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

[Dmitry Baryshkov]

On Thu, 5 May 2022 at 20:30, Kuogee Hsieh <quic_khsieh@xxxxxxxxxxx> wrote:
>
>
> On 5/5/2022 10:20 AM, Abhinav Kumar wrote:
> > Hi Doug
> >
> > On 5/5/2022 8:44 AM, Doug Anderson wrote:
> >> Ville,
> >>
> >> On Tue, Apr 26, 2022 at 11:47 AM Douglas Anderson
> >> <dianders@xxxxxxxxxxxx> 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(-)
> >>
> >> I think this patch is fairly safe / non-controversial, but someone
> >> suggested you might have an opinion on it and another patch I posted
> >> recently [1] so I wanted to double-check. Just to be clear: I'm hoping
> >> to land _both_ this patch and [1]. If you don't have an opinion,
> >> that's OK too.
> >>
> >> Abhinav: I think maybe you're happy with this now? Would you be
> >> willing to give a Reviewed-by?
> >
> > Yes, I have no concerns with this approach from DP spec standpoint and
> > in addition, kuogee has tested this out and this does help us to pass
> > the tests.
> >
> > Although, I might be missing some historical context on why this is
> > not already done.
> >
> > But apart from that, LGTM. Hence,
> >
> > Reviewed-by: Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx>
> > Tested-by: Kuogee Hsieh <quic_khsieh@xxxxxxxxxxx>

This line got wrong quotation level, so it will not be noticed by
patchwork (and can be easily missed by other people too). Please
resend.

> >>
> >> [1]
> >> https://lore.kernel.org/r/20220426132121.RFC.1.I31ec454f8d4ffce51a7708a8092f8a6f9c929092@changeid
> >>
> >> -Doug



-- 
With best wishes
Dmitry