Hi,
On Wed, May 11, 2022 at 3:58 PM 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@quic...
Signed-off-by: Douglas Anderson dianders@chromium.org Tested-by: Kuogee Hsieh quic_khsieh@quicinc.com Reviewed-by: Abhinav Kumar quic_abhinavk@quicinc.com Reviewed-by: Dmitry Baryshkov dmitry.baryshkov@linaro.org
Changes in v3:
- Removed WARN_ON
Changes in v2:
- Two underscores for __drm_helper_update_and_validate().
- Return err and use WARN_ON instead of returning a bool.
drivers/gpu/drm/drm_probe_helper.c | 27 ++++++++++++++++++++++----- 1 file changed, 22 insertions(+), 5 deletions(-)
Pushed to drm-misc-next:
e7c254d75d16 drm/probe-helper: For DP, add 640x480 if all other modes are bad