.10 for ; Tue, 26 Apr 2022 14:17:41 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=9kkw3V+5MLt1m5hoiWQpBjrcDCFgA9bH0l/Cgz/5i0Y=; b=Txcqq+E8b1nvNJ4CwcrWa26tfFMXvfHJ7iCCQN/y84apAGGwg+cTZiM6tNGnwi8NN8 YqCHSDs1QTWXOVdIb8o3vpidV6J1KG5vhrvazNJyCXuPp2Z8bk7YMerXHaiOcz7SJ9As zSn+Ek/XBNkz/Ip6nSNmEAvc0fpd6eP0+7OLw= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=9kkw3V+5MLt1m5hoiWQpBjrcDCFgA9bH0l/Cgz/5i0Y=; b=7IsjVb+E7Hi+nsdObWSPDIVFbeycSgFFWjyHVYnshioTjsNY5hgHj+tE+OCGo0w/Pu UMVA34cFWO6X4DRm/EyaGWggOTC9Ybs5x98UqFtv2d0nLjLiKRAvcVceYR/On2qUYX6i B9PeD/EPOB977gaxdNdt+LhIwLjwDxdYKf6Y/vUuZKUucqI7uH/TIogluvmfAibdM6vW ObmIZMFFhapXIGUzcghB0QkOilsR//wEuZSjK3BYv86Boyd49QEaAG9LKk4Tq5yxBrNs 6WAKLTETkKLaNQi5gY7q3Hw9/O6RuyQMGCgk/yt5cFnZW/WlQgTXbq7iytYvhQOkzYLn Y7Aw== X-Gm-Message-State: AOAM532bJ0xplZxtLplOFolDNFzK1csKSUoAhfOd4byB3Kdyo1QhSHdP 7vyQjs70sDEJ8EaDVBv+jWFzf9Cc6Mp97wcob9g= X-Google-Smtp-Source: ABdhPJxOxBe1PIywUhxsd2A5E8lD5138UiiQ8zAjeh4yCRl06D1nZI/7VTnknOmX7qqrXdMHgS8ytg== X-Received: by 2002:a17:907:7f94:b0:6da:64ec:fabc with SMTP id qk20-20020a1709077f9400b006da64ecfabcmr23116565ejc.717.1651007859348; Tue, 26 Apr 2022 14:17:39 -0700 (PDT) Received: from mail-wr1-f47.google.com (mail-wr1-f47.google.com. [209.85.221.47]) by smtp.gmail.com with ESMTPSA id s23-20020a508d17000000b00425d41ab7afsm5601371eds.17.2022.04.26.14.17.37 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 26 Apr 2022 14:17:37 -0700 (PDT) Received: by mail-wr1-f47.google.com with SMTP id u3so27199388wrg.3 for ; Tue, 26 Apr 2022 14:17:37 -0700 (PDT) X-Received: by 2002:a05:6000:1acb:b0:20a:a4b0:dbc8 with SMTP id i11-20020a0560001acb00b0020aa4b0dbc8mr19325351wry.513.1651007856493; Tue, 26 Apr 2022 14:17:36 -0700 (PDT) MIME-Version: 1.0 References: <20220426114627.1.I2dd93486c6952bd52f2020904de0133970d11b29@changeid> <20220426114627.2.I4ac7f55aa446699f8c200a23c10463256f6f439f@changeid> <517f71e4-785f-ef6f-d30e-fb18974eed57@quicinc.com> <37b16e86-f726-bf26-98ba-6146b6835c04@quicinc.com> In-Reply-To: <37b16e86-f726-bf26-98ba-6146b6835c04@quicinc.com> From: Doug Anderson Date: Tue, 26 Apr 2022 14:17:24 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 2/2] drm/probe-helper: For DP, add 640x480 if all other modes are bad To: Abhinav Kumar Cc: Sankeerth Billakanti , David Airlie , linux-arm-msm , LKML , dri-devel , "Kuogee Hsieh (QUIC)" , Thomas Zimmermann , Dmitry Baryshkov , "Aravind Venkateswaran (QUIC)" , Stephen Boyd Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-arm-msm@vger.kernel.org Hi, On Tue, Apr 26, 2022 at 2:11 PM Abhinav Kumar wrote: > > > > On 4/26/2022 1:26 PM, Doug Anderson wrote: > > Hi, > > > > On Tue, Apr 26, 2022 at 12:20 PM Abhinav Kumar > > wrote: > >> > >> Missed one more comment. > >> > >> On 4/26/2022 12:16 PM, Abhinav Kumar wrote: > >>> Hi Doug > >>> > >>> One minor comment below. > >>> > >>> But otherwise, looking at this change this should work for us acc to me. > >>> > >>> We will test this out with our equipment and then provide R-b. > >>> > >>> Thanks > >>> > >>> Abhinav > >>> On 4/26/2022 11:46 AM, Douglas Anderson wrote: > >>>> As per Displayport spec section 5.2.1.2 ("Video Timing Format") says > >>>> that all detachable sinks shall support 640x480 @60Hz as a fail safe > >>>> mode. > >>>> > >>>> A DP compliance test expected us to utilize the above fact when all > >>>> modes it presented to the DP source were