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

[Kuogee Hsieh <quic_khsieh@quicinc.com>]

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