Date:   Wed, 27 Apr 2022 14:24:35 -0700
From:   Kuogee Hsieh <quic_khsieh@...cinc.com>
To:     Doug Anderson <dianders@...omium.org>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>
CC:     Sankeerth Billakanti <quic_sbillaka@...cinc.com>,
        David Airlie <airlied@...ux.ie>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        "Aravind Venkateswaran (QUIC)" <quic_aravindh@...cinc.com>,
        Stephen Boyd <swboyd@...omium.org>
Subject: Re: [PATCH 2/2] drm/probe-helper: For DP, add 640x480 if all other
 modes are bad

Tested-by: Kuogee Hsieh <quic_khsieh@...cinc.com>

On 4/26/2022 2:17 PM, Doug Anderson wrote:
> Hi,
>
> On Tue, Apr 26, 2022 at 2:11 PM Abhinav Kumar <quic_abhinavk@...cinc.com> wrote:
>>
>>
>> On 4/26/2022 1:26 PM, Doug Anderson wrote:
>>> Hi,
>>>
>>> On Tue, Apr 26, 2022 at 12:20 PM Abhinav Kumar
>>> <quic_abhinavk@...cinc.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@...omium.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
> 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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