Re: [PATCH 2/2] drm/probe-helper: For DP, add 640x480 if all other modes are bad

[Doug Anderson]

Hi,

On Tue, Apr 26, 2022 at 2:11 PM Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx> 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@xxxxxxxxxxx> 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@xxxxxxxxxxx
> >>>>
> >>>>
> >>>> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> >>>> ---
> >>>>
> >>>>    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.