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