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    SubjectRe: [RFC PATCH] drm/edid: drm_add_modes_noedid() should set lowest resolution as preferred
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    On 4/26/2022 1:52 PM, Doug Anderson wrote:
    > Hi,
    >
    > On Tue, Apr 26, 2022 at 1:46 PM Abhinav Kumar <quic_abhinavk@quicinc.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@chromium.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@quicinc.com>

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

    Thanks

    Abhinav

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