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Message-ID: <37b16e86-f726-bf26-98ba-6146b6835c04@quicinc.com>
Date:   Tue, 26 Apr 2022 14:11:29 -0700
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Subject: Re: [PATCH 2/2] drm/probe-helper: For DP, add 640x480 if all other
 modes are bad
Content-Language: en-US
To:     Doug Anderson <dianders@chromium.org>
CC:     Sankeerth Billakanti <quic_sbillaka@quicinc.com>,
        David Airlie <airlied@linux.ie>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        "Kuogee Hsieh (QUIC)" <quic_khsieh@quicinc.com>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
        "Aravind Venkateswaran (QUIC)" <quic_aravindh@quicinc.com>,
        Stephen Boyd <swboyd@chromium.org>
References: <20220426114627.1.I2dd93486c6952bd52f2020904de0133970d11b29@changeid>
 <20220426114627.2.I4ac7f55aa446699f8c200a23c10463256f6f439f@changeid>
 <fa49384c-76a5-6686-7d4d-cf11f3e98c75@quicinc.com>
 <517f71e4-785f-ef6f-d30e-fb18974eed57@quicinc.com>
 <CAD=FV=Xn_uTOxvwTZsKnQN4651T1OuZNd-EQG0WqvDsFPz+4hQ@mail.gmail.com>
From:   Abhinav Kumar <quic_abhinavk@quicinc.com>
In-Reply-To: <CAD=FV=Xn_uTOxvwTZsKnQN4651T1OuZNd-EQG0WqvDsFPz+4hQ@mail.gmail.com>
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On 4/26/2022 1:26 PM, Doug Anderson wrote:
> Hi,
> 
> On Tue, Apr 26, 2022 at 12:20 PM Abhinav Kumar
> <quic_abhinavk@quicinc.com> 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 <dianders@chromium.org>
>>>> ---
>>>>
>>>>    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


> -Doug