Messages in this thread | | | Date | Thu, 5 May 2022 10:20:49 -0700 | Subject | Re: [PATCH 2/2] drm/probe-helper: For DP, add 640x480 if all other modes are bad | From | Abhinav Kumar <> |
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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>
> > [1] https://lore.kernel.org/r/20220426132121.RFC.1.I31ec454f8d4ffce51a7708a8092f8a6f9c929092@changeid > > -Doug
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