not achievable. It presented > >>>> only modes that would be achievable with more lanes and/or higher > >>>> speeds than we had available and expected that when we couldn't do > >>>> that then we'd fall back to 640x480 even though it didn't advertise > >>>> this size. > >>>> > >>>> In order to pass the compliance test (and also support any users who > >>>> might fall into a similar situation with their display), we need to > >>>> add 640x480 into the list of modes. However, we don't want to add > >>>> 640x480 all the time. Despite the fact that the DP spec says all sinks > >>>> _shall support_ 640x480, they're not guaranteed to support it > >>>> _well_. Continuing to read the spec you can see that the display is > >>>> not required to really treat 640x480 equal to all the other modes. It > >>>> doesn't need to scale or anything--just display the pixels somehow for > >>>> failsafe purposes. It should also be noted that it's not hard to find > >>>> a display hooked up via DisplayPort that _doesn't_ support 640x480 at > >>>> all. The HP ZR30w screen I'm sitting in front of has a native DP port > >>>> and doesn't work at 640x480. I also plugged in a tiny 800x480 HDMI > >>>> display via a DP to HDMI adapter and that screen definitely doesn't > >>>> support 640x480. > >>>> > >>>> As a compromise solution, let's only add the 640x480 mode if: > >>>> * We're on DP. > >>>> * All other modes have been pruned. > >>>> > >>>> This acknowledges that 640x480 might not be the best mode to use but, > >>>> since sinks are _supposed_ to support it, we will at least fall back > >>>> to it if there's nothing else. > >>>> > >>>> Note that we _don't_ add higher resolution modes like 1024x768 in this > >>>> case. We only add those modes for a failed EDID read where we have no > >>>> idea what's going on. In the case where we've pruned all modes then > >>>> instead we only want 640x480 which is the only defined "Fail Safe" > >>>> resolution. > >>>> > >>>> This patch originated in response to Kuogee Hsieh's patch [1]. > >>>> > >>>> [1] > >>>> https://lore.kernel.org/r/1650671124-14030-1-git-send-email-quic_khsieh@quicinc.com > >>>> > >>>> > >>>> Signed-off-by: Douglas Anderson > >>>> --- > >>>> > >>>> drivers/gpu/drm/drm_probe_helper.c | 26 +++++++++++++++++++++----- > >>>> 1 file changed, 21 insertions(+), 5 deletions(-) > >>>> > >>>> diff --git a/drivers/gpu/drm/drm_probe_helper.c > >>>> b/drivers/gpu/drm/drm_probe_helper.c > >>>> index 819225629010..90cd46cbfec1 100644 > >>>> --- a/drivers/gpu/drm/drm_probe_helper.c > >>>> +++ b/drivers/gpu/drm/drm_probe_helper.c > >>>> @@ -476,7 +476,6 @@ int drm_helper_probe_single_connector_modes(struct > >>>> drm_connector *connector, > >>>> const struct drm_connector_helper_funcs *connector_funcs = > >>>> connector->helper_private; > >>>> int count = 0, ret; > >>>> - bool verbose_prune = true; > >>>> enum drm_connector_status old_status; > >>>> struct drm_modeset_acquire_ctx ctx; > >>>> @@ -556,8 +555,8 @@ int drm_helper_probe_single_connector_modes(struct > >>>> drm_connector *connector, > >>>> DRM_DEBUG_KMS("[CONNECTOR:%d:%s] disconnected\n", > >>>> connector->base.id, connector->name); > >>>> drm_connector_update_edid_property(connector, NULL); > >>>> - verbose_prune = false; > >>>> - goto prune; > >>>> + drm_mode_prune_invalid(dev, &connector->modes, false); > >>>> + goto exit; > >>>> } > >>>> count = (*connector_funcs->get_modes)(connector); > >>>> @@ -580,9 +579,26 @@ int > >>>> drm_helper_probe_single_connector_modes(struct drm_connector *connector, > >>>> } > >>>> } > >>>> -prune: > >>>> - drm_mode_prune_invalid(dev, &connector->modes, verbose_prune); > >>>> + drm_mode_prune_invalid(dev, &connector->modes, true); > >>>> + /* > >>>> + * Displayport spec section 5.2.1.2 ("Video Timing Format") says > >>>> that > >>>> + * all detachable sinks shall support 640x480 @60Hz as a fail safe > >>>> + * mode. If all modes were pruned, perhaps because they need more > >>>> + * lanes or a higher pixel clock than available, at least try to add > >>>> + * in 640x480. > >>>> + */ > >>>> + if (list_empty(&connector->modes) && > >>>> + connector->connector_type == DRM_MODE_CONNECTOR_DisplayPort) { > >>>> + count = drm_add_modes_noedid(connector, 640, 480); > >>>> + if (_drm_helper_update_and_validate(connector, maxX, maxY, > >>>> &ctx)) { > >>>> + drm_modeset_backoff(&ctx); > >>>> + goto retry; > >>> > >>> Do we need another retry here? This will again repeat everything from > >>> get_modes(). > >>> The fact that we are hitting this code is because we have already tried > >>> that and this is already a second-pass. So I think another retry isnt > >>> needed? > >> > >> This will help cover the case of 4.2.2.6 but not fix 4.2.2.1. > >> > >> For 4.2.2.1, we will have 0 modes and so the original DRM fwk code of > >> adding all modes <= 1024x768 will kick in. > >> > >> Now, in that list, we will still need to pick/mark 640x480 as the > >> preferred mode. > >> > >> We still need IGT for that. > > > > Are you sure you don't have those backwards? It seems like 4.2.2.6 is > > the test case dealing with corrupt EDID and that's the one that will > > still be broken, no? ...and corrupt EDID is still the case where we > > have 0 modes. > > Yes indeed, sorry, I did have the numbers backwards. > 4.2.2.6 will still be broken. > > > > > In any case, let's see what people think about: > > > > https://lore.kernel.org/r/20220426132121.RFC.1.I31ec454f8d4ffce51a7708a8092f8a6f9c929092@changeid > > Yes sure. If it gets accepted, it will save us some IGT work. > > > > > I've marked that one as RFC just because it seems like a bigger change > > to existing behavior, though it still seems correct to me. > > > > NOTE: reading 4.2.2.6 more closely, it actually looks as if we're > > actually supposed to be able to try various video modes one at a time > > until we find one that works (or land on 640x480). Seems as if we're > > supposed to be able to try the higher resolutions one at a time and we > > can tell whether the sink "accepted" it by seeing if SINK_STATUS goes > > to 1? I have no idea how that works with all the Linux APIs, though. > > > > hmmm .... our equipment throws a warning if we dont sent 640x480. So > perhaps just go with the "or land on 640x480" option. > > 0006.392.232: [WARNING] Source DUT failed to transmit a video stream > using fail-safe mode > 0006.392.491: Received 1344 Htotal differs from fail-safe 800 > 0006.392.621: Received 1024 Hactive differs from fail-safe 640 > 0006.392.750: Received 296 Hstart differs from fail-safe 144 > 0006.392.868: Received 136 Hsync width differs from fail-safe 96 > 0006.392.975: Received 806 Vtotal differs from fail-safe 525 > 0006.393.099: Received 768 Vactive differs from fail-safe 480 > 0006.393.229: Received 6 Vsync width differs from fail-safe 2 Do you actually have code to implement the checking of SINK_STATUS? I'm not aware of how that would work in Linux, which is why just defaulting to 640x480 seems like a reasonable thing to do for now. The test case actually says that you're allowed to try clock rates one at a time (polling SINK_STATUS in DPCT) as long as you don't spend more than 5 seconds on each clock rate. According to the test case if you never saw SINK_STATUS in DPCT go to 1 then you should end at 640x480. 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[209.85.221.41]) by smtp.gmail.com with ESMTPSA id n14-20020a50934e000000b0042053e79386sm6999679eda.91.2022.04.26.14.17.37 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 26 Apr 2022 14:17:37 -0700 (PDT) Received: by mail-wr1-f41.google.com with SMTP id w4so27126509wrg.12 for ; Tue, 26 Apr 2022 14:17:37 -0700 (PDT) X-Received: by 2002:a05:6000:1acb:b0:20a:a4b0:dbc8 with SMTP id i11-20020a0560001acb00b0020aa4b0dbc8mr19325351wry.513.1651007856493; Tue, 26 Apr 2022 14:17:36 -0700 (PDT) MIME-Version: 1.0 References: <20220426114627.1.I2dd93486c6952bd52f2020904de0133970d11b29@changeid> <20220426114627.2.I4ac7f55aa446699f8c200a23c10463256f6f439f@changeid> <517f71e4-785f-ef6f-d30e-fb18974eed57@quicinc.com> <37b16e86-f726-bf26-98ba-6146b6835c04@quicinc.com> In-Reply-To: <37b16e86-f726-bf26-98ba-6146b6835c04@quicinc.com> From: Doug Anderson Date: Tue, 26 Apr 2022 14:17:24 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 2/2] drm/probe-helper: For DP, add 640x480 if all other modes are bad To: Abhinav Kumar Content-Type: text/plain; charset="UTF-8" X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Sankeerth Billakanti , David Airlie , linux-arm-msm , LKML , dri-devel , "Kuogee Hsieh \(QUIC\)" , Thomas Zimmermann , Dmitry Baryshkov , "Aravind Venkateswaran \(QUIC\)" , Stephen Boyd Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" Hi, On Tue, Apr 26, 2022 at 2:11 PM Abhinav Kumar wrote: > > > > On 4/26/2022 1:26 PM, Doug Anderson wrote: > > Hi, > > > > On Tue, Apr 26, 2022 at 12:20 PM Abhinav Kumar > > wrote: > >> > >> Missed one more comment. > >> > >> On 4/26/2022 12:16 PM, Abhinav Kumar wrote: > >>> Hi Doug > >>> > >>> One minor comment below. > >>> > >>> But otherwise, looking at this change this should work for us acc to me. > >>> > >>> We will test this out with our equipment and then provide R-b. > >>> > >>> Thanks > >>> > >>> Abhinav > >>> On 4/26/2022 11:46 AM, Douglas Anderson wrote: > >>>> As per Displayport spec section 5.2.1.2 ("Video Timing Format") says > >>>> that all detachable sinks shall support 640x480 @60Hz as a fail safe > >>>> mode. > >>>> > >>>> A DP compliance test expected us to utilize the above fact when all > >>>> modes it presented to the DP source were not achievable. It presented > >>>> only modes that would be achievable with more lanes and/or higher > >>>> speeds than we had available and expected that when we couldn't do > >>>> that then we'd fall back to 640x480 even though it didn't advertise > >>>> this size. > >>>> > >>>> In order to pass the compliance test (and also support any users who > >>>> might fall into a similar situation with their display), we need to > >>>> add 640x480 into the list of modes. However, we don't want to add > >>>> 640x480 all the time. Despite the fact