linux-kernel - Re: [RFC PATCH] drm/edid: drm_add_modes_noedid() should set lowest resolution as preferred

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Date:   Tue, 26 Apr 2022 14:21:16 -0700
From:   Abhinav Kumar <quic_abhinavk@...cinc.com>
To:     Doug Anderson <dianders@...omium.org>
CC:     Sankeerth Billakanti <quic_sbillaka@...cinc.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        "Aravind Venkateswaran (QUIC)" <quic_aravindh@...cinc.com>,
        "Kuogee Hsieh (QUIC)" <quic_khsieh@...cinc.com>
Subject: Re: [RFC PATCH] drm/edid: drm_add_modes_noedid() should set lowest
 resolution as preferred



On 4/26/2022 1:52 PM, Doug Anderson wrote:
> Hi,
> 
> On Tue, Apr 26, 2022 at 1:46 PM Abhinav Kumar <quic_abhinavk@...cinc.com> wrote:
>>
>> On 4/26/2022 1:21 PM, Douglas Anderson wrote:
>>> If we're unable to read the EDID for a display because it's corrupt /
>>> bogus / invalid then we'll add a set of standard modes for the
>>> display. When userspace looks at these modes it doesn't really have a
>>> good concept for which mode to pick and it'll likely pick the highest
>>> resolution one by default. That's probably not ideal because the modes
>>> were purely guesses on the part of the Linux kernel.
>>>
>>> Let's instead set 640x480 as the "preferred" mode when we have no EDID.
>>>
>>> Signed-off-by: Douglas Anderson <dianders@...omium.org>
>>
>> drm_dmt_modes array is sorted but you are also relying on this check to
>> eliminate the non-60fps modes
>>
>> 5611            if (drm_mode_vrefresh(ptr) > 61)
>> 5612                    continue;
>>
>> I am not sure why we filter out the modes > 61 vrefresh.
>>
>> If that check will remain this is okay.
>>
>> If its not, its not reliable that the first mode will be 640x480@60
> 
> I suspect that the check will remain. I guess I could try to do
> something fancier if people want, but I'd be interested in _what_
> fancier thing I should do if so. Do we want the rule to remain that we
> always prefer 640x480, or do we want to prefer the lowest resolution?
> ...do we want to prefer 60 Hz or the lowest refresh rate? Do we do
> this only for DP (which explicitly calls out 640x480 @60Hz as the best
> failsafe) or for everything?
> 
> For now, the way it's coded up seems reasonable (to me). It's the
> lowest resolution _and_ it's 640x480 just because of the current
> values of the table. I suspect that extra lower resolution failsafe
> modes won't be added, but we can always change the rules here if/when
> they are.
> 
> -Doug

Alright, agreed. The way the API is today, I dont see anything getting 
broken with this.

So typically, as per spec, when a preferred mode is not set by the sink, 
the first entry becomes the preferred mode.

This also aligns with that expectation.

So FWIW,

Reviewed-by: Abhinav Kumar <quic_abhinavk@...cinc.com>

We will test this one also out with our equipment, then give tested-by tags.

Thanks

Abhinav