Messages in this thread | | | Date | Thu, 5 May 2022 10:30:22 -0700 | Subject | Re: [PATCH 2/2] drm/probe-helper: For DP, add 640x480 if all other modes are bad | From | Kuogee Hsieh <> |
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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@chromium.org> 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(-) >> >> 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@quicinc.com> > Tested-by: Kuogee Hsieh <quic_khsieh@quicinc.com> >> >> [1] >> https://lore.kernel.org/r/20220426132121.RFC.1.I31ec454f8d4ffce51a7708a8092f8a6f9c929092@changeid >> >> -Doug
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