that the DP spec says all sinks > >>>> _shall support_ 640x480, they're not guaranteed to support it > >>>> _well_. Continuing to read the spec you can see that the display is > >>>> not required to really treat 640x480 equal to all the other modes. It > >>>> doesn't need to scale or anything--just display the pixels somehow for > >>>> failsafe purposes. It should also be noted that it's not hard to find > >>>> a display hooked up via DisplayPort that _doesn't_ support 640x480 at > >>>> all. The HP ZR30w screen I'm sitting in front of has a native DP port > >>>> and doesn't work at 640x480. I also plugged in a tiny 800x480 HDMI > >>>> display via a DP to HDMI adapter and that screen definitely doesn't > >>>> support 640x480. > >>>> > >>>> As a compromise solution, let's only add the 640x480 mode if: > >>>> * We're on DP. > >>>> * All other modes have been pruned. > >>>> > >>>> This acknowledges that 640x480 might not be the best mode to use but, > >>>> since sinks are _supposed_ to support it, we will at least fall back > >>>> to it if there's nothing else. > >>>> > >>>> Note that we _don't_ add higher resolution modes like 1024x768 in this > >>>> case. We only add those modes for a failed EDID read where we have no > >>>> idea what's going on. In the case where we've pruned all modes then > >>>> instead we only want 640x480 which is the only defined "Fail Safe" > >>>> resolution. > >>>> > >>>> This patch originated in response to Kuogee Hsieh's patch [1]. > >>>> > >>>> [1] > >>>> https://lore.kernel.org/r/1650671124-14030-1-git-send-email-quic_khsieh@quicinc.com > >>>> > >>>> > >>>> Signed-off-by: Douglas Anderson > >>>> --- > >>>> > >>>> drivers/gpu/drm/drm_probe_helper.c | 26 +++++++++++++++++++++----- > >>>> 1 file changed, 21 insertions(+), 5 deletions(-) > >>>> > >>>> diff --git a/drivers/gpu/drm/drm_probe_helper.c > >>>> b/drivers/gpu/drm/drm_probe_helper.c > >>>> index 819225629010..90cd46cbfec1 100644 > >>>> --- a/drivers/gpu/drm/drm_probe_helper.c > >>>> +++ b/drivers/gpu/drm/drm_probe_helper.c > >>>> @@ -476,7 +476,6 @@ int drm_helper_probe_single_connector_modes(struct > >>>> drm_connector *connector, > >>>> const struct drm_connector_helper_funcs *connector_funcs = > >>>> connector->helper_private; > >>>> int count = 0, ret; > >>>> - bool verbose_prune = true; > >>>> enum drm_connector_status old_status; > >>>> struct drm_modeset_acquire_ctx ctx; > >>>> @@ -556,8 +555,8 @@ int drm_helper_probe_single_connector_modes(struct > >>>> drm_connector *connector, > >>>> DRM_DEBUG_KMS("[CONNECTOR:%d:%s] disconnected\n", > >>>> connector->base.id, connector->name); > >>>> drm_connector_update_edid_property(connector, NULL); > >>>> - verbose_prune = false; > >>>> - goto prune; > >>>> + drm_mode_prune_invalid(dev, &connector->modes, false); > >>>> + goto exit; > >>>> } > >>>> count = (*connector_funcs->get_modes)(connector); > >>>> @@ -580,9 +579,26 @@ int > >>>> drm_helper_probe_single_connector_modes(struct drm_connector *connector, > >>>> } > >>>> } > >>>> -prune: > >>>> - drm_mode_prune_invalid(dev, &connector->modes, verbose_prune); > >>>> + drm_mode_prune_invalid(dev, &connector->modes, true); > >>>> + /* > >>>> + * Displayport spec section 5.2.1.2 ("Video Timing Format") says > >>>> that > >>>> + * all detachable sinks shall support 640x480 @60Hz as a fail safe > >>>> + * mode. If all modes were pruned, perhaps because they need more > >>>> + * lanes or a higher pixel clock than available, at least try to add > >>>> + * in 640x480. > >>>> + */ > >>>> + if (list_empty(&connector->modes) && > >>>> + connector->connector_type == DRM_MODE_CONNECTOR_DisplayPort) { > >>>> + count = drm_add_modes_noedid(connector, 640, 480); > >>>> + if (_drm_helper_update_and_validate(connector, maxX, maxY, > >>>> &ctx)) { > >>>> + drm_modeset_backoff(&ctx); > >>>> + goto retry; > >>> > >>> Do we need another retry here? This will again repeat everything from > >>> get_modes(). > >>> The fact that we are hitting this code is because we have already tried > >>> that and this is already a second-pass. So I think another retry isnt > >>> needed? > >> > >> This will help cover the case of 4.2.2.6 but not fix 4.2.2.1. > >> > >> For 4.2.2.1, we will have 0 modes and so the original DRM fwk code of > >> adding all modes <= 1024x768 will kick in. > >> > >> Now, in that list, we will still need to pick/mark 640x480 as the > >> preferred mode. > >> > >> We still need IGT for that. > > > > Are you sure you don't have those backwards? It seems like 4.2.2.6 is > > the test case dealing with corrupt EDID and that's the one that will > > still be broken, no? ...and corrupt EDID is still the case where we > > have 0 modes. > > Yes indeed, sorry, I did have the numbers backwards. > 4.2.2.6 will still be broken. > > > > > In any case, let's see what people think about: > > > > https://lore.kernel.org/r/20220426132121.RFC.1.I31ec454f8d4ffce51a7708a8092f8a6f9c929092@changeid > > Yes sure. If it gets accepted, it will save us some IGT work. > > > > > I've marked that one as RFC just because it seems like a bigger change > > to existing behavior, though it still seems correct to me. > > > > NOTE: reading 4.2.2.6 more closely, it actually looks as if we're > > actually supposed to be able to try various video modes one at a time > > until we find one that works (or land on 640x480). Seems as if we're > > supposed to be able to try the higher resolutions one at a time and we > > can tell whether the sink "accepted" it by seeing if SINK_STATUS goes > > to 1? I have no idea how that works with all the Linux APIs, though. > > > > hmmm .... our equipment throws a warning if we dont sent 640x480. So > perhaps just go with the "or land on 640x480" option. > > 0006.392.232: [WARNING] Source DUT failed to transmit a video stream > using fail-safe mode > 0006.392.491: Received 1344 Htotal differs from fail-safe 800 > 0006.392.621: Received 1024 Hactive differs from fail-safe 640 > 0006.392.750: Received 296 Hstart differs from fail-safe 144 > 0006.392.868: Received 136 Hsync width differs from fail-safe 96 > 0006.392.975: Received 806 Vtotal differs from fail-safe 525 > 0006.393.099: Received 768 Vactive differs from fail-safe 480 > 0006.393.229: Received 6 Vsync width differs from fail-safe 2 Do you actually have code to implement the checking of SINK_STATUS? I'm not aware of how that would work in Linux, which is why just defaulting to 640x480 seems like a reasonable thing to do for now. The test case actually says that you're allowed to try clock rates one at a time (polling SINK_STATUS in DPCT) as long as you don't spend more than 5 seconds on each clock rate. According to the test case if you never saw SINK_STATUS in DPCT go to 1 then you should end at 640x